Why is "Using 'base' and 'alkali' as if they mean the same thing" a common mistake in Aqueous solution?

Why it happens: Both neutralise acids, so students treat the words as interchangeable. How to avoid it: An **alkali is a SOLUBLE base** that gives OH⁻ ions. Every alkali is a base, but an insoluble base (e.g. CuO) is **not** an alkali. Source: Edexcel Chemistry examiner reports — recurring error

Why is "Saying acids are a source of OH⁻ (or alkalis a source of H⁺)" a common mistake in Aqueous solution?

Why it happens: Students mix up which ion goes with which. How to avoid it: Memorise: **acid → H⁺** (positive); **alkali → OH⁻** (hydroxide, negative). Acids and the H⁺ ion both start with 'H'. Source: Edexcel Chemistry examiner reports

Why is "Writing the ions without their charges (e.g. 'H' instead of 'H⁺')" a common mistake in Aqueous solution?

Why it happens: Students rush and forget the superscript charge. How to avoid it: Always include the charge: **H⁺** and **OH⁻**. An ion without a charge is just an atom/group and can be marked wrong. Source: Edexcel Chemistry examiner reports

Why is "Writing the neutralisation ionic equation as H⁺ + OH⁻ without the H₂O product" a common mistake in Aqueous solution?

Why it happens: Students recall the reacting ions but forget to show what is formed. How to avoid it: The full ionic equation is **H⁺ + OH⁻ → H₂O**. The product (water) is essential for the mark. Source: Edexcel Chemistry examiner reports

Why is "Forgetting water as a product of neutralisation (writing 'acid + base → salt' only)" a common mistake in Aqueous solution?

Why it happens: Students focus on the salt and overlook the water. How to avoid it: Neutralisation **always** gives **salt AND water**. Name both products every time. Source: Edexcel Chemistry examiner reports

Why is "Thinking a 'salt' means only sodium chloride (table salt)" a common mistake in Aqueous solution?

Why it happens: In everyday life 'salt' means the salt on food. How to avoid it: In chemistry a **salt** is any compound where the acid's H⁺ is replaced by a metal/ammonium ion — e.g. copper sulfate and potassium nitrate are also salts. Source: Edexcel Chemistry examiner reports

Acids, bases and salt preparationsEdexcel IGCSE Science(Double Award)-Chemistry (4WSD0-1C)

Aqueous solution

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Acids and Alkalis in Aqueous Solution — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

What 'acid' and 'alkali' really mean in terms of ions in water — acids are sources of H⁺ ions, alkalis are sources of OH⁻ ions — plus the difference between a base and an alkali, and neutralisation (acid + base → salt + water; H⁺ + OH⁻ → H₂O). Covers Double Award Chemistry spec 2.36 and 2.40, assessed on Chemistry Paper 1 (4WSD0/1C).

What you’ll learn

Mapped to the Edexcel IGCSE 4WSD0-1C syllabus (2026 onwards).

2.36 — Recall that acids in aqueous solution are sources of H+ ions and alkalis are sources of OH- ions.
2.40 — Understand neutralisation (acid + base → salt + water; H+ + OH- → H2O) and the meaning of acid, base, alkali and salt.

Acids are sources of H⁺ ions (spec 2.36)

When an acid dissolves in water it releases H⁺ ions — that is what makes it acidic.

The word 'aqueous' simply means dissolved in water — that is what the state symbol (aq) tells you.

An acid is a substance that is a source of H⁺ ions (hydrogen ions, also called protons) when it dissolves in water. The H⁺ ions are what give an acid all its acidic behaviour.

For example, hydrochloric acid splits up (ionises) in water:

$\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

Common acids and the ions they release:

Acid	Formula	Ions released in water
Hydrochloric acid	HCl	H⁺ and Cl⁻
Sulfuric acid	H₂SO₄	H⁺ and SO₄²⁻
Nitric acid	HNO₃	H⁺ and NO₃⁻

Key idea: an acid only behaves as an acid when it is dissolved in water. A dry acid molecule, with no water, has not released its H⁺ ions yet — so the H⁺(aq) ion is the active part.

'Aqueous' (aq) = dissolved in water.
Acid = a source of H⁺ ions in aqueous solution.
H⁺ ions (protons) are what make a solution acidic.
HCl(aq) → H⁺(aq) + Cl⁻(aq).

See the full worked example for aqueous solution →

Base vs alkali — alkalis are sources of OH⁻ ions (spec 2.36, 2.40)

A base neutralises an acid; an alkali is a base that is SOLUBLE and gives OH⁻ ions.

Students lose marks every series by muddling base and alkali. They are linked but not the same.

Base. A base is a substance that neutralises an acid. Bases are usually metal oxides (e.g. CuO, MgO) or metal hydroxides (e.g. Fe(OH)₃). Many bases are insoluble in water.

Alkali. An alkali is a base that is SOLUBLE in water. When it dissolves it is a source of OH⁻ ions (hydroxide ions). For example:

$\text{NaOH(aq)} \rightarrow \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}$

Common alkalis: sodium hydroxide NaOH, potassium hydroxide KOH, calcium hydroxide Ca(OH)₂.

So the relationship is:

Every alkali IS a base (it neutralises acids).
Not every base is an alkali — copper(II) oxide neutralises acid (a base) but is insoluble, so it cannot give OH⁻ ions in solution and is not an alkali.

Substance	Base?	Soluble?	Alkali?	Gives OH⁻(aq)?
Sodium hydroxide NaOH	yes	yes	yes	yes
Potassium hydroxide KOH	yes	yes	yes	yes
Copper(II) oxide CuO	yes	no	no	no
Magnesium oxide MgO	yes	almost insoluble	no	no

Key idea: "alkali" = a soluble base that releases OH⁻ ions. If a base does not dissolve, it cannot be an alkali.

Base = a substance that neutralises an acid (metal oxide / hydroxide).
Alkali = a SOLUBLE base → source of OH⁻ ions in water.
Every alkali is a base; not every base is an alkali.
NaOH(aq) → Na⁺(aq) + OH⁻(aq).

See the full worked example for aqueous solution →

Neutralisation: H⁺ + OH⁻ → H₂O (spec 2.40)

Acid + base → salt + water; the H⁺ and OH⁻ ions join to make water.

When an acid reacts with a base, they cancel each other out — this is neutralisation. The general word equation is:

acid + base → salt + water

The reason it always works is the same in every case: the H⁺ ions from the acid join with the OH⁻ ions from the alkali/base to make water. This is the ionic equation you must learn:

$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$

In every neutralisation the H⁺ ion from the acid joins the OH⁻ ion from the alkali to make a molecule of water.

Some worked word equations:

Acid	Base / alkali	Salt + water
HCl	NaOH	NaCl + H₂O
H₂SO₄	CuO	CuSO₄ + H₂O
HNO₃	KOH	KNO₃ + H₂O

In each case the metal from the base takes the place of the hydrogen in the acid → a salt — and the leftover H⁺ and OH⁻ make water.

Why the ionic equation is always H⁺ + OH⁻ → H₂O: the metal ion (e.g. Na⁺) and the acid's anion (e.g. Cl⁻) stay dissolved and unchanged — they are spectator ions. Only the H⁺ and OH⁻ actually react, so the underlying reaction is identical every time.

Neutralisation word equation: acid + base → salt + water.
Neutralisation ionic equation: H⁺ + OH⁻ → H₂O.
The H⁺ comes from the acid; the OH⁻ comes from the alkali.
Metal ion and acid anion are spectator ions (unchanged).

See the full worked example for aqueous solution →

What a salt is (spec 2.40)

A salt forms when the H⁺ of an acid is replaced by a metal (or ammonium) ion.

A salt is the compound made when the hydrogen ion(s) of an acid are replaced by a metal ion (or an ammonium ion, NH₄⁺). The salt is the "acid + base → salt + water" product.

The name of the salt comes from two parts:

the metal (from the base) gives the first part of the name;
the acid gives the second part of the name (the anion).

Acid used	Salt ending	Example salt
Hydrochloric acid (HCl)	chloride	sodium chloride (NaCl)
Sulfuric acid (H₂SO₄)	sulfate	copper(II) sulfate (CuSO₄)
Nitric acid (HNO₃)	nitrate	potassium nitrate (KNO₃)

For example, when hydrochloric acid is neutralised by sodium hydroxide, the H⁺ of the acid is replaced by Na⁺, giving the salt sodium chloride (NaCl) plus water.

Key idea: the salt is not an extra ingredient you add — it is made during the neutralisation from the metal of the base and the anion of the acid.

Salt = compound where the acid's H⁺ is replaced by a metal (or NH₄⁺) ion.
Salt name = metal (from base) + anion (from acid).
HCl → chloride; H₂SO₄ → sulfate; HNO₃ → nitrate.
The salt is produced during neutralisation, not added.

See the full worked example for aqueous solution →

How it’s examined

Double Award Chemistry is assessed on one untiered paper — Chemistry Paper 1 (4WSD0/1C), 2 hours, 110 marks, 33.3% of the Double Award (calculator allowed). This subtopic appears as short recall and definition marks: state what acids/alkalis are sources of (H⁺ / OH⁻), distinguish a base from an alkali, write the neutralisation word equation and the ionic equation H⁺ + OH⁻ → H₂O, and identify the products (salt + water). The biggest discriminators are the exact ionic equation and the base-vs-alkali distinction.

Sources: Pearson Edexcel International GCSE Science (Double Award) 4SD0 specification — Issue 4 (valid for 2026 onwards); Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4 (shared Chemistry Paper 1 content); Pearson Edexcel 4SD0 / 4CH1 examiner reports 2022–2024. Last reviewed 2026-06-07.

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Step-by-step worked examples — Aqueous solution

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

1What are acids a source of? (Getting started)
Getting started• 4WSD0/1C style — short answer• acids, spec-2.36
Question
Name the ion that acids are a source of when they are dissolved in water, and name the ion that alkalis are a source of. (2 marks)
Step-by-step solution
1. Step 1
  Acids (1). Acids in aqueous solution are a source of H⁺ ions (hydrogen ions / protons).
2. Step 2
  Alkalis (1). Alkalis in aqueous solution are a source of OH⁻ ions (hydroxide ions).
3. Step 3
  Watch the charge. The marks need the correct sign: H⁺ (positive) and OH⁻ (negative). 'Hydrogen' without the charge is risky.
Answer
Acids are a source of H⁺ ions; alkalis are a source of OH⁻ ions.
Examiner tip
Two independent marks. Write the formula of the ion with its charge (H⁺, OH⁻). Naming the wrong ion (e.g. saying acids give OH⁻) loses both.
2Base vs alkali (Getting started)
Getting started• 4WSD0/1C style — short answer• bases, alkalis, spec-2.40
Question
Sodium hydroxide and copper(II) oxide both neutralise acids. Only one of them is an alkali. State which one is an alkali and explain why the other one is not. (2 marks)
Step-by-step solution
1. Step 1
  Which is the alkali (1). Sodium hydroxide is the alkali — it is a base that is soluble in water.
2. Step 2
  Why CuO is not (1). Copper(II) oxide is a base (it neutralises acid) but it is insoluble in water, so it cannot dissolve to give OH⁻ ions — therefore it is not an alkali.
3. Step 3
  The rule. An alkali = a soluble base. If a base does not dissolve, it is not an alkali.
Answer
Sodium hydroxide is the alkali because it is a soluble base that gives OH⁻ ions in water. Copper(II) oxide is a base but is insoluble, so it is not an alkali.
Examiner tip
The mark is for the idea of SOLUBILITY. An alkali is a soluble base; bases such as CuO neutralise acids but do not dissolve, so they are not alkalis.
3Write the neutralisation ionic equation (Building confidence)
Building confidence• 4WSD0/1C style — structured• neutralisation, spec-2.40, equations
Question
Hydrochloric acid reacts with sodium hydroxide solution. (a) Write the word equation. (b) Write the ionic equation for the neutralisation. (3 marks)
Step-by-step solution
1. Step 1
  (a) Word equation (1). acid + base → salt + water, so: hydrochloric acid + sodium hydroxide → sodium chloride + water.
2. Step 2
  (b) Identify what actually reacts (1). The H⁺ from the acid and the OH⁻ from the alkali combine; Na⁺ and Cl⁻ are spectator ions.
3. Step 3
  (b) Ionic equation (1). H⁺(aq) + OH⁻(aq) → H₂O(l) — this is the same for every acid–alkali neutralisation.
Answer
(a) hydrochloric acid + sodium hydroxide → sodium chloride + water. (b) H⁺(aq) + OH⁻(aq) → H₂O(l).
Examiner tip
The ionic equation H⁺ + OH⁻ → H₂O is worth a guaranteed mark and is the same in every neutralisation — learn it exactly, including the H₂O product.
4Identify the products of neutralisation (Building confidence)
Building confidence• 4WSD0/1C style — structured• salts, spec-2.40, products
Question
Sulfuric acid is neutralised by potassium hydroxide. (a) Name the two products formed. (b) Name the salt produced. (3 marks)
Step-by-step solution
1. Step 1
  (a) Products (1 + 1). Neutralisation gives a salt and water.
2. Step 2
  (b) Name the salt (1). Sulfuric acid → a sulfate; the metal is potassium → the salt is potassium sulfate (K₂SO₄).
3. Step 3
  Check the pattern. Salt name = metal (from base) + anion (from acid): potassium + sulfate.
Answer
(a) a salt and water. (b) potassium sulfate.
Examiner tip
Both products (salt AND water) are needed. The salt name comes from the acid (sulfuric → sulfate) and the metal in the base (potassium).
5Classify substances as acid, base or alkali (Stretch)
Stretch• 4WSD0/1C style — extended• classification, spec-2.36, spec-2.40
Question
For each substance, state whether it is an acid, a base, an alkali, or none: (a) HNO₃; (b) KOH; (c) MgO (insoluble); (d) NaCl. Give a brief reason for each. (4 marks)
Step-by-step solution
1. Step 1
  (a) HNO₃ — acid (1). Nitric acid is a source of H⁺ ions in water.
2. Step 2
  (b) KOH — alkali (1). Potassium hydroxide is a soluble base, so it gives OH⁻ ions → it is an alkali (and also a base).
3. Step 3
  (c) MgO — base (1). Magnesium oxide neutralises acids (a base) but is essentially insoluble, so it is not an alkali.
4. Step 4
  (d) NaCl — none (1). Sodium chloride is a salt; it is neither acidic nor basic — it is a neutralisation product.
Answer
(a) acid (source of H⁺); (b) alkali (soluble base → OH⁻); (c) base only (neutralises acid but insoluble); (d) none — it is a salt.
Examiner tip
The trap is (b) vs (c): KOH is soluble so it is an alkali; MgO neutralises acid but is insoluble so it is only a base. (d) NaCl is a salt, not a base.
6Explain why one ionic equation fits all neutralisations (Stretch)
Stretch• 4WSD0/1C style — extended• neutralisation, spec-2.40, reasoning
Question
Hydrochloric acid neutralises sodium hydroxide, and nitric acid neutralises potassium hydroxide. Both reactions have the same ionic equation. Write that ionic equation and explain why it is the same for both reactions. (4 marks)
Step-by-step solution
1. Step 1
  Ionic equation (1). Both give: H⁺(aq) + OH⁻(aq) → H₂O(l).
2. Step 2
  What reacts (1). In both cases it is the H⁺ ions from the acid and the OH⁻ ions from the alkali that react together to form water.
3. Step 3
  Spectator ions (1). The metal ions (Na⁺, K⁺) and the acid anions (Cl⁻, NO₃⁻) do not take part — they are spectator ions and stay dissolved unchanged.
4. Step 4
  Conclusion (1). Because only H⁺ and OH⁻ actually react, the underlying ionic equation is identical for every acid–alkali neutralisation.
Answer
H⁺(aq) + OH⁻(aq) → H₂O(l). It is the same for both because only the H⁺ ions (from the acid) and OH⁻ ions (from the alkali) react to make water; the metal ions and acid anions are spectator ions that do not change.
Examiner tip
Top marks need the term 'spectator ions' and the idea that only H⁺ and OH⁻ react. Stating the equation alone caps you at 1 mark on a 4-mark 'explain' question.

Model Answers — Aqueous solution

High-scoring sample answers for aqueous solution on the Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
4WSD0/1C style2 marks
Complete the sentences. (a) Acids in aqueous solution are a source of ____ ions. (b) Alkalis in aqueous solution are a source of ____ ions. (2 marks)
Model answer
(a) Acids are a source of H⁺ ions (hydrogen ions / protons).

(b) Alkalis are a source of OH⁻ ions (hydroxide ions).
Why this scores
1 mark each. Include the charge: H⁺ and OH⁻. 'Hydrogen' / 'hydroxide' written without the correct sign can be marked wrong.
Question 2
4WSD0/1C style3 marks
Write the general word equation for the neutralisation of an acid by a base, and write the ionic equation for neutralisation. (3 marks)
Model answer
Word equation: acid + base → salt + water.

Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l).
Why this scores
2 marks for the word equation (salt AND water), 1 mark for the ionic equation. The H₂O product in the ionic equation must be present.
Question 3
4WSD0/1C style4 marks
Explain the difference between a base and an alkali. Use copper(II) oxide and sodium hydroxide as examples. (4 marks)
Model answer
A base is a substance that neutralises an acid — usually a metal oxide or metal hydroxide.

An alkali is a base that is soluble in water, so it can dissolve to give OH⁻ ions.

Copper(II) oxide is a base because it neutralises acids, but it is insoluble, so it is not an alkali.

Sodium hydroxide is a base that is soluble, giving OH⁻ ions in water, so it is an alkali (every alkali is also a base).
Why this scores
Marks for: base = neutralises acid; alkali = soluble base / gives OH⁻; CuO insoluble so base only; NaOH soluble so alkali. The discriminator is solubility.
Question 4
4WSD0/1C style4 marks
Hydrochloric acid is added to magnesium oxide until the acid is neutralised. (a) Name the type of reaction. (b) Name the two products. (c) Name the salt formed. (4 marks)
Model answer
(a) The reaction is neutralisation (acid + base → salt + water).

(b) The two products are a salt and water.

(c) The salt is magnesium chloride (MgCl₂) — hydrochloric acid gives a chloride and the metal is magnesium.
Why this scores
1 mark for 'neutralisation'; 1 each for salt and water; 1 for the correct salt name. Magnesium oxide is a base (insoluble), but it still neutralises acid to give a salt + water.
Question 5
4WSD0/1C style — extended6 marks
Nitric acid solution reacts with potassium hydroxide solution. Write the ionic equation for the reaction and explain, in terms of the ions present, why this ionic equation does not include potassium or nitrate ions. (6 marks)
Model answer
Ionic equation. H⁺(aq) + OH⁻(aq) → H₂O(l). (2)

The ions present. Nitric acid in water provides H⁺ and NO₃⁻ ions; potassium hydroxide in water provides K⁺ and OH⁻ ions. (1)

What actually reacts. Only the H⁺ ions (from the acid) and the OH⁻ ions (from the alkali) react together — they combine to form water. (1)

Spectator ions. The K⁺ ions and NO₃⁻ ions do not take part in the reaction; they remain dissolved in solution unchanged. They are called spectator ions. (1)

Conclusion. Because spectator ions appear on both sides of the equation and do not change, they are left out of the ionic equation, leaving only H⁺ + OH⁻ → H₂O. (1)
Why this scores
Mark scheme rewards: correct ionic equation (2), identifying H⁺ and OH⁻ as the reacting ions, naming K⁺/NO₃⁻ as spectator ions, and explaining spectators are removed. 'Spectator ions' is the key term.
Question 6
4WSD0/1C style — extended8 marks
Classify each of the following as an acid, an alkali, a base that is not an alkali, or a salt, and give a reason for each: sulfuric acid (H₂SO₄), potassium hydroxide (KOH), iron(III) oxide (Fe₂O₃, insoluble), sodium nitrate (NaNO₃). (8 marks)
Model answer
Sulfuric acid (H₂SO₄) — acid. It is a source of H⁺ ions in aqueous solution. (2)

Potassium hydroxide (KOH) — alkali. It is a soluble base, so it dissolves to give OH⁻ ions in water (it is also a base). (2)

Iron(III) oxide (Fe₂O₃) — base (not an alkali). It is a metal oxide that neutralises acid (a base), but it is insoluble, so it cannot give OH⁻ ions and is not an alkali. (2)

Sodium nitrate (NaNO₃) — salt. It is the product of neutralisation (the H⁺ of nitric acid replaced by Na⁺); it is neither acidic nor basic. (2)
Why this scores
2 marks each: classification + reason. The hardest mark is recognising that an insoluble metal oxide is a base but NOT an alkali, and that a sodium salt is neither acid nor base.

Key Definitions and Keywords — Aqueous solution

Definitions to memorise and the exact keywords mark schemes credit for aqueous solution answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) sitting.

Aqueous solution (aq)
A solution made by dissolving a substance in water. The state symbol (aq) means 'dissolved in water'.
Acid
Examiner keyword
A substance that is a source of H⁺ ions (hydrogen ions / protons) when dissolved in water.
Base
Examiner keyword
A substance that neutralises an acid, usually a metal oxide or metal hydroxide. Bases may be soluble or insoluble.
Alkali
Examiner keyword
A base that is soluble in water; it dissolves to be a source of OH⁻ ions (hydroxide ions). Every alkali is a base, but not every base is an alkali.
Neutralisation
Examiner keyword
The reaction of an acid with a base to form a salt and water: acid + base → salt + water; ionic equation H⁺ + OH⁻ → H₂O.
Salt
Examiner keyword
A compound formed when the hydrogen ion(s) of an acid are replaced by a metal ion (or an ammonium ion, NH₄⁺).
Hydrogen ion (H⁺)
Examiner keyword
A positively charged ion (a proton) released by acids in aqueous solution; it is responsible for acidic behaviour.
Hydroxide ion (OH⁻)
Examiner keyword
A negatively charged ion released by alkalis in aqueous solution; it is responsible for alkaline behaviour.
Spectator ion
An ion that is present in a reaction but does not take part in it (e.g. Na⁺ and Cl⁻ in a neutralisation); it is left out of the ionic equation.
Ionic equation
An equation showing only the ions and molecules that actually take part in a reaction, with spectator ions removed (e.g. H⁺ + OH⁻ → H₂O).

Common Mistakes and Misconceptions — Aqueous solution

The traps other students keep falling into on aqueous solution questions — taken from recent Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) examiner reports and mark schemes — and how to avoid them.

✕Using 'base' and 'alkali' as if they mean the same thing
Edexcel Chemistry examiner reports — recurring error
Why it happens
Both neutralise acids, so students treat the words as interchangeable.
How to avoid it
An alkali is a SOLUBLE base that gives OH⁻ ions. Every alkali is a base, but an insoluble base (e.g. CuO) is not an alkali.
✕Saying acids are a source of OH⁻ (or alkalis a source of H⁺)
Edexcel Chemistry examiner reports
Why it happens
Students mix up which ion goes with which.
How to avoid it
Memorise: acid → H⁺ (positive); alkali → OH⁻ (hydroxide, negative). Acids and the H⁺ ion both start with 'H'.
✕Writing the ions without their charges (e.g. 'H' instead of 'H⁺')
Edexcel Chemistry examiner reports
Why it happens
Students rush and forget the superscript charge.
How to avoid it
Always include the charge: H⁺ and OH⁻. An ion without a charge is just an atom/group and can be marked wrong.
✕Writing the neutralisation ionic equation as H⁺ + OH⁻ without the H₂O product
Edexcel Chemistry examiner reports
Why it happens
Students recall the reacting ions but forget to show what is formed.
How to avoid it
The full ionic equation is H⁺ + OH⁻ → H₂O. The product (water) is essential for the mark.
✕Forgetting water as a product of neutralisation (writing 'acid + base → salt' only)
Edexcel Chemistry examiner reports
Why it happens
Students focus on the salt and overlook the water.
How to avoid it
Neutralisation always gives salt AND water. Name both products every time.
✕Thinking a 'salt' means only sodium chloride (table salt)
Edexcel Chemistry examiner reports
Why it happens
In everyday life 'salt' means the salt on food.
How to avoid it
In chemistry a salt is any compound where the acid's H⁺ is replaced by a metal/ammonium ion — e.g. copper sulfate and potassium nitrate are also salts.

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Acids and Alkalis in Aqueous Solution — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

Acid

Formula

Ions released in water

Hydrochloric acid

HCl

H⁺ and Cl⁻

Sulfuric acid

H₂SO₄

H⁺ and SO₄²⁻

Nitric acid

HNO₃

H⁺ and NO₃⁻

Substance

Base?

Soluble?

Alkali?

Gives OH⁻(aq)?

Sodium hydroxide NaOH

yes

yes

yes

Potassium hydroxide KOH

yes

yes

yes

Copper(II) oxide CuO

yes

Magnesium oxide MgO

yes

almost insoluble

Acid

Base / alkali

Salt + water

HCl

NaOH

NaCl + H₂O

H₂SO₄

CuO

CuSO₄ + H₂O

HNO₃

KOH

KNO₃ + H₂O

Acid used

Salt ending

Example salt

Hydrochloric acid (HCl)

chloride

sodium chloride (NaCl)

Sulfuric acid (H₂SO₄)

sulfate

copper(II) sulfate (CuSO₄)

Nitric acid (HNO₃)

nitrate

potassium nitrate (KNO₃)

Ionic equation. H⁺(aq) + OH⁻(aq) → H₂O(l). (2)

The ions present. Nitric acid in water provides H⁺ and NO₃⁻ ions; potassium hydroxide in water provides K⁺ and OH⁻ ions. (1)

What actually reacts. Only the H⁺ ions (from the acid) and the OH⁻ ions (from the alkali) react together — they combine to form water. (1)

Spectator ions. The K⁺ ions and NO₃⁻ ions do not take part in the reaction; they remain dissolved in solution unchanged. They are called spectator ions. (1)

Conclusion. Because spectator ions appear on both sides of the equation and do not change, they are left out of the ionic equation, leaving only H⁺ + OH⁻ → H₂O. (1)

Aqueous solution

Detailed Notes

What you’ll learn

How it’s examined

1What are acids a source of? (Getting started)

2Base vs alkali (Getting started)

3Write the neutralisation ionic equation (Building confidence)

4Identify the products of neutralisation (Building confidence)

5Classify substances as acid, base or alkali (Stretch)

6Explain why one ionic equation fits all neutralisations (Stretch)