What are alcohols in the context of Edexcel IGCSE Chemistry?

In Edexcel IGCSE Chemistry, alcohols are a group of organic compounds characterized by the presence of a hydroxyl group (-OH) attached to a carbon atom. They are part of the homologous series with the general formula CnH2n+1OH. Common examples include methanol and ethanol.

How are alcohols named according to IUPAC nomenclature?

In IUPAC nomenclature, alcohols are named by identifying the longest carbon chain containing the hydroxyl group and replacing the '-e' ending of the corresponding alkane with '-ol'. The position of the hydroxyl group is indicated by a number. For example, CH3CH2OH is named ethanol, and CH3CH(OH)CH3 is named propan-2-ol.

What are the physical properties of alcohols studied in IGCSE Chemistry?

Alcohols generally have higher boiling points than alkanes of similar molecular weight due to hydrogen bonding between molecules. They are also polar, making them soluble in water, especially the lower alcohols like methanol and ethanol. As the carbon chain length increases, solubility in water decreases.

What are the common reactions involving alcohols in organic chemistry?

In organic chemistry, alcohols can undergo several reactions. They can be oxidized to form aldehydes, ketones, or carboxylic acids. Alcohols also react with carboxylic acids to form esters in a process called esterification. Additionally, they can undergo dehydration to form alkenes.

How is ethanol produced industrially, and why is it significant?

Ethanol is produced industrially through the fermentation of sugars by yeast or by the hydration of ethene. It is significant due to its wide range of uses, including as a solvent, in alcoholic beverages, and as a biofuel. Understanding its production and uses is part of the Edexcel IGCSE Chemistry curriculum.

Why is "Saying the colour change for ethanol oxidation is 'orange to colourless' or 'orange to yellow'" a common mistake in Alcohols?

Why it happens: Confusing with bromine water or other decolourisation reactions. How to avoid it: Acidified potassium dichromate goes from ORANGE (Cr₂O₇²⁻) to GREEN (Cr³⁺) — NOT colourless. Memorise: 'dichromate orange → chromium GREEN'. Source: 4CH1 Examiner Reports 2022-2024

Why is "Saying fermentation uses oxygen / is aerobic" a common mistake in Alcohols?

Why it happens: Confusion with respiration or with the bacterial oxidation of wine → vinegar. How to avoid it: Fermentation is ANAEROBIC (no oxygen). The vessel must be sealed (airlock lets CO₂ out, keeps air out). If O₂ is present, ethanol is oxidised to ethanoic acid (vinegar) instead → wine 'goes off'. The mark scheme wants 'anaerobic' or 'no air/oxygen'. Source: 4CH1 Examiner Reports 2023-2024

Why is "Saying fermentation must be done at a high temperature" a common mistake in Alcohols?

Why it happens: Confusion with industrial / petrochemical conditions. How to avoid it: Fermentation is at 25-35 °C — WARM but not hot. If T > ~ 40 °C, the yeast enzymes are DENATURED and the yeast dies → reaction stops. If T < 20 °C, the rate is too slow. The narrow optimum is one disadvantage of fermentation. Source: 4CH1 Examiner Reports 2022-2024

Why is "Confusing methanol and ethanol — writing 'methanol' as the product of fermentation, or 'ethanol' as poisonous" a common mistake in Alcohols?

Why it happens: The names sound similar. How to avoid it: **Methanol** = CH₃OH = poisonous, NOT for drinking. **Ethanol** = C₂H₅OH = the alcohol in drinks, made by fermentation, can be safely consumed. Always count the carbon atoms before naming. C₁ = meth-; C₂ = eth-; C₃ = prop-. Source: 4CH1 Examiner Reports 2022-2024

Why is "Saying ethanol is soluble in water because it 'has H atoms' or 'is small'" a common mistake in Alcohols?

Why it happens: Vague answer. How to avoid it: The correct reason: ethanol has a polar –OH group that forms HYDROGEN BONDS with water molecules → highly soluble. Use the phrase 'hydrogen bonding between the –OH and water'.

Why is "Writing C₆H₁₂O₆ → C₂H₅OH + CO₂ (unbalanced)" a common mistake in Alcohols?

Why it happens: Forgetting that glucose has 6 carbons → must split into 2 ethanol (2 C each) + 2 CO₂ (1 C each) to balance. How to avoid it: Always balance the C atoms: glucose has 6 C → 2 ethanol (2 C × 2 = 4 C) + 2 CO₂ (1 C × 2 = 2 C) = 6 C total. H: glucose 12 → 2C₂H₅OH gives 12 (2 × 6) ✓. O: glucose 6 → 2 ethanol (1 × 2 = 2) + 2 CO₂ (2 × 2 = 4) = 6 ✓. Correct equation: **C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂**. Source: 4CH1 Examiner Reports 2023-2024

Organic ChemistryEdexcel IGCSE Chemistry (4CH1)

Alcohols

Q: Why is "Confusing methanol and ethanol — writing 'methanol' as the product of fermentation, or 'ethanol' as poisonous" a common mistake in Alcohols?

Why it happens: The names sound similar. How to avoid it: **Methanol** = CH₃OH = poisonous, NOT for drinking. **Ethanol** = C₂H₅OH = the alcohol in drinks, made by fermentation, can be safely consumed. Always count the carbon atoms before naming. C₁ = meth-; C₂ = eth-; C₃ = prop-. Source: 4CH1 Examiner Reports 2022-2024

Q: Why is "Saying ethanol is soluble in water because it 'has H atoms' or 'is small'" a common mistake in Alcohols?

Why it happens: Vague answer. How to avoid it: The correct reason: ethanol has a polar –OH group that forms HYDROGEN BONDS with water molecules → highly soluble. Use the phrase 'hydrogen bonding between the –OH and water'.

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

Previous Next

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Page 1 / 0

Detailed Study Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Alcohols, ready to print or save offline.

Alcohols — Pearson Edexcel International GCSE Chemistry 4CH1 Study Notes (2026 onwards, Spec Issue 4)

Homologous series with –OH (hydroxyl) functional group. General formula CₙH₂ₙ₊₁OH. First three: methanol CH₃OH, ethanol CH₃CH₂OH, propan-1-ol CH₃CH₂CH₂OH (isomer: propan-2-ol). Soluble in water (–OH hydrogen-bonds with H₂O). Ethanol made TWO ways: HYDRATION of ethene (continuous, fast, pure, non-renewable — H₃PO₄, 300 °C, 60 atm) or FERMENTATION of glucose by yeast (renewable, slow, batch, dilute — 25-35 °C, anaerobic). Reactions of ethanol: combustion (CO₂ + H₂O — biofuel); oxidation with acidified K₂Cr₂O₇ → ethanoic acid (ORANGE → GREEN colour change); dehydration over Al₂O₃ at 600 °C → ethene. Methanol is POISONOUS.

At a glance

Alcohol = organic compound with –OH (hydroxyl) group. General formula CₙH₂ₙ₊₁OH.
First three: methanol CH₃OH, ethanol CH₃CH₂OH, propan-1-ol CH₃CH₂CH₂OH (isomer: propan-2-ol CH₃CH(OH)CH₃).
Short-chain alcohols: COLOURLESS LIQUIDS at room T, SOLUBLE in water (–OH hydrogen-bonds with H₂O).
Naming: alkane stem + -ol suffix. For C₃+ use locant (propan-1-ol vs propan-2-ol).
Two industrial methods to make ethanol:
1. HYDRATION of ethene: C₂H₄ + H₂O ⇌ C₂H₅OH. H₃PO₄, 300 °C, 60 atm. Fast, continuous, pure, NON-RENEWABLE.
1. FERMENTATION of glucose: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Yeast, 25-35 °C, anaerobic. Slow, batch, dilute (~14%), RENEWABLE.
Key reactions of ethanol: combustion (CO₂ + H₂O — fuel); oxidation with acidified K₂Cr₂O₇ → ethanoic acid (ORANGE → GREEN); dehydration over hot Al₂O₃ → ethene.
Uses: ethanol = drinks, solvent, fuel, sanitiser. Methanol = solvent, feedstock, antifreeze (POISONOUS).

What you’ll learn

Mapped to the Cambridge IGCSE 4CH1 syllabus (2026 onwards).

4.27 — Recall the general formula CₙH₂ₙ₊₁OH and state the functional group –OH of alcohols.
4.28 — Name and write the structural formula of the first three members of the alcohol homologous series (methanol, ethanol, propan-1-ol).
4.29 — Describe the manufacture of ethanol by: (a) hydration of ethene over phosphoric acid catalyst at 300 °C and 60 atm; (b) fermentation of glucose by yeast at 25-35 °C under anaerobic conditions. Compare advantages and disadvantages of each method.
4.30P — (Paper 2) Describe the OXIDATION of ethanol to ethanoic acid using acidified potassium dichromate(VI), including the colour change from orange to green; or by bacteria in air (vinegar-making).
4.31 — Recall key uses of methanol (industrial solvent, chemical feedstock) and ethanol (alcoholic drinks, solvent, biofuel, hand sanitiser).

Alcohol structure and the –OH functional group (spec 4.27, 4.28)

CₙH₂ₙ₊₁OH. Methanol, ethanol, propan-1-ol. Soluble in water (–OH H-bonds).

Alcohols are a homologous series of organic compounds characterised by the HYDROXYL group –OH bonded to a carbon atom.

General formula: $C_n H_{2n+1} OH$ .

This represents an alkyl group (CₙH₂ₙ₊₁ from the corresponding alkane) with one H replaced by an OH group.

First three alcohols.

n	Name	Structural formula	Molecular formula	bp (°C)
1	Methanol	CH₃OH	CH₄O	65
2	Ethanol	CH₃CH₂OH (or C₂H₅OH)	C₂H₆O	78
3	Propan-1-ol	CH₃CH₂CH₂OH	C₃H₈O	97

Naming pattern. Take the alkane name (methane, ethane, propane) → drop the '-e' → add '-ol'. For 3+ carbons, include a LOCANT (number) indicating which carbon carries the –OH: propan-1-ol (OH on C1) vs propan-2-ol (OH on C2, the middle carbon).

Positional isomerism — propan-1-ol vs propan-2-ol.

Both have the same molecular formula C₃H₈O but different positions of the –OH group:

Property	Propan-1-ol	Propan-2-ol
Structural formula	CH₃CH₂CH₂OH	(CH₃)₂CHOH or CH₃CH(OH)CH₃
–OH position	C1 (end carbon)	C2 (middle carbon)
Boiling point	97 °C	82 °C
Smell	Sharp, alcoholic	Slightly sweet

Both are alcohols, both have the –OH functional group, both share most of the same chemistry — they just differ slightly in physical properties because of the different molecular shape.

Displayed formula of ethanol.

   H   H
   |   |
H-C-C-O-H
   |   |
   H   H

The –OH group is at the end. Ethanol has 2 C, 6 H, 1 O = C₂H₆O.

Why short-chain alcohols are LIQUIDS at room temperature.

Compare with the corresponding alkanes:

Alcohol	bp (°C)	Alkane	bp (°C)
Methanol	65	Methane	–162
Ethanol	78	Ethane	–89
Propan-1-ol	97	Propane	–42

Each alcohol has a MUCH HIGHER bp than the corresponding alkane (despite similar molecular masses). Why?

HYDROGEN BONDING. The –OH group of alcohols is polar (O is much more electronegative than H, so the O has a δ⁻ charge and the H has a δ⁺ charge). The δ⁻ O of one molecule attracts the δ⁺ H of another molecule → forms a hydrogen bond. Hydrogen bonds are much stronger than the van der Waals forces between alkane molecules → much more energy required to separate alcohol molecules → much higher boiling points.

Alkane molecules have only weak van der Waals forces (no polar groups) → much lower bp.

Why short-chain alcohols are SOLUBLE in water.

Water (H₂O) is also a polar molecule that forms hydrogen bonds. Alcohols can hydrogen-bond DIRECTLY with water (alcohol's –O-H to water's O, or vice versa) → the alcohol molecules disperse easily into water → high solubility.

Alcohol	Solubility in water
Methanol	Fully miscible (any ratio)
Ethanol	Fully miscible
Propan-1-ol	Fully miscible
Butan-1-ol	Partially soluble (~ 8 g/100 mL)
Pentan-1-ol	Slightly soluble (~ 2 g/100 mL)

As the alkyl chain gets longer, the non-polar hydrocarbon part dominates → water solubility drops. Long-chain alcohols (C₁₀+) are essentially insoluble in water.

The dual nature of alcohols.

Alcohols have BOTH polar (–OH) and non-polar (alkyl chain) regions → they can dissolve in both water (via –OH H-bonding) AND non-polar solvents (via van der Waals on the alkyl part). This makes them excellent SOLVENTS that can dissolve a wide range of substances — useful for perfumes, cleaning agents, paint thinners, antiseptics.

Industrial production: methanol vs ethanol.

Methanol: made on huge scale by catalytic conversion of synthesis gas (CO + 2H₂ → CH₃OH, Cu/ZnO/Al₂O₃ catalyst, 250 °C, 50-100 atm). Used as chemical feedstock for plastics + biodiesel.
Ethanol: made by TWO routes — hydration of ethene (industrial petrochemical) and fermentation of glucose (biological, used for drinks + biofuel). Detail in the next section.

Common uses.

Alcohol	Uses
Methanol	Industrial solvent, chemical feedstock, biodiesel production, antifreeze (windscreen washer). POISONOUS to humans.
Ethanol	Alcoholic drinks (beer ~5%, wine ~12%, spirits ~40%); solvent (perfumes, paints, mouthwash); biofuel; antiseptic / hand sanitiser; chemical feedstock.
Propan-1-ol & propan-2-ol	Solvents, antiseptics, screen cleaners, automotive products.

Safety: methanol is POISONOUS.

NEVER drink methanol. In the body, methanol is metabolised in the liver to METHANAL (HCHO) and METHANOIC ACID (HCOOH), which damage:

The optic nerve → causes BLINDNESS (just 10 mL can cause permanent damage).
Metabolic acidosis → coma and death (30 mL can be fatal).

This is why methanol is added to industrial ethanol to make 'methylated spirits' (denatured alcohol) — undrinkable but tax-free for industrial use.

Alcohols: CₙH₂ₙ₊₁OH, –OH functional group.
Methanol CH₃OH, ethanol CH₃CH₂OH, propan-1-ol CH₃CH₂CH₂OH.
Liquids at room T, soluble in water (hydrogen bonding with H₂O).
Higher bp than corresponding alkanes — H-bonding between alcohol molecules.
Positional isomerism: propan-1-ol vs propan-2-ol (same molecular formula, different OH position).
Methanol POISONOUS (causes blindness and death — methylated spirits).
Ethanol = drinks, solvent, fuel, sanitiser. Methanol = industrial solvent + feedstock.

See the full worked example for alcohols →

Two methods to make ethanol (spec 4.29)

HYDRATION (ethene + H₂O, H₃PO₄, 300°C/60atm — fast, non-renewable) vs FERMENTATION (glucose + yeast, 25-35°C, anaerobic — slow, renewable).

Ethanol is one of the most important industrial chemicals. There are TWO main ways to make it.

Method 1 — Hydration of ethene.

Ethene (from cracking crude oil) reacts with steam over a phosphoric acid catalyst:

$\text{CH}_2{=}\text{CH}_2(\text{g}) + \text{H}_2\text{O(g)} \rightleftharpoons \text{CH}_3\text{CH}_2\text{OH(g)}$

Conditions:

Catalyst: concentrated phosphoric acid (H₃PO₄), typically adsorbed on silica.
Temperature: 300 °C.
Pressure: 60 atm.

The reaction is REVERSIBLE — only ~5% conversion per pass. Unreacted ethene + steam are RECYCLED back to the reactor. Over many passes, overall conversion reaches ~97%. CONTINUOUS process (24/7).

Why these conditions?

Forward reaction is exothermic and has fewer moles of gas (2 → 1), so high P favours product (Le Chatelier).
300 °C is a compromise: too low → slow rate; too high → equilibrium shifts back.
60 atm: compromise between yield and equipment cost.

Pros and cons.

Advantages:

FAST (minutes per pass) — high throughput.
CONTINUOUS — 24/7 operation.
PURE ethanol (~95%) directly — no further purification needed.
Reliable, predictable industrial chemistry.

Disadvantages:

Uses NON-RENEWABLE crude oil (source of ethene).
HIGH ENERGY: 300 °C + 60 atm — expensive electricity / steam + high CO₂ emissions from heating.
Capital-intensive: cracker + reactor + recycling = expensive equipment.

Method 2 — Fermentation of glucose.

Yeast contains enzymes (zymase complex) that convert glucose to ethanol + CO₂ under anaerobic conditions:

$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$

Conditions:

Yeast — supplies enzymes that catalyse the reaction.
Temperature 25-35 °C — optimum for enzymes.
Anaerobic (no oxygen) — otherwise yeast respires aerobically (no ethanol produced) OR bacteria oxidise the ethanol to ethanoic acid (vinegar).
Aqueous solution of glucose / sucrose (from sugar cane, sugar beet, fruit, grain).

Why temperature is critical.

Temperature	Effect
Below ~20 °C	Enzymes work slowly → slow rate
25-35 °C	OPTIMUM — fast enzyme activity
Above ~40 °C	Enzymes DENATURED — protein shape destroyed, active site lost, yeast dies

The optimum is narrow because the yeast is a LIVING ORGANISM with PROTEIN ENZYMES.

Why anaerobic.

If O₂ is present:

Yeast respires AEROBICALLY: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$ — no ethanol.
OR bacteria (Acetobacter) in air oxidise any ethanol to ethanoic acid: $\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$ — vinegar.

So the vessel is SEALED with an AIRLOCK that lets CO₂ escape but keeps O₂ out.

The 15% limit.

Yeast dies when the ethanol concentration reaches ~14-15%. Ethanol is toxic to yeast at higher concentrations → reaction stops. To get pure ethanol, the fermented mixture must be FRACTIONALLY DISTILLED (ethanol bp 78 °C; water bp 100 °C — ethanol vapour rises first).

Pros and cons.

Advantages:

Uses RENEWABLE sugars from plants (sugar cane, beet, corn).
LOW ENERGY (near room T, atm P).
CARBON-NEUTRAL — CO₂ released absorbed by next crop (photosynthesis).
Cheap to start up (vessels + airlock + yeast).
Only way to make alcoholic beverages.

Disadvantages:

SLOW (days to weeks).
BATCH process — must drain, clean, restart.
DILUTE product (~14%) needs distillation.
Yeast dies at ~15% → can't push concentration further.
Uses food crops (competes with food production).
Requires arable land (deforestation pressure).

Side-by-side comparison.

Feature	Hydration of ethene	Fermentation
Source	Crude oil (ethene)	Sugar crops (glucose)
Renewable?	NO (fossil fuel)	YES (plants regrow)
Mode	Continuous	Batch
Speed	Fast (minutes/pass)	Slow (days/weeks)
Conditions	300 °C, 60 atm, H₃PO₄	25-35 °C, atm P, yeast
Energy	High	Low
Purity of product	~95% direct	~14% (needs distillation)
Carbon footprint	High (fossil)	Carbon-neutral
Yield	High (after recycling, ~97%)	Limited (~15% max)
Capital cost	Very high	Low

Which one is used when?

Both methods are used commercially:

Hydration of ethene dominates industrial ethanol (solvents, sanitiser, chemical feedstock, ethyl acetate production) — pure ethanol needed in bulk.
Fermentation is the route for alcoholic beverages (the ONLY route for food/drink ethanol) and increasingly for BIOETHANOL (motor fuel, especially in Brazil where ~30% of motor fuel is ethanol from sugar cane).

As fossil fuels become scarcer and climate policies tighten, fermentation will likely grow relative to hydration. The shift is already visible in many countries.

Real-world example: Brazil's bioethanol industry.

Brazil produces ~30 billion litres of bioethanol per year from sugar cane fermentation. About 30% of all motor fuel in Brazil is ethanol (either as E10 blends with petrol or as neat ethanol in flex-fuel vehicles). The industry employs hundreds of thousands of workers and reduces fossil fuel imports. Critics argue about deforestation pressures and land-use changes, but for sugar cane specifically (efficient crop), bioethanol can have a much lower carbon footprint than petrol.

Two methods to make ethanol: hydration of ethene + fermentation of glucose.
Hydration: C₂H₄ + H₂O ⇌ C₂H₅OH. H₃PO₄, 300 °C, 60 atm. Continuous, fast, pure, NON-renewable.
Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Yeast, 25-35 °C, anaerobic. Batch, slow, dilute (~14%), RENEWABLE.
Fermentation T is critical: <20 °C slow; >40 °C enzymes denatured.
Anaerobic essential: O₂ would either stop ethanol formation OR oxidise it to ethanoic acid (vinegar).
Ethanol from fermentation is carbon-neutral (CO₂ released = CO₂ absorbed by next crop).
Hydration used for industrial ethanol; fermentation for drinks + bioethanol fuel.

See the full worked example for alcohols →

Key reactions of ethanol (spec 4.30P, 4.31)

COMBUSTION (CO₂ + H₂O + energy — biofuel). OXIDATION with K₂Cr₂O₇ (orange→green) → ethanoic acid. DEHYDRATION with Al₂O₃ → ethene.

Ethanol's –OH functional group + the alkyl chain make it react in several useful ways.

Reaction 1: COMBUSTION (burning in air).

Ethanol burns in oxygen to give CO₂ + H₂O + energy:

$\text{C}_2\text{H}_5\text{OH(l)} + 3\text{O}_2(\text{g}) \rightarrow 2\text{CO}_2(\text{g}) + 3\text{H}_2\text{O(l)} + \text{energy}$

The flame is clean and blue (compared with the smoky yellow flame of incomplete combustion of larger hydrocarbons).

Uses:

Biofuel — bioethanol blended with petrol (E10 = 10% ethanol; E85 = 85%; or pure E100 in flex-fuel cars).
Camping fuel — small portable cookers.
Alcohol stoves in labs (spirit burner — fewer fumes than Bunsen).
Flambé — alcohol on food, lit dramatically in restaurants.

Why ethanol is good as a fuel:

High energy content per kg (~ 27 MJ/kg — about 70% of petrol's ~ 44 MJ/kg).
Clean burn (full combustion gives only CO₂ + H₂O — no soot, no SO₂, less CO).
Renewable if made by fermentation → carbon-neutral.
Burns efficiently in modified petrol engines.

Compared with petrol: ethanol has lower energy density (need more litres for same distance), but produces less air pollution and is renewable.

Reaction 2: OXIDATION to ethanoic acid (Paper 2 content).

When ethanol is heated with an OXIDISING AGENT, it is converted to ETHANOIC ACID.

Laboratory method. Heat ethanol with acidified POTASSIUM DICHROMATE(VI):

$\text{C}_2\text{H}_5\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$

(where [O] represents 'oxidising agent' — a simplified notation).

Reagents:

Oxidising agent: acidified potassium dichromate(VI), K₂Cr₂O₇. The active ion is the dichromate Cr₂O₇²⁻ — orange in solution.
Acid: dilute sulfuric acid (H₂SO₄) — provides H⁺ ions needed for the reduction half-reaction.

Observation — COLOUR CHANGE: ORANGE → GREEN.

Cr₂O₇²⁻ (dichromate, Cr in +6 oxidation state) is ORANGE.
Cr³⁺ (chromium(III) ion, +3 state) is GREEN.

The orange dichromate is REDUCED to green Cr³⁺ as it oxidises the ethanol. Watch the colour change to confirm the reaction is happening.

Conditions.

Heat under REFLUX for ~ 20-30 minutes (vertical condenser ensures no volatile reactants/products are lost).
Temperature ~ 60-80 °C.

The chemistry happening.

In the ethanol molecule CH₃CH₂OH, the OH on the END carbon is converted to a –COOH group:

Remove 2 H atoms from –CH₂–OH (one from the OH, one from the CH₂).
Insert an O atom to form –C(=O)–OH = –COOH.
Net: ethanol's –CH₂–OH → –COOH.

The 2 H atoms removed combine with one [O] from the oxidiser to form H₂O. Another [O] is inserted into the bond.

Industrial / natural route (Paper 2).

In nature, bacteria (e.g. Acetobacter) carry out the SAME overall transformation using O₂ from air:

$\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \xrightarrow{\text{bacteria}} \text{CH}_3\text{COOH} + \text{H}_2\text{O}$

This is the basis of:

Wine turning to vinegar when exposed to air.
Industrial vinegar production: aerobic fermentation of dilute ethanol over a few weeks.
Sour beer / mead — same chemistry.

Why is wine sealed with corks and bottles? To keep O₂ out (and prevent bacterial oxidation to vinegar).

Reaction 3: DEHYDRATION to ethene.

Pass ethanol vapour over a HOT CATALYST → loses water → forms ethene.

$\text{C}_2\text{H}_5\text{OH(g)} \xrightarrow{\text{cat, } \sim 600 °\text{C}} \text{CH}_2{=}\text{CH}_2(\text{g}) + \text{H}_2\text{O(g)}$

Catalyst options:

Aluminium oxide (Al₂O₃) at ~ 600 °C.
Pumice stone (porous catalyst) heated red-hot.
Concentrated sulfuric acid (used in older methods, less safe).

The dehydration is the REVERSE of the hydration reaction used to MAKE ethanol industrially:

$\text{C}_2\text{H}_5\text{OH} \rightleftharpoons \text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O}$

The conditions (T, P, catalyst) chosen determine which direction is favoured.

Industrial relevance. Dehydration of bioethanol provides a 'green' source of ethene for plastic manufacture — useful as bioethanol becomes more competitive with petrochemical ethene from cracking.

Summary of ethanol's three reactions.

Reaction	Conditions	Equation	Product	Use
Combustion	O₂ + spark	C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O	Heat + CO₂ + H₂O	Biofuel, cooking
Oxidation	Acidified K₂Cr₂O₇, reflux	C₂H₅OH + 2[O] → CH₃COOH + H₂O	Ethanoic acid (orange→green colour)	Vinegar, chemical feedstock
Dehydration	Al₂O₃ catalyst, 600 °C	C₂H₅OH → C₂H₄ + H₂O	Ethene	Green ethene for plastics

These three reactions show the versatility of the –OH functional group: it can be oxidised (to acid), removed (giving alkene), or simply burnt (as fuel).

Uses of ethanol in everyday life.

Alcoholic drinks: beer ~ 5%, wine ~ 12%, spirits (vodka, whisky, rum) ~ 40%. Made by fermentation. Effects on the body: depressant on the central nervous system; metabolised by the liver via alcohol dehydrogenase to ethanal then to ethanoic acid then to CO₂ + H₂O.
Solvent: dissolves both polar and non-polar substances → used in perfumes, paints, varnishes, lacquers, mouthwash, tinctures.
Biofuel: bioethanol from fermentation, blended with petrol (E10, E85) or used pure (E100). Carbon-neutral, renewable.
Hand sanitiser: 60-95% ethanol kills bacteria + viruses by denaturing their proteins. Widely used post-COVID.
Chemical feedstock: starting material for ethanal, ethanoic acid, ethyl esters, ethene (dehydration), and many other industrial chemicals.
Cosmetics + toiletries: dissolves fragrance oils + evaporates cleanly → in perfumes, aftershaves, hairsprays, deodorants.
Medical applications: antiseptic wipes, surgical disinfectant, ear-drop solvent.

Uses of methanol.

Industrial solvent — similar to ethanol but cheaper.
Chemical feedstock — to make methanal (HCHO, used in plastics like Bakelite); biodiesel (vegetable oils + methanol → fatty acid methyl esters); methanoic acid.
Antifreeze + de-icing fluid — methanol-water mix has low freezing point; used in car windscreen washer fluid, runway de-icers.
Fuel — methanol racing fuel (high-performance engines, clean burn) and small-scale fuel cells. CAUTION: methanol burns with a near-INVISIBLE flame — dangerous to handle without UV-blocking goggles.
Methylated spirits — denatured industrial ethanol (mixed with methanol to make it undrinkable).

Safety: methanol is POISONOUS. Drinking methanol can cause blindness or death because the liver metabolises it to methanal + methanoic acid, both highly toxic. Methylated spirits is bitter and contains methanol specifically to deter drinking.

Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + energy. Clean blue flame. Used as biofuel.
Oxidation: acidified K₂Cr₂O₇, reflux → ethanoic acid. ORANGE → GREEN (Cr₂O₇²⁻ → Cr³⁺).
Industrial / natural: bacteria + O₂ → wine becomes vinegar.
Dehydration: Al₂O₃ catalyst, ~600 °C → ethene + H₂O. Reverse of industrial hydration.
Ethanol uses: drinks, solvent, biofuel, sanitiser, chemical feedstock.
Methanol uses: industrial solvent, biodiesel feedstock, antifreeze, methylated spirits.
Methanol POISONOUS — causes blindness (metabolised to methanal + methanoic acid).

See the full worked example for alcohols →

Bioethanol — the carbon-neutral fuel (Paper 2 context)

Fermentation ethanol from plants → CO₂ from combustion reabsorbed by next crop → carbon-neutral.

'Bioethanol' is ethanol made by FERMENTATION of plant sugars — used as a transport fuel (alone or blended with petrol). It is described as 'renewable' and 'carbon-neutral' because of the carbon cycle.

The carbon cycle of bioethanol.

Step 1: PLANT GROWTH (photosynthesis). Sugar cane (or corn, sugar beet, etc.) absorbs CO₂ from the atmosphere using sunlight + water:

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

The carbon in the glucose was just absorbed from the atmosphere.

Step 2: FERMENTATION. Yeast converts the glucose to ethanol + CO₂:

$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$

About 1/3 of the glucose's carbon is released as CO₂ at this stage.

Step 3: COMBUSTION of ethanol as fuel.

$\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

CO₂ is released.

Step 4: NEXT SEASON. The next sugar cane crop GROWS, absorbing the CO₂ back from the atmosphere (Step 1 repeats).

Net effect. All the CO₂ released in Steps 2 and 3 was originally absorbed in Step 1. There is NO NET ADDITION of CO₂ to the atmosphere. The carbon cycles between atmosphere → plant → ethanol → atmosphere → plant. The system is CARBON-NEUTRAL.

Why fossil fuels are different.

Crude oil contains carbon that was locked underground for ~ 100 million years. When we burn fossil fuels, we release that ANCIENT carbon to the modern atmosphere → NET ADDITION of CO₂ → contributes to climate change.

In contrast, bioethanol just recycles carbon that is already part of the modern atmospheric carbon cycle.

Real numbers — Brazil's sugar cane ethanol.

Metric	Sugar cane ethanol	Petrol
Energy content	~ 27 MJ/litre	~ 35 MJ/litre
CO₂ emissions (full life cycle, g/MJ)	~ 20	~ 95
% carbon-neutral	~ 80-85% (the rest is fossil energy used in cultivation + transport)	0%

So bioethanol is NOT 100% carbon-neutral in practice — the agriculture uses some fossil fuels — but it is dramatically better than petrol (about 80% reduction in CO₂ emissions per unit energy).

Other advantages of bioethanol fuel.

Energy security — reduces dependence on imported crude oil. Brazil's bioethanol industry has freed it from oil imports.
Cleaner combustion — fewer particulates and less CO than petrol.
Higher octane — supports high-performance engines (e.g. racing).
Domestic agriculture — supports rural employment and farming.
No sulfur — no SO₂ pollution (sulfur is a contaminant in crude oil).

Disadvantages of bioethanol fuel.

Lower energy density — about 60% of petrol's per litre → need bigger fuel tanks.
Engine modifications for E85 (85% ethanol) — special seals + fuel system.
Cold-weather starting — pure ethanol doesn't vaporise well at low T.
Competition with food — sugar cane / corn could feed people; in some cases biofuel production has driven up food prices.
Land use — large-scale bioethanol production uses significant arable land; concerns about Amazon deforestation in Brazil for new sugar cane areas.
Energy in vs energy out — corn ethanol (USA) has only modest net energy gain because growing + processing corn uses fossil fuels. Sugar cane (Brazil) is much more efficient.
Water use — sugar cane / corn farming uses significant water resources.

The bigger picture.

Bioethanol is one part of the transition away from fossil fuels. It's not a perfect solution — large-scale production has environmental + social costs — but it's a meaningful step. The same arguments apply to biodiesel (from vegetable oils + methanol → fatty acid methyl esters).

The most efficient bioethanol comes from:

Crops grown without irrigation (sugar cane in Brazil — rain-fed).
Crops with high sugar yield per hectare (sugar cane >> corn >> wheat).
Use of agricultural waste (straw, bagasse — the leftover after juice extraction is burnt as fuel for the bioethanol plant — second-generation bioethanol).

Second-generation bioethanol.

Modern research focuses on producing bioethanol from NON-FOOD plant materials:

Cellulose (in grass, wood, straw, agricultural waste) can be broken down by enzymes to glucose, then fermented to ethanol.
This avoids competition with food crops AND uses abundant waste material.
Currently more expensive than corn / sugar cane bioethanol but improving rapidly.

Final thoughts.

Bioethanol = renewable + carbon-neutral fuel from plant sugars. The chemistry is the same fermentation reaction known for thousands of years; the application as a motor fuel is the modern twist. As climate policies tighten and crude oil becomes scarcer, expect bioethanol (and other biofuels) to grow.

Bioethanol = ethanol from FERMENTATION of plant sugars.
Renewable: plants regrow each season; carbon-neutral cycle.
Photosynthesis absorbs CO₂; fermentation + combustion release CO₂; cycle closed.
Net effect: no addition of CO₂ to atmosphere (unlike fossil fuels which release stored ancient carbon).
Used as transport fuel: E10 (10%), E85 (85%), or pure E100 in flex-fuel cars.
Advantages: renewable, low CO₂, energy security, cleaner combustion.
Disadvantages: lower energy density, land + water use, competition with food crops.

See the full worked example for alcohols →

Quick recap

Alcohols: CₙH₂ₙ₊₁OH, –OH functional group, hydrogen-bond with water → soluble.
First three: methanol (CH₃OH), ethanol (CH₃CH₂OH), propan-1-ol (CH₃CH₂CH₂OH). Positional isomer: propan-2-ol.
Two methods for ethanol: HYDRATION of ethene (fast, pure, non-renewable, 300 °C/60 atm/H₃PO₄) vs FERMENTATION of glucose (slow, dilute, renewable, 25-35 °C, anaerobic, yeast).
Fermentation equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
Yeast killed at ~15% ethanol → fermentation product needs distillation.
Ethanol oxidation: heat with acidified K₂Cr₂O₇ → ethanoic acid. ORANGE → GREEN colour change.
Ethanol combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Clean burn — used as bioethanol fuel.
Ethanol dehydration: Al₂O₃ catalyst, 600 °C → ethene + water.
Methanol POISONOUS — methylated spirits contains methanol to make industrial ethanol undrinkable.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Alcohol general formula: CₙH₂ₙ₊₁OH with –OH functional group.
First three alcohols: methanol CH₃OH, ethanol CH₃CH₂OH (C₂H₅OH), propan-1-ol CH₃CH₂CH₂OH.
Hydration of ethene: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. Conditions: H₃PO₄, 300 °C, 60 atm.
Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Conditions: yeast, 25-35 °C, anaerobic.
Combustion of ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
Oxidation of ethanol: C₂H₅OH + 2[O] → CH₃COOH + H₂O. Colour change: ORANGE → GREEN (Cr₂O₇²⁻ → Cr³⁺).
Reagent for oxidation: acidified potassium dichromate(VI) (K₂Cr₂O₇ + dilute H₂SO₄). Heat under reflux.
Dehydration: ethanol → ethene + water. Catalyst: Al₂O₃ at ~ 600 °C.
Why fermentation is renewable: 'glucose comes from plants that absorb CO₂ via photosynthesis → carbon-neutral'.
Why temperature controlled in fermentation: 'enzymes denatured > 40 °C; yeast dies. Below 20 °C: slow.'
Methanol is POISONOUS — causes blindness + death. Methylated spirits = ethanol + methanol to deter drinking.

How it’s examined

Alcohols appear on every 4CH1 paper, typically worth 5-8 marks per session. Paper 1 (4CH1/1C) items: (a) define + give general formula + first three names (3-4 marks); (b) compare hydration vs fermentation (4-5 marks); (c) ethanol uses + methanol toxicity (2-3 marks). Paper 2 (4CH1/2C) items: deeper analysis of fermentation (temperature control, anaerobic conditions); industrial ethanol manufacture with conditions; OXIDATION of ethanol to ethanoic acid with acidified potassium dichromate(VI) — colour change ORANGE → GREEN is THE most common Paper 2 alcohol question (always 4-5 marks). Mark scheme keywords: 'partially ionised' (acids), 'denatured enzymes', 'anaerobic', 'orange to green', 'phosphoric acid catalyst', '300 °C / 60 atm'.

Sources: Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4CH1 Examiner Reports 2022-2024; 4CH1/1C May/Jun 2024 question paper and mark scheme; 4CH1/1C January 2024 question paper and mark scheme; 4CH1/2C May/Jun 2024 question paper and mark scheme; 4CH1/2C January 2024 question paper and mark scheme. Last reviewed 2026-05-12.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Alcohols, ready to print or save as PDF.

Step-by-step worked examples — Alcohols

Step-by-step solutions to past-paper-style questions on alcohols, written exactly the way a tutor would explain them at the board.

1Define the alcohol homologous series and give first three members (4 marks, Paper 1)
Core• Adapted from 4CH1/1C Jan 2024 Q3• alcohols, general formula, spec-4.27
Question
(a) State the functional group of an alcohol. (b) State the general formula of the alcohol homologous series. (c) Name and give the structural formula of the first THREE members. (d) State and explain ONE physical property that all three share. (4 marks)
Step-by-step solution
1. Step 1
  (a) Functional group (1 M). All alcohols contain the HYDROXYL group –OH attached to a carbon atom. This single shared group is what gives every alcohol its characteristic chemistry.
2. Step 2
  (b) General formula (1 A). Alcohols follow the pattern:
  $C_n H_{2n+1} OH$
3. Step 3
  (c) First three alcohols (1 A). Naming uses the parent alkane stem and the suffix -ol (with a locant for C₃ and higher).
  
  n Name Structural formula
  1 Methanol CH₃OH
  2 Ethanol CH₃CH₂OH (often written C₂H₅OH)
  3 Propan-1-ol CH₃CH₂CH₂OH
  
  Propan-1-ol has a positional isomer propan-2-ol (CH₃CH(OH)CH₃) — same molecular formula C₃H₈O, different position of the –OH.
4. Step 4
  (d) Shared physical property + reason (1 B). All three are LIQUIDS at room temperature and are SOLUBLE in water. Reason: the polar –OH group can form HYDROGEN BONDS with water molecules → high solubility. The hydrogen bonds between alcohol molecules also raise the boiling point above the corresponding alkane (e.g. ethanol bp 78 °C vs ethane bp –89 °C).
Answer
(a) –OH (hydroxyl). (b) CₙH₂ₙ₊₁OH. (c) Methanol CH₃OH; ethanol CH₃CH₂OH; propan-1-ol CH₃CH₂CH₂OH (isomer: propan-2-ol CH₃CH(OH)CH₃). (d) Liquids at room T AND soluble in water — the polar –OH group hydrogen-bonds with water.
Examiner tip
Edexcel 4-mark mark scheme: 1 for –OH; 1 for CₙH₂ₙ₊₁OH; 1 for naming + structures of all three; 1 for property + reason. Mark scheme keyword: 'hydroxyl group' and 'hydrogen bonding with water'.
2Fermentation of glucose by yeast — equation and conditions (5 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q6• fermentation, ethanol, spec-4.28, spec-4.29
Question
Ethanol can be made by the fermentation of glucose. (a) Write a balanced equation for the fermentation reaction. (b) State the conditions required: temperature, organism, oxygen requirement. (c) Explain why the temperature must be carefully controlled. (d) Explain why fermentation is described as using a renewable resource. (5 marks)
Step-by-step solution
1. Step 1
  (a) Equation (1 M). Yeast contains enzymes that convert glucose into ethanol and carbon dioxide under anaerobic conditions.
  $\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$
2. Step 2
  (b) Conditions (1 A). • Yeast (a single-celled fungus — supplies the enzymes that catalyse the reaction). • Temperature 25-35 °C — warm enough for fast enzyme activity, but not too hot. • Anaerobic (no oxygen). If O₂ is present, the ethanol is oxidised to ethanoic acid (vinegar) instead. • Sugar source — glucose from fruit, grains, or sugar cane.
3. Step 3
  (c) Why temperature is controlled (1 B). Yeast is a LIVING ORGANISM with PROTEIN ENZYMES. If T is too LOW (< ~ 20 °C) the enzymes work slowly → slow rate of fermentation. If T is too HIGH (> ~ 40 °C) the enzymes are DENATURED — the protein structure changes irreversibly, the active site loses its shape, and the yeast dies. 25-35 °C is the optimum where enzyme activity is high but the yeast survives.
4. Step 4
  (d) Renewable resource (1 B). Glucose comes from PLANTS (sugar cane, sugar beet, fruit, grains). Plants grow back each season by photosynthesis, absorbing CO₂ from the atmosphere as they grow. So the carbon released as CO₂ during fermentation (and combustion of the ethanol) was originally taken from the atmosphere by the plants → CARBON-NEUTRAL overall. Compare with crude-oil-derived ethene (hydration route): crude oil takes millions of years to form → non-renewable.
5. Step 5
  (d) Additional context (1 B). The yeast eventually dies when the ethanol concentration reaches ~ 14-15% (alcohol is toxic to yeast). The dilute aqueous ethanol can then be purified by FRACTIONAL DISTILLATION — boiling point of ethanol 78 °C is lower than water 100 °C, so ethanol vapour rises and is condensed first.
Answer
(a) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (catalysed by yeast enzymes). (b) Yeast, 25-35 °C, anaerobic, glucose substrate. (c) T too low → slow enzymes; T too high → enzymes denatured + yeast killed. (d) Glucose comes from plants (sugar cane, beet, fruit) that regrow each season → renewable.
Examiner tip
Edexcel 5-mark mark scheme: 1 for balanced equation; 1 for the three conditions (yeast / 25-35 °C / anaerobic); 1 each for explaining temperature control AND renewable resource; 1 for context (yeast dies at ~ 15% — purification needed). Mark-scheme phrases: 'enzymes denatured', 'anaerobic conditions', 'renewable plant source'.

n	Name	Structural formula
1	Methanol	CH₃OH
2	Ethanol	CH₃CH₂OH (often written C₂H₅OH)
3	Propan-1-ol	CH₃CH₂CH₂OH

3Compare hydration of ethene vs fermentation (6 marks, Paper 2)

Higher• Adapted from 4CH1/2C May/Jun 2024 Q5• ethanol, industrial, comparison, spec-4.29

Question

Ethanol can be manufactured industrially by TWO different routes: (i) hydration of ethene, and (ii) fermentation of glucose. Compare these methods. Discuss speed, energy requirements, source of raw material, purity of product, and sustainability. Give a balanced conclusion. (6 marks)

Step-by-step solution

Step 1
(a) Hydration of ethene — overview (1 M). Ethene from cracking crude oil reacts with steam over H₃PO₄ catalyst at 300 °C, 60 atm.
$\text{C}_2\text{H}_4(\text{g}) + \text{H}_2\text{O(g)} \rightleftharpoons \text{C}_2\text{H}_5\text{OH(g)}$
Step 2
(a) Fermentation overview (1 A). Glucose from sugar crops is converted to ethanol by yeast at 25-35 °C anaerobically.
$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$

Step 3

(c) Side-by-side comparison (1 A).

Feature	Hydration of ethene	Fermentation
Source	Crude oil (ethene)	Sugar crops (glucose)
Renewable	NO (fossil fuel)	YES (plants regrow)
Mode	Continuous	Batch
Speed	Fast (minutes/pass)	Slow (days/weeks)
Conditions	300 °C, 60 atm, H₃PO₄	25-35 °C, atm P, yeast
Energy	High	Low
Purity	High (~95% ethanol)	Low (~14% — needs distillation)
Yield	High (after recycling)	Limited (yeast dies at ~15%)
CO₂ footprint	High (fossil)	Carbon-neutral (plant CO₂)

Step 4
(d) Advantages of hydration (1 B). Very PURE ethanol; CONTINUOUS process (24/7); fast throughput; high overall conversion via recycling of unreacted ethene + steam (~ 97% overall conversion).
Step 5
(e) Advantages of fermentation (1 B). Uses RENEWABLE sugars from plants → carbon-neutral; LOW energy requirements (near room T, atm pressure); simple equipment; can be done at small scale.
Step 6
(f) Balanced conclusion (1 B). Both methods are used commercially: hydration of ethene is preferred for INDUSTRIAL ethanol (solvents, sanitiser, chemical feedstock — pure ethanol needed in bulk). Fermentation is the route for ALCOHOLIC BEVERAGES (beer, wine, spirits) and for BIOETHANOL fuel in countries like Brazil. As crude oil becomes scarcer and climate concerns grow, fermentation is becoming relatively more important.

Answer

Hydration: fast, continuous, pure (~95%) ethanol BUT uses non-renewable crude oil + high energy (300 °C, 60 atm). Fermentation: slow, batch, dilute (~14%) ethanol BUT uses renewable sugars + low energy + carbon-neutral. Conclusion: hydration suits industrial bulk pure ethanol; fermentation suits drinks + bioethanol fuel. Both have a role; sustainability favours fermentation long-term.

Examiner tip

Edexcel 6-mark mark scheme: 1 for equation of each method (or named conditions); 1 for one advantage of each; 1 for one disadvantage of each; 1 for clear comparison structure; 1 for balanced conclusion. Mark scheme accepts 'renewable / non-renewable' and 'continuous / batch' as keyword phrases.

4Oxidation of ethanol to ethanoic acid — colour change (5 marks, Paper 2)
Higher• Adapted from 4CH1/2C Jan 2024 Q7• oxidation, ethanoic acid, spec-4.30P, spec-4.31P
Question
Ethanol can be oxidised to ethanoic acid in the laboratory. (a) Name the oxidising agent and the acid used to acidify it. (b) State the colour change observed. (c) Write a simplified equation for the oxidation of ethanol (using [O] to represent the oxidising agent). (d) Describe the industrial bacterial route by which ethanol is oxidised to ethanoic acid. (5 marks)
Step-by-step solution
1. Step 1
  (a) Reagents (1 M). Oxidising agent: acidified POTASSIUM DICHROMATE(VI) (K₂Cr₂O₇). The acid used to acidify it is dilute SULFURIC ACID (H₂SO₄). The dichromate ion Cr₂O₇²⁻ is the active oxidiser.
2. Step 2
  (b) Colour change (1 A). The solution changes from ORANGE → GREEN as the oxidation proceeds. The orange colour is from the dichromate ion Cr₂O₇²⁻; the green colour is from the chromium(III) ion Cr³⁺, formed when the dichromate has been reduced. This colour change is the key OBSERVATION that proves oxidation has occurred.
3. Step 3
  (c) Simplified equation (1 A). Using [O] to represent the oxidising agent, ethanol loses 2 H atoms and gains an O atom overall (one [O] for the H₂O byproduct + one [O] inserted as =O):
  $\text{C}_2\text{H}_5\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$
4. Step 4
  (c) Reaction conditions (1 B). The mixture (ethanol + K₂Cr₂O₇ + dilute H₂SO₄) is heated under reflux (a condenser is fitted vertically so vapours condense back into the flask — this allows extended boiling without losing the volatile ethanol or product). Hot mixture; heated for ~ 20-30 min for full conversion.
5. Step 5
  (d) Industrial bacterial route (1 B). Some species of BACTERIA (e.g. Acetobacter) naturally oxidise ethanol to ethanoic acid in the presence of OXYGEN (air). This is the basis of VINEGAR production: aerobic fermentation of wine or cider over a few weeks at warm temperatures gives a dilute solution of ethanoic acid (~ 4-8%) — vinegar.
  
  $\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \xrightarrow{\text{bacteria}} \text{CH}_3\text{COOH} + \text{H}_2\text{O}$
  
  This is essentially the same overall transformation as the lab route but slower and at lower T.
Answer
(a) Acidified POTASSIUM DICHROMATE(VI) (K₂Cr₂O₇) with dilute SULFURIC ACID. (b) Orange → green (Cr₂O₇²⁻ orange; Cr³⁺ green). (c) C₂H₅OH + 2[O] → CH₃COOH + H₂O (heat under reflux). (d) Industrial / wine→vinegar: bacteria (Acetobacter) oxidise ethanol with O₂ in air over weeks: C₂H₅OH + O₂ → CH₃COOH + H₂O.
Examiner tip
Edexcel 5-mark mark scheme: 1 for naming acidified potassium dichromate; 1 for orange → green colour change; 1 for balanced equation; 1 for heating under reflux; 1 for bacterial vinegar route. Mark-scheme keywords: 'potassium dichromate', 'orange to green'. Common error: writing 'orange to yellow' or 'orange to colourless' — the correct change is to green.
5Propan-1-ol and its isomer propan-2-ol (3 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q4• isomerism, propanol, spec-4.27
Question
Propan-1-ol has the structural formula CH₃CH₂CH₂OH. (a) Draw the displayed formula of propan-1-ol. (b) Name and give the structural formula of a positional isomer of propan-1-ol. (c) Explain why both compounds are described as alcohols. (3 marks)
Step-by-step solution
1. Step 1
  (a) Displayed formula of propan-1-ol (1 M). Three-carbon chain with –OH on C1 (an end carbon).
  
  H H H | | | H-C-C-C-O-H | | | H H H
  
  All bonds shown explicitly; –OH at the end carbon.
2. Step 2
  (b) Positional isomer (1 A). Propan-2-ol — same molecular formula C₃H₈O, but the –OH is on the MIDDLE carbon (C2) instead of an end carbon (C1).
  
  Structural formula: CH₃CH(OH)CH₃ (also written (CH₃)₂CHOH).
  
  H H H | | | H-C-C-C-H | | | H O H | H
  
  The –OH is now on the central carbon.
3. Step 3
  (c) Both are alcohols (1 B). Both compounds contain the HYDROXYL group –OH bonded to a carbon atom of an alkyl chain. The presence of the –OH functional group is what defines them as members of the alcohol homologous series. They share the same general formula CₙH₂ₙ₊₁OH (here both are C₃H₇OH).
Answer
(a) Displayed: three C atoms in a chain with all bonds; –OH on first C. (b) Propan-2-ol, CH₃CH(OH)CH₃, with –OH on the middle (C2) carbon. (c) Both contain the –OH (hydroxyl) functional group → both members of the alcohol series.
Examiner tip
Edexcel 3-mark mark scheme: 1 for displayed formula of propan-1-ol with –OH at end; 1 for naming propan-2-ol + correct structural formula with –OH on C2; 1 for explaining both are alcohols (–OH functional group). Common error: drawing the same molecule twice (–OH on end-carbon = both propan-1-ol). The isomer MUST have –OH on the middle carbon.

Model Answers — Alcohols

High-scoring sample answers for alcohols on the Pearson Edexcel IGCSE 4CH1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question

4CH1/2C-style — fermentation of glucose5 marks

Describe in detail how ethanol can be manufactured by the fermentation of glucose. Include the equation, the conditions required, an explanation of why each condition is needed, and how the dilute ethanol product is concentrated. (5 marks)

Model answer

Equation for fermentation.

Yeast contains enzymes (zymase complex) that convert glucose into ethanol and carbon dioxide under anaerobic conditions:

$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$

One mole of glucose gives two moles of ethanol and two moles of CO₂ gas (bubbles seen rising during fermentation).

Conditions required and why.

Condition	Specific value	Why required
Yeast	A single-celled fungus (e.g. Saccharomyces cerevisiae)	Yeast supplies the ENZYMES that catalyse glucose → ethanol. Without yeast no reaction.
Temperature	25-35 °C	Optimal for enzyme activity. Too LOW (< 20 °C) → enzymes work too slowly → low rate of reaction. Too HIGH (> 40 °C) → enzymes DENATURED (protein structure breaks down, active site loses shape) → yeast dies → reaction stops.
Anaerobic	NO oxygen / air excluded	If O₂ is present, bacteria + yeast oxidise the ethanol to ethanoic acid (vinegar). The fermentation vessel is sealed with an airlock that lets CO₂ escape but stops air entering.
Sugar source	Glucose, sucrose or fructose (from sugar cane, sugar beet, grapes, barley, etc.)	The substrate the yeast enzymes act on.
Aqueous solution	Water added to dissolve sugar	The enzymes work in aqueous solution.
Time	A few days to a couple of weeks	Fermentation is SLOW — enzymes work at modest rates.

Limit to fermentation yield.

The reaction stops naturally when the ethanol concentration reaches about 14-15% by volume. At this concentration the ethanol is TOXIC to the yeast — the cell membranes are disrupted and the yeast dies. So fermentation gives DILUTE ethanol; pure ethanol cannot be obtained directly from fermentation.

Concentrating the ethanol — fractional distillation.

To produce more concentrated ethanol for industrial use, drinks (spirits), or biofuel, the fermented mixture is FRACTIONALLY DISTILLED:

The fermented liquid (~ 14% ethanol, ~ 86% water + sugars + impurities) is heated in a still.
Ethanol has a lower boiling point (78 °C) than water (100 °C), so ethanol vapour rises faster and is enriched in the column.
The fractionating column is packed with glass beads or trays — vapour and liquid equilibrate at each level, with the more volatile ethanol rising and the less volatile water falling back.
The vapour at the top (rich in ethanol) is condensed and collected.
Repeated distillation can produce ethanol up to about 96% (the ethanol-water azeotrope) by simple distillation. Anhydrous ethanol requires chemical drying agents.

Why fermentation matters.

Fermentation is the route used for:

Alcoholic beverages: beer (~ 5% ethanol — from barley + hops), wine (~ 12% — from grapes), spirits (~ 40% — from distilled mash of grains/potatoes).
Bioethanol fuel: Brazil produces ~ 30 billion litres of bioethanol per year from sugar cane fermentation — used as a petrol additive or pure fuel.
Green industrial ethanol: as crude oil costs rise and climate policies tighten, fermentation-derived ethanol is becoming relatively more cost-competitive with hydration of ethene.

The chemistry behind the colour-coded conditions.

25-35 °C is the 'sweet spot' for ALL enzyme-catalysed reactions (close to physiological / body temperature for many organisms).
Anaerobic conditions are critical because yeast can ALSO respire aerobically: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$ — which produces only CO₂ + water, no ethanol! The yeast 'chooses' anaerobic respiration only when O₂ is absent.

So in summary, fermentation is a SLOW, BATCH, BIOLOGICAL route to DILUTE ethanol from RENEWABLE plant sugars. Distillation concentrates the product. The advantages are sustainability and low energy; the disadvantages are speed, scale, and dilute output.

Why this scores

Edexcel 5-mark mark scheme: 1 for balanced equation; 1 for yeast + temperature range; 1 for anaerobic conditions; 1 for explaining why temperature matters (enzymes denature); 1 for fractional distillation to concentrate (lower bp of ethanol = 78 °C). Mark-scheme keywords: 'enzymes', 'anaerobic', 'denatured', 'fractional distillation'.

Question

4CH1/2C-style — ethanol manufacture comparison6 marks

Compare the advantages and disadvantages of producing ethanol by FERMENTATION of glucose vs HYDRATION of ethene. Discuss in terms of speed, energy, sustainability, purity and economics. Reach a balanced conclusion about when each method should be preferred. (6 marks)

Model answer

Overview. There are two industrial routes to ethanol. They differ in raw material, conditions, speed, purity, and environmental impact.

Route 1: Hydration of ethene (petrochemical):

$\text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightleftharpoons \text{C}_2\text{H}_5\text{OH}$ Conditions: H₃PO₄ catalyst, 300 °C, 60 atm. Ethene comes from cracking crude oil.

Route 2: Fermentation of glucose (biological):

$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$ Conditions: yeast, 25-35 °C, atm pressure, anaerobic. Glucose from sugar cane, beet, corn, etc.

Side-by-side comparison.

Feature	Hydration of ethene	Fermentation
Speed	Fast (minutes per pass through reactor)	Slow (days to weeks per batch)
Mode	CONTINUOUS (24/7)	BATCH (start, stop, restart)
Energy required	HIGH (heat to 300 °C, compress to 60 atm)	LOW (near room T, atm P)
Raw material	Ethene (from crude oil)	Glucose (from sugar crops)
Renewable?	NO — fossil fuel	YES — plants regrow
Carbon footprint	High (fossil CO₂ released)	LOW / NEUTRAL — CO₂ released = CO₂ taken in by plants when growing
Purity of product	High (~ 95% ethanol direct)	Low (~ 14% — limited by yeast death) → needs distillation
Equipment cost	Very high (reactor, compressors, catalyst beds, recycling)	Low (vessels, distillation column)
Operating cost	High (energy intensive)	Low (only heat for distillation)
Scale	Industrial (millions of tonnes/year)	Variable (home brewer to multi-million-litre biorefinery)
Land use	Minimal	Large — competes with food crops

Advantages of fermentation.

Renewable: sugar crops regrow annually; carbon-neutral.
Low energy: near room T, no high-pressure equipment.
Cheap to start up: vessels + airlock + yeast.
Produces alcoholic beverages: the ONLY method that makes beer, wine, spirits.
Bioethanol fuel: used as motor fuel in many countries (Brazil, USA, EU).

Disadvantages of fermentation.

Slow: days to weeks per batch.
Batch process: must drain, clean, restart between batches → time lost.
Dilute product: ~ 14% ethanol — purification by distillation needed.
Yeast dies at high ethanol (~ 15%) → cannot push concentration further.
Competes with food: sugar cane / corn could feed people instead.
Land + water intensive: large agricultural footprint.

Advantages of hydration.

Fast: minutes per pass; production line operates continuously.
Pure product: ~ 95% ethanol directly; minimal further purification.
Continuous process: 24/7 operation → high throughput.
No food competition: doesn't take crops from human consumption.
Reliable: predictable industrial chemistry; not dependent on weather/crops.

Disadvantages of hydration.

Non-renewable: ethene from crude oil — a finite fossil resource.
High energy: 300 °C, 60 atm — high electricity / steam costs + high CO₂ emissions from heating.
Capital-intensive: very expensive cracker + reactor + recycling equipment.
Carbon footprint: fossil-derived → net CO₂ release.
Will become more expensive as oil supply tightens and climate policies tighten.

Balanced conclusion — when to prefer each.

Prefer HYDRATION of ethene when:

High-purity ethanol is needed for industrial use (solvent, chemical feedstock, sanitiser).
Continuous, large-scale production is required.
Energy is cheap (e.g. near oil refineries that supply ethene as a co-product).
Speed and reliability matter more than carbon footprint.

Prefer FERMENTATION when:

The product is an alcoholic beverage (the only way — beer/wine/spirits).
Renewable feedstock is required (climate policy, customer expectation).
A small/local plant is needed (bioethanol co-op, brewery).
Sugar crops are abundant (Brazil — sugar cane; USA — corn).
Long-term sustainability is the priority.

Real-world outcome. Both methods currently produce billions of litres per year of ethanol. Bioethanol fuel is growing rapidly (Brazil mandates 27% ethanol in petrol; USA E15 = 15%; EU has biofuel targets). As fossil fuels decline and climate policies tighten, fermentation will likely become the dominant route by mid-century. For ethyl ethanoate, ethanal, ethanoic acid (chemical feedstocks made from ethanol), the choice depends on whether 'green ethanol' is mandated or whether the cheapest source (currently petrochemical) is used.

The bottom line for 4CH1: hydration = fast/pure/non-renewable; fermentation = slow/dilute/renewable. Both have a role; the balance is shifting towards fermentation as concerns about fossil fuels grow.

Why this scores

Edexcel 6-mark mark scheme: 1 for both equations / conditions; 1 each for one advantage of each route (max 2); 1 each for one disadvantage of each route (max 2); 1 for balanced conclusion. Mark-scheme keywords: 'renewable / non-renewable', 'continuous / batch', 'high energy', 'carbon-neutral'.

Question

4CH1/2C-style — ethanol oxidation5 marks

Describe how a sample of ethanol can be oxidised in the laboratory to ethanoic acid. Include the reagents, observed colour change, equation, and the conditions needed. (5 marks)

Model answer

Reagents and reaction.

To oxidise ethanol in the laboratory we use acidified potassium dichromate(VI) as the oxidising agent:

Oxidising agent: potassium dichromate(VI), K₂Cr₂O₇. The active ion is the dichromate, Cr₂O₇²⁻, which is ORANGE in solution.
Acidifying agent: dilute sulfuric acid (H₂SO₄). This provides the H⁺ ions needed by the redox half-equations and ensures the reaction proceeds to completion.

Reaction observed.

The mixture (a few cm³ of ethanol + acidified potassium dichromate solution) is heated gently in a flask. As oxidation proceeds:

The ORANGE colour of Cr₂O₇²⁻ fades.
A GREEN colour appears as the Cr³⁺ ion (chromium(III)) is formed.

The observed colour change is ORANGE → GREEN. This is the diagnostic test that oxidation has occurred. The green Cr³⁺ is the reduced form of the chromium (the chromium has gone from +6 oxidation state in Cr₂O₇²⁻ to +3 in Cr³⁺).

Equation for the oxidation of ethanol.

The full balanced equation involving the dichromate ion is complex (involves H⁺ + electrons). At 4CH1, the simplified equation using [O] to represent 'one oxygen atom from the oxidising agent' is:

$\text{C}_2\text{H}_5\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$

In words: ethanol + oxygen (from oxidising agent) → ethanoic acid + water.

What's happening atom-by-atom:

Ethanol C₂H₅OH = CH₃CH₂OH.
The end carbon's H atoms are replaced/rearranged so that the –CH₂–OH group becomes a –COOH group.
Net: one H atom is removed from the OH; one H atom from the CH₂; one O atom is inserted (between the C and the H) → forming the C=O of the COOH group. The two H atoms removed combine with one [O] to form H₂O.

Conditions — heating under reflux.

The reaction is heated to about 60-80 °C for ~ 20-30 minutes.
Reflux apparatus is used: a vertical condenser is fitted to the flask so that volatile reactants and products (ethanol bp 78 °C; ethanoic acid bp 118 °C) condense back into the flask rather than escaping. This allows extended heating with no loss of volatile material.
The mixture is stirred or shaken to ensure good contact.

Why it works — the redox chemistry.

The chromium in Cr₂O₇²⁻ is in oxidation state +6. It gains electrons (is reduced) to become Cr³⁺ (+3 state). The colour change reflects the electronic transitions of the chromium ion.

Meanwhile the ethanol gains an O (or loses 2H) and is OXIDISED to ethanoic acid. The C of the –CH₂–OH group goes from oxidation state −1 to +3 (a change of +4) — corresponding to the addition of an O and loss of 2 H atoms.

The two half-reactions balance:

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (reduction)
C₂H₅OH + H₂O → CH₃COOH + 4H⁺ + 4e⁻ (oxidation)

Multiplying and adding gives the overall ionic equation, but this is beyond 4CH1.

Industrial / natural route — vinegar.

In industry (and in nature), ethanol can also be oxidised to ethanoic acid by bacteria (e.g. Acetobacter) using atmospheric oxygen:

$\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \xrightarrow{\text{bacteria}} \text{CH}_3\text{COOH} + \text{H}_2\text{O}$

This is the process by which wine turns sour (becomes vinegar) when exposed to air. Slow and at room temperature.

Why this matters.

The oxidation of ethanol to ethanoic acid is the key step in:

Vinegar production — industrial bacterial oxidation of dilute ethanol.
Chemical industry — ethanoic acid is a major industrial chemical (used to make polyvinyl acetate, paints, dyes, esters).
Wine spoilage — ageing/oxidation gives off-flavours.
Liver metabolism — alcohol is metabolised in the body to ethanal (CH₃CHO) and then ethanoic acid via similar oxidation (using NAD⁺ as the oxidising agent).

Summary.

Item	Detail
Starting material	Ethanol C₂H₅OH
Reagent	Acidified K₂Cr₂O₇ (orange)
Conditions	Heat under reflux ~ 60-80 °C, dilute H₂SO₄
Colour change	ORANGE → GREEN
Product	Ethanoic acid CH₃COOH
Equation	C₂H₅OH + 2[O] → CH₃COOH + H₂O
Industrial route	Bacteria + O₂ (vinegar making)

Why this scores

Edexcel 5-mark mark scheme: 1 for naming acidified potassium dichromate as oxidising agent; 1 for ORANGE → GREEN colour change; 1 for balanced equation with [O]; 1 for product = ethanoic acid; 1 for heat / reflux condition. Mark-scheme keyword: 'orange to green'. Common errors: 'orange to colourless' (incorrect — green); writing 'methanoic acid' as product (incorrect — ethanoic).

Question

4CH1/2C-style — bioethanol sustainability5 marks

Explain why ethanol produced by fermentation is described as a RENEWABLE FUEL. Include the carbon cycle in your answer, and describe TWO advantages and ONE disadvantage of using bioethanol as a transport fuel. (5 marks)

Model answer

Why fermentation-ethanol is renewable.

Ethanol made by fermentation is produced from PLANT SUGARS (glucose, sucrose) from crops such as sugar cane, sugar beet, corn or wheat. These plants grow back each season. The fundamental cycle is:

Plant growth (photosynthesis). Plants absorb CO₂ from the atmosphere using sunlight + water: $6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ . The carbon in the sugar comes ultimately from atmospheric CO₂.
Fermentation. Yeast converts the sugar to ethanol + CO₂: $\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$ . Some CO₂ is released back, but this CO₂ was just absorbed by the plant.
Combustion of ethanol as fuel. $\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$ . CO₂ is released.

The carbon cycle is CLOSED. All the CO₂ released (in fermentation + combustion) was originally absorbed by the plant. There is NO net addition of CO₂ to the atmosphere → bioethanol is described as 'carbon-neutral'.

When the next season's crop grows, it absorbs the CO₂ back, ready for another fermentation cycle. The system is RENEWABLE — it can be sustained indefinitely as long as sunlight and arable land are available.

Contrast with fossil fuels.

Fossil fuels (oil, coal, gas) come from carbon that was locked underground for millions of years. Burning them releases CO₂ that has NOT been part of the active atmospheric carbon cycle in modern times → NET ADDITION of CO₂ to the atmosphere → contributes to global warming.

Two advantages of bioethanol as a transport fuel.

Advantage 1 — Carbon neutrality. Combustion of bioethanol returns CO₂ to the atmosphere that was just absorbed by the crop. Net CO₂ emissions from bioethanol are far lower than from petrol/diesel (~ 30-90% reduction depending on agricultural practices). This helps mitigate climate change.

Advantage 2 — Reduces dependence on imported oil. Many countries (Brazil, USA, EU) have policies to blend bioethanol with petrol (E10 = 10% bioethanol, E85 = 85%) — this reduces dependence on imported crude oil, improving energy security and supporting local agriculture / rural economies. Brazil's ethanol industry from sugar cane is a textbook example of a successful biofuel programme.

Other valid advantages:

Burns more cleanly than petrol (fewer particulates and CO emissions).
Higher octane rating → can be used in high-performance engines.
Supports rural employment (sugar cane / corn farming).
Domestic production (no import vulnerability).

One disadvantage of bioethanol as a transport fuel.

Disadvantage — Land use and food competition. Producing bioethanol on a large scale requires huge areas of arable land for sugar cane / corn / wheat. This land could otherwise be used for FOOD production. As biofuel demand grows, food prices may rise (especially for staple crops like corn and wheat). In some cases, rainforest has been cleared to grow biofuel crops, undermining the carbon-neutrality argument and damaging biodiversity.

Other valid disadvantages:

Energy in vs energy out: growing + harvesting + fermenting + distilling crops uses energy (often from fossil fuels) — net energy gain can be small.
Requires fertiliser, water, pesticides — environmental impact of intensive agriculture.
Engines may need modification for high ethanol blends.
Ethanol has lower energy density than petrol (60% of the energy per litre) → bigger fuel tanks needed.

The overall picture.

Bioethanol is RENEWABLE because the feedstock (plant sugar) regrows annually using atmospheric CO₂. The carbon cycle is closed → carbon-neutral. This is a fundamental advantage over fossil fuels (oil, coal, gas) which release stored ancient carbon.

But bioethanol is NOT a perfect solution — large-scale production competes with food, uses land + water, and the energy efficiency of the whole life cycle must be carefully assessed. Best practice uses non-food crops (sugar cane is good; corn is less so) or agricultural waste (straw, husks) to make 'second-generation' bioethanol that doesn't compete with food.

Edexcel-friendly summary:

Feature	Bioethanol (fermentation)	Petrol (from crude oil)
Source	Plant sugars (annual crop)	Crude oil (geological time)
Renewable?	YES	NO
CO₂ cycle	Closed (carbon-neutral)	Open (releases stored carbon)
Sustainability	High (with careful management)	Low (finite resource)
Land use	High	Low
Per-litre energy	~ 60% of petrol	100% (baseline)

This is why bioethanol is described as renewable: it is part of the modern atmospheric carbon cycle, recycled annually by plants.

Why this scores

Edexcel 5-mark mark scheme: 1 for renewable = plants regrow annually; 1 for carbon-neutral = CO₂ released absorbed by next crop; 1 for advantage 1 (carbon footprint / climate); 1 for advantage 2 (energy security / supports farmers); 1 for disadvantage (land competition with food). Mark-scheme keywords: 'carbon-neutral', 'photosynthesis absorbs CO₂'.

Question

4CH1/1C-style — alcohol uses4 marks

Describe the main uses of methanol and ethanol. Explain how the chemical properties of alcohols make them suitable for these uses. Mention safety considerations where appropriate. (4 marks)

Model answer

Uses of ETHANOL.

(1) Alcoholic drinks. Ethanol is the only alcohol safe for human consumption (in moderation). Beer (~ 5%), wine (~ 12%), spirits (~ 40% — vodka, whisky, gin, rum). Produced by FERMENTATION of plant sugars (grains, fruit, sugar cane). Ethanol's mild psychoactive effect underpins a large global beverage industry.

(2) Solvent. Ethanol dissolves both POLAR (water-soluble) and NON-POLAR (oil-soluble) substances because it has both a polar –OH group (H-bonds with water) and a non-polar alkyl chain –CH₂CH₃ (dissolves in non-polar oils). This makes ethanol an excellent SOLVENT for:

Perfumes and aftershaves (carries the fragrance oils + evaporates cleanly).
Mouthwash and tinctures (medicines dissolved in ethanol).
Inks, paints, varnishes (carries pigments + dries fast).
Cosmetics (lotions, deodorants).

(3) Fuel / biofuel. Ethanol burns with a clean, hot flame:

$\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

Used as:

BIOETHANOL — blended with petrol (E10, E85) to reduce CO₂ emissions and dependence on crude oil.
Pure fuel — Brazil runs millions of cars on neat ethanol from sugar cane.
Camping fuel and small portable cookers (alcohol stoves).
Laboratory burners (spirit burner / Bunsen substitute).

Renewable when made by FERMENTATION → described as 'carbon-neutral'.

(4) Hand sanitiser / antiseptic. Ethanol at 60-95% concentration kills bacteria and viruses by denaturing their proteins and disrupting cell membranes. Hand sanitisers, surgical wipes, and disinfectants rely on ethanol. (Demand soared during the 2020-2022 COVID-19 pandemic.)

(5) Chemical feedstock. Ethanol is the starting material for many other chemicals: ethanoic acid (oxidation), ethyl esters (esterification), ethene (dehydration), ethanal (partial oxidation).

Uses of METHANOL.

(1) Industrial solvent. Similar to ethanol (small polar + non-polar molecule) — dissolves paints, varnishes, inks, and many organic compounds.

(2) Chemical feedstock. Methanol is converted into:

Methanal (formaldehyde, HCHO) — used to make plastics (Bakelite, urea-formaldehyde resins) and as a preservative.
Methanoic acid (HCOOH) — small industrial scale.
Methyl esters and dimethyl ether (DME, a fuel substitute).
Biodiesel (methanol reacts with vegetable oils to make fatty acid methyl esters — biodiesel).

(3) Fuel. Methanol is used as a clean-burning fuel in racing cars (high-performance engines), model aeroplanes (mixed with castor oil), and small-scale fuel cells. It is cheaper than ethanol but burns with a near-invisible flame (dangerous because you can't see it).

(4) Antifreeze and de-icing fluid. Methanol mixed with water has a much lower freezing point than pure water — used in car windscreen washer fluid and runway de-icer.

SAFETY: methanol is POISONOUS.

Methanol must NEVER be drunk. It is metabolised in the liver to methanal and methanoic acid, both of which are TOXIC:

Methanal damages the optic nerve → blindness (10 mL can cause permanent blindness).
Methanoic acid causes severe metabolic acidosis → coma and death (~ 30 mL can be fatal).

This is why methanol is sometimes added to industrial ethanol ('denatured alcohol' or 'methylated spirits') to make it undrinkable. The denatured alcohol can be sold tax-free for industrial use because it's not suitable for beverages.

Why these properties make alcohols useful.

Property	Use it enables
Liquid at room T + volatile	Solvent that evaporates cleanly
Polar –OH + non-polar chain	Dissolves both polar and non-polar substances
Soluble in water (H-bonding)	Mixes with aqueous drinks/sprays
Flammable	Fuel (high energy density per kg)
Denatures proteins	Antiseptic
Reacts to form esters	Flavours, fragrances
Reacts to form ethanoic acid	Vinegar production

The same –OH functional group makes alcohols both useful AND reactive — central to the petrochemical and food industries.

Why this scores

Edexcel 4-mark mark scheme: 1 for ethanol use 1 (beverage/solvent/fuel/sanitiser — any valid); 1 for ethanol use 2; 1 for methanol use (solvent or chemical feedstock); 1 for safety / methanol toxicity. Mark-scheme keyword: 'methanol POISONOUS / causes blindness'.

Key Definitions and Keywords — Alcohols

Definitions to memorise and the exact keywords mark schemes credit for alcohols answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4CH1 sitting.

Alcohol
Examiner keyword
A homologous series of organic compounds containing the –OH (hydroxyl) functional group attached to a carbon atom. General formula CₙH₂ₙ₊₁OH. Examples: methanol CH₃OH, ethanol CH₃CH₂OH, propan-1-ol CH₃CH₂CH₂OH.
Hydroxyl group (–OH)
Examiner keyword
The functional group of alcohols. An oxygen atom bonded to a hydrogen atom and to a carbon atom. Polar — forms hydrogen bonds with water → alcohols soluble in water.
Fermentation
Examiner keyword
Anaerobic conversion of glucose (or other sugars) into ethanol and carbon dioxide by yeast enzymes. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Conditions: yeast, 25-35 °C, anaerobic. Used to make beer, wine, spirits, bioethanol.
Hydration of ethene (industrial ethanol)
Examiner keyword
Industrial manufacture of ethanol from ethene + steam over a phosphoric acid catalyst at 300 °C and 60 atm. Reversible reaction, ~5% conversion per pass; unreacted gases recycled. Continuous process producing pure (~95%) ethanol.
Renewable / bioethanol
Examiner keyword
Ethanol from fermentation is renewable because the plant sugar source regrows each season. Combustion releases CO₂ that was absorbed during plant growth (photosynthesis) → carbon-neutral fuel.
Combustion of ethanol
Examiner keyword
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + energy. Burns with a clean blue flame, releasing energy. Used as fuel (biofuel, alcohol stoves) and in alcoholic-drink-related combustion (flambé).
Oxidation of ethanol (to ethanoic acid)
Examiner keyword
Heating ethanol with acidified potassium dichromate(VI) under reflux converts it to ethanoic acid. C₂H₅OH + 2[O] → CH₃COOH + H₂O. Observed colour change: orange (Cr₂O₇²⁻) → green (Cr³⁺). Industrial / natural route: bacteria oxidise ethanol with air to make vinegar.
Dehydration (of an alcohol)
Examiner keyword
Removal of water from an alcohol to give an alkene. Conditions: pass alcohol vapour over hot Al₂O₃ catalyst at ~ 600 °C, or heat with concentrated H₂SO₄. Example: ethanol → ethene + H₂O.
Positional isomer (alcohol)
Examiner keyword
Compounds with the same molecular formula but different positions of the functional group on the carbon chain. Example: propan-1-ol (CH₃CH₂CH₂OH) and propan-2-ol (CH₃CH(OH)CH₃) are both C₃H₈O — same molecular formula, different positions of –OH.
Acidified potassium dichromate(VI)
Examiner keyword
K₂Cr₂O₇ + dilute H₂SO₄. Strong oxidising agent. ORANGE Cr₂O₇²⁻ → GREEN Cr³⁺ when reduced. Used in 4CH1 to oxidise ethanol to ethanoic acid.
Methanol (toxicity)
Examiner keyword
CH₃OH — the simplest alcohol. Useful industrial solvent and feedstock for plastics + biodiesel. POISONOUS to humans — metabolised to methanal + methanoic acid → causes blindness and death. Added to ethanol to make 'methylated spirits' (denatured alcohol) — not drinkable but tax-free for industry.

Common Mistakes and Misconceptions — Alcohols

The traps other students keep falling into on alcohols questions — taken from recent Pearson Edexcel IGCSE 4CH1 examiner reports and mark schemes — and how to avoid them.

✕Saying the colour change for ethanol oxidation is 'orange to colourless' or 'orange to yellow'
4CH1 Examiner Reports 2022-2024
Why it happens
Confusing with bromine water or other decolourisation reactions.
How to avoid it
Acidified potassium dichromate goes from ORANGE (Cr₂O₇²⁻) to GREEN (Cr³⁺) — NOT colourless. Memorise: 'dichromate orange → chromium GREEN'.
✕Saying fermentation uses oxygen / is aerobic
4CH1 Examiner Reports 2023-2024
Why it happens
Confusion with respiration or with the bacterial oxidation of wine → vinegar.
How to avoid it
Fermentation is ANAEROBIC (no oxygen). The vessel must be sealed (airlock lets CO₂ out, keeps air out). If O₂ is present, ethanol is oxidised to ethanoic acid (vinegar) instead → wine 'goes off'. The mark scheme wants 'anaerobic' or 'no air/oxygen'.
✕Saying fermentation must be done at a high temperature
4CH1 Examiner Reports 2022-2024
Why it happens
Confusion with industrial / petrochemical conditions.
How to avoid it
Fermentation is at 25-35 °C — WARM but not hot. If T > ~ 40 °C, the yeast enzymes are DENATURED and the yeast dies → reaction stops. If T < 20 °C, the rate is too slow. The narrow optimum is one disadvantage of fermentation.
✕Confusing methanol and ethanol — writing 'methanol' as the product of fermentation, or 'ethanol' as poisonous
4CH1 Examiner Reports 2022-2024
Why it happens
The names sound similar.
How to avoid it
Methanol = CH₃OH = poisonous, NOT for drinking. Ethanol = C₂H₅OH = the alcohol in drinks, made by fermentation, can be safely consumed. Always count the carbon atoms before naming. C₁ = meth-; C₂ = eth-; C₃ = prop-.
✕Saying ethanol is soluble in water because it 'has H atoms' or 'is small'
Why it happens
Vague answer.
How to avoid it
The correct reason: ethanol has a polar –OH group that forms HYDROGEN BONDS with water molecules → highly soluble. Use the phrase 'hydrogen bonding between the –OH and water'.
✕Writing C₆H₁₂O₆ → C₂H₅OH + CO₂ (unbalanced)
4CH1 Examiner Reports 2023-2024
Why it happens
Forgetting that glucose has 6 carbons → must split into 2 ethanol (2 C each) + 2 CO₂ (1 C each) to balance.
How to avoid it
Always balance the C atoms: glucose has 6 C → 2 ethanol (2 C × 2 = 4 C) + 2 CO₂ (1 C × 2 = 2 C) = 6 C total. H: glucose 12 → 2C₂H₅OH gives 12 (2 × 6) ✓. O: glucose 6 → 2 ethanol (1 × 2 = 2) + 2 CO₂ (2 × 2 = 4) = 6 ✓. Correct equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Alcohols — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Alcohols

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Define the alcohol homologous series and give first three members (4 marks, Paper 1)

2Fermentation of glucose by yeast — equation and conditions (5 marks, Paper 1)

3Compare hydration of ethene vs fermentation (6 marks, Paper 2)

4Oxidation of ethanol to ethanoic acid — colour change (5 marks, Paper 2)

5Propan-1-ol and its isomer propan-2-ol (3 marks, Paper 1)

Alcohol

Hydroxyl group (–OH)

Fermentation

Hydration of ethene (industrial ethanol)

Renewable / bioethanol

Combustion of ethanol

Oxidation of ethanol (to ethanoic acid)

Dehydration (of an alcohol)

Positional isomer (alcohol)

Acidified potassium dichromate(VI)

Methanol (toxicity)

✕Saying the colour change for ethanol oxidation is 'orange to colourless' or 'orange to yellow'

✕Saying fermentation uses oxygen / is aerobic

✕Saying fermentation must be done at a high temperature

✕Confusing methanol and ethanol — writing 'methanol' as the product of fermentation, or 'ethanol' as poisonous

✕Saying ethanol is soluble in water because it 'has H atoms' or 'is small'

✕Writing C₆H₁₂O₆ → C₂H₅OH + CO₂ (unbalanced)

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Alcohols — frequently asked questions