Why is "Thinking pH 7 is slightly acidic or 'an acid'" a common mistake in Using the pH scale?

Why it happens: Students remember 'acids have low pH' and forget that 7 is the exact neutral point. How to avoid it: Learn the three regions sharply: ** 7 alkaline**. 7 is neutral, never acidic. Source: Edexcel Chemistry examiner reports — recurring error

Why is "Calling a high pH a 'strong acid'" a common mistake in Using the pH scale?

Why it happens: Students associate any extreme pH with 'strong acid' without checking the direction. How to avoid it: A **high** pH (e.g. 13, 14) is strongly **ALKALINE**, not acidic. Only **low** pH values are acidic. Source: Edexcel Chemistry examiner reports

Why is "Saying only pure water can have a pH of 7" a common mistake in Using the pH scale?

Why it happens: Water is the standard example of a neutral substance, so students assume it is the only one. How to avoid it: Neutral means **equal H⁺ and OH⁻ ions** — other solutions (e.g. sodium chloride solution) are neutral too. Source: Edexcel Chemistry examiner reports

Why is "Mixing up which ion belongs to acids and which to alkalis" a common mistake in Using the pH scale?

Why it happens: H⁺ and OH⁻ look similar and both involve hydrogen, so they get swapped. How to avoid it: Memorise: **Acid → H⁺** (hydrogen ion); **Alkali → OH⁻** (hydroxide ion). 'A' for Acid and the H⁺ comes first. Source: Edexcel Chemistry examiner reports

Why is "Claiming universal indicator gives an exact/accurate pH" a common mistake in Using the pH scale?

Why it happens: Students see a number on the colour chart and assume it is precise. How to avoid it: Universal indicator only gives an **estimate/approximate** pH (colour judged by eye). For an accurate value use a **pH probe/meter**. Source: Edexcel Chemistry examiner reports

Why is "Saying universal indicator is 'colourless' when neutral" a common mistake in Using the pH scale?

Why it happens: Students confuse universal indicator with phenolphthalein, which is colourless in acid. How to avoid it: Universal indicator is **green at pH 7 (neutral)** — never colourless. Red/orange = acid, green = neutral, blue/purple = alkali. Source: Edexcel Chemistry examiner reports

Acids, alkalis and titrationsEdexcel IGCSE Science(Double Award)-Chemistry (4WSD0-1C)

Using the pH scale

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Using the pH Scale — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

What the pH scale 0–14 tells you, how acids are a source of H⁺ ions and alkalis a source of OH⁻ ions, and how to measure pH using indicator solutions, pH paper, universal indicator and a pH probe. Covers Double Award Chemistry spec 2.36–2.39, 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 are a source of H+ ions and alkalis are a source of OH- ions in aqueous solution.
2.37 — Understand how to measure pH using indicator solutions, pH paper and pH probes.
2.39 — Describe how universal indicator estimates pH on the 0–14 scale.

The pH scale 0–14 (spec 2.39)

Below 7 acidic, 7 neutral, above 7 alkaline — lower pH means more acidic.

pH is a number from 0 to 14 that tells you how acidic or alkaline a solution is. It is one of the most common ideas in IGCSE Chemistry, so learn the scale until it is automatic.

pH less than 7 → the solution is ACIDIC.
pH exactly 7 → the solution is NEUTRAL.
pH greater than 7 → the solution is ALKALINE.

The key direction: the lower the pH, the more acidic the solution (it has more H⁺ ions). The higher the pH, the more alkaline the solution (it has more OH⁻ ions). A pH 1 acid is more acidic than a pH 5 acid; a pH 14 alkali is more alkaline than a pH 9 alkali.

pH below 7 is acidic, exactly 7 is neutral, above 7 is alkaline — colours match the universal indicator chart.

pH	Description	Example solution
0–3	Strongly acidic	hydrochloric acid (HCl)
4–6	Weakly acidic	vinegar, lemon juice, rainwater
7	Neutral	pure water, sodium chloride solution
8–11	Weakly alkaline	soap solution, baking soda solution
12–14	Strongly alkaline	sodium hydroxide (NaOH)

pH < 7 acidic; pH = 7 neutral; pH > 7 alkaline.
Lower pH = more acidic (more H⁺); higher pH = more alkaline (more OH⁻).
The scale runs 0–14 — memorise it.
Order solutions by pH to compare how acidic or alkaline they are.

See the full worked example for using the ph scale →

Acids give H⁺ ions, alkalis give OH⁻ ions (spec 2.36)

Acid = source of H⁺ in water; alkali = source of OH⁻ in water.

pH is really a measure of ions in solution. When a substance dissolves in water it can release ions, and the type of ion decides whether the solution is acidic or alkaline.

An ACID in aqueous solution is a source of hydrogen ions, H⁺. The more H⁺ ions, the lower the pH (the more acidic).
An ALKALI in aqueous solution is a source of hydroxide ions, OH⁻. The more OH⁻ ions, the higher the pH (the more alkaline).

For example, hydrochloric acid releases H⁺ ions in water, and sodium hydroxide (an alkali) releases OH⁻ ions in water:

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

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

Neutral (pH 7) does NOT mean "no ions". A neutral solution contains equal numbers of H⁺ and OH⁻ ions, so neither dominates. Pure water is the most famous neutral example, but other solutions (such as sodium chloride solution) can be neutral too — neutral is about a balance of H⁺ and OH⁻, not about being water.

The link to remember:

Solution	Main ion released	pH region
Acid	H⁺ (hydrogen ion)	below 7
Neutral	equal H⁺ and OH⁻	exactly 7
Alkali	OH⁻ (hydroxide ion)	above 7

Acid = source of H⁺ ions in aqueous solution.
Alkali = source of OH⁻ ions in aqueous solution.
More H⁺ → lower pH; more OH⁻ → higher pH.
Neutral = equal H⁺ and OH⁻, not just pure water.

See the full worked example for using the ph scale →

Universal indicator and pH paper (spec 2.37, 2.39)

Universal indicator gives a colour you match to a chart to estimate pH.

An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline. Some indicators only tell you acid-or-alkali (one colour change). Universal indicator is more useful because it tells you the approximate pH.

How universal indicator works.

Universal indicator is a mixture of several indicators blended together.
Add a few drops to the solution (or use pH paper, which is paper soaked in universal indicator).
It turns a colour that depends on the pH, then you match the colour to a printed pH colour chart to read off the value.

The colour sequence across the scale is:

Red / orange at low pH → strongly/weakly acidic (pH 0–6).
Green at pH 7 → neutral.
Blue / purple at high pH → weakly/strongly alkaline (pH 8–14).

Universal indicator changes through the rainbow — red (acid) to green (neutral) to purple (alkali).

Important limitation. Universal indicator only gives an estimate of pH because you are matching a colour by eye. It is great for a quick whole-number reading, but it is not precise — you cannot easily tell pH 4 from pH 5 from the colour alone.

Universal indicator = mixture of indicators → colour shows approximate pH.
Red/orange (acid) → green (neutral) → blue/purple (alkali).
pH paper works the same way as universal indicator solution.
Match the colour to the printed chart to estimate the pH value.

See the full worked example for using the ph scale →

Choosing how to measure pH (spec 2.37)

Universal indicator/pH paper = quick estimate; pH probe = accurate number.

Spec 2.37 wants you to know three ways to measure pH and when each is suitable.

Method	What you do	What you get
Indicator solution (e.g. litmus)	Add drops, watch colour	Tells you acid OR alkali (less detail)
Universal indicator / pH paper	Add drops or dip paper, match colour to chart	Approximate pH value (a colour band)
pH probe / pH meter	Dip the electrode into the solution, read the digital display	Accurate numerical pH (e.g. 3.7)

Universal indicator / pH paper.

Advantages: cheap, quick, simple, no electricity needed.
Disadvantage: only an estimate — judging colour by eye is not precise, and coloured solutions can hide the colour.

pH probe / pH meter.

Advantages: gives a precise decimal number (more accurate than colour matching) and is not affected by the colour of the solution.
Disadvantages: more expensive, must be calibrated before use.

Exam tip: if a question asks for an accurate pH value, choose a pH probe/meter. If it asks for a quick estimate or there is no power supply, choose universal indicator / pH paper.

Indicator solution → acid or alkali only.
Universal indicator / pH paper → approximate pH (colour band).
pH probe / meter → accurate numerical pH.
Need an exact value → pick the probe; quick estimate → universal indicator.

See the full worked example for using the ph scale →

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). The pH scale appears as quick recall and short application marks: state whether a solution is acidic/neutral/alkaline from its pH, link a low/high pH to H⁺/OH⁻ ions, predict the universal indicator colour at a given pH (or read the pH back from a colour), and choose a measuring method (universal indicator for a quick estimate vs a pH probe for an accurate value). The most common discriminators are remembering that neutral means EQUAL H⁺ and OH⁻ (not just water) and that universal indicator only ESTIMATES pH.

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 — Using the pH scale

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

1Classify solutions from their pH (Getting started)
Getting started• 4WSD0/1C style — short answer• ph-scale, spec-2.39
Question
Three solutions have pH 2, pH 7 and pH 11. State whether each is acidic, neutral or alkaline. (3 marks)
Step-by-step solution
1. Step 1
  pH 2 (1). Below 7 → acidic.
2. Step 2
  pH 7 (1). Exactly 7 → neutral.
3. Step 3
  pH 11 (1). Above 7 → alkaline.
Answer
pH 2 = acidic; pH 7 = neutral; pH 11 = alkaline.
Examiner tip
One mark each. The rule is simply < 7 acidic, = 7 neutral, > 7 alkaline. Learn it so well you never hesitate at the 7 boundary.
2Predict the universal indicator colour (Getting started)
Getting started• 4WSD0/1C style — short answer• universal-indicator, spec-2.39
Question
Universal indicator is added to three solutions. State the colour you would expect for a solution of: (a) pH 2; (b) pH 7; (c) pH 13. (3 marks)
Step-by-step solution
1. Step 1
  (a) pH 2 (1). Strongly acidic → red.
2. Step 2
  (b) pH 7 (1). Neutral → green.
3. Step 3
  (c) pH 13 (1). Strongly alkaline → purple (blue/purple acceptable).
Answer
(a) red; (b) green; (c) purple.
Examiner tip
Remember the order: red/orange (acid) → green (neutral) → blue/purple (alkali). Green is the give-away for neutral — many students wrongly say 'colourless'.
3Link pH to H⁺ and OH⁻ ions (Building confidence)
Building confidence• 4WSD0/1C style — structured• ions, spec-2.36, application
Question
Solution X has pH 1 and solution Y has pH 13. State which ion makes each solution acidic or alkaline, and which solution contains the most of that ion. (3 marks)
Step-by-step solution
1. Step 1
  Solution X (1). pH 1 is acidic → it is a source of H⁺ (hydrogen) ions.
2. Step 2
  Solution Y (1). pH 13 is alkaline → it is a source of OH⁻ (hydroxide) ions.
3. Step 3
  Most ions (1). The lower the pH, the more H⁺; the higher the pH, the more OH⁻. So X has the most H⁺ and Y has the most OH⁻.
Answer
X (pH 1) is acidic — a source of H⁺ ions, with a high H⁺ concentration. Y (pH 13) is alkaline — a source of OH⁻ ions, with a high OH⁻ concentration.
Examiner tip
The marks are for naming the correct ion (H⁺ for acids, OH⁻ for alkalis) AND linking the extreme pH to a large amount of that ion. Use the ion symbols, not just the words.
4Order solutions by acidity (Building confidence)
Building confidence• 4WSD0/1C style — structured• ph-scale, spec-2.39, data
Question
Four solutions have the following pH values: lemon juice pH 2, milk pH 6, blood pH 7, oven cleaner pH 13. Place them in order from MOST acidic to MOST alkaline, and state which is neutral. (3 marks)
Step-by-step solution
1. Step 1
  Most acidic first (1). Lowest pH is most acidic → lemon juice (pH 2), then milk (pH 6).
2. Step 2
  Neutral (1). Blood (pH 7) is neutral — equal H⁺ and OH⁻.
3. Step 3
  Most alkaline last (1). Highest pH is most alkaline → oven cleaner (pH 13).
Answer
Most acidic → most alkaline: lemon juice (2), milk (6), blood (7), oven cleaner (13). Blood (pH 7) is neutral.
Examiner tip
'Most acidic' = lowest pH; 'most alkaline' = highest pH. Students who think a high pH is 'strong acid' lose the ordering marks — high pH means strongly alkaline.
5Choose a method to measure pH (Stretch)
Stretch• 4WSD0/1C style — extended• measuring-ph, spec-2.37, reasoning
Question
A scientist needs the exact pH of a colourless solution, recorded to one decimal place. Universal indicator gives a colour that could be matched to either pH 4 or pH 5. State which method she should use instead and give two reasons why it is better here. (3 marks)
Step-by-step solution
1. Step 1
  Method (1). She should use a pH probe / pH meter.
2. Step 2
  Reason 1 (1). A pH probe gives an accurate numerical value (a decimal, e.g. pH 4.6) rather than an estimated colour band, so she can record it to one decimal place.
3. Step 3
  Reason 2 (1). Reading universal indicator relies on judging colour by eye, which is imprecise (she cannot tell pH 4 from pH 5); the probe removes this uncertainty.
Answer
Use a pH probe/meter. It gives an accurate numerical pH to one decimal place, and it avoids the imprecision of matching the universal indicator colour by eye.
Examiner tip
When a question stresses 'exact' or 'to one decimal place', the answer is almost always a pH probe/meter. Universal indicator only ever gives an estimate.
6Explain what neutral really means (Stretch)
Stretch• 4WSD0/1C style — extended• ions, spec-2.36, common-error
Question
A student says 'only pure water can be neutral'. Explain, in terms of ions, what a pH of 7 actually tells you, and why the student is wrong. (4 marks)
Step-by-step solution
1. Step 1
  Meaning of pH 7 (1). A pH of 7 means the solution is neutral — neither acidic nor alkaline.
2. Step 2
  In terms of ions (1 + 1). It contains equal numbers of H⁺ ions and OH⁻ ions, so neither ion is in excess.
3. Step 3
  Why the student is wrong (1). Pure water is neutral, but it is not the only neutral solution — for example, a sodium chloride (salt) solution is also neutral because it too has equal H⁺ and OH⁻ ions.
Answer
pH 7 means neutral: the solution has equal numbers of H⁺ and OH⁻ ions, so neither dominates. The student is wrong because other solutions (e.g. sodium chloride solution) are also neutral — neutral is about a balance of H⁺ and OH⁻ ions, not about being pure water.
Examiner tip
Top marks require the ion idea: neutral = EQUAL H⁺ and OH⁻. A correct named second example (e.g. salt solution) secures the final mark.

Model Answers — Using the pH scale

High-scoring sample answers for using the ph scale 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 style3 marks
Complete the sentences. A solution with a pH below 7 is ____. A solution with a pH of exactly 7 is ____. A solution with a pH above 7 is ____. (3 marks)
Model answer
A pH below 7 → acidic.

A pH of exactly 7 → neutral.

A pH above 7 → alkaline.
Why this scores
1 mark each. This is pure recall — the easiest marks on the topic, so they must be banked. Watch the 7 boundary: 7 is neutral, not acidic.
Question 2
4WSD0/1C style2 marks
Name the ion that acids produce in aqueous solution, and name the ion that alkalis produce in aqueous solution. (2 marks)
Model answer
Acids are a source of hydrogen ions, H⁺.

Alkalis are a source of hydroxide ions, OH⁻.
Why this scores
1 mark each. Use the correct ion symbols (H⁺ and OH⁻). Writing 'hydrogen' but drawing the symbol as OH⁻ for the acid is a common slip — keep them straight.
Question 3
4WSD0/1C style4 marks
Describe how universal indicator can be used to estimate the pH of a solution, and name one colour it would turn for an acid and one colour for an alkali. (4 marks)
Model answer
Add a few drops of universal indicator to the solution (or dip pH paper soaked in it). (1)

The indicator changes colour depending on the pH; you then match this colour to a printed pH colour chart to read off the approximate pH value. (1)

In an acid it turns red / orange / yellow (depending on how acidic). (1)

In an alkali it turns blue / purple. (1)
Why this scores
Key marking points: add indicator/dip paper, MATCH colour to a chart, plus an acid colour and an alkali colour. The word 'estimate' or 'approximate' shows you understand it is not precise.
Question 4
4WSD0/1C style4 marks
A table shows four household liquids: vinegar pH 3, water pH 7, washing-up liquid pH 8, drain cleaner pH 14. (a) Which liquid is the most acidic? (b) Which is neutral? (c) Which contains the most OH⁻ ions? Explain your answer to part (c). (4 marks)
Model answer
(a) Most acidic = the lowest pH = vinegar (pH 3). (1)

(b) Neutral = water (pH 7). (1)

(c) Most OH⁻ ions = drain cleaner (pH 14). (1)

Explanation: the higher the pH, the more alkaline the solution, and alkalis are a source of OH⁻ ions — so the solution with the highest pH has the most OH⁻ ions. (1)
Why this scores
Parts (a)–(c) are recall; the explanation mark needs the link 'higher pH → more alkaline → more OH⁻ ions'. Just stating the answer without the reasoning loses the final mark.
Question 5
4WSD0/1C style — extended6 marks
A student measures the pH of an orange juice. Compare using universal indicator with using a pH probe to measure its pH. In your answer give an advantage and a disadvantage of each method, and recommend which is better for getting an accurate value. (6 marks)
Model answer
Universal indicator (or pH paper).

Advantage: it is cheap, quick and simple, and needs no electricity. (1)

Disadvantage: it only gives an estimate of pH because you judge the colour by eye, which is imprecise — and the orange colour of the juice can mask the indicator colour. (1+1)

pH probe / pH meter.

Advantage: it gives an accurate numerical pH (a decimal value) and is not affected by the colour of the juice. (1+1)

Disadvantage: it is more expensive and must be calibrated before use. (1)

Recommendation: for an accurate value, the pH probe/meter is better, especially because the coloured juice would make the universal indicator difficult to read.
Why this scores
Top band needs a balanced comparison: a genuine advantage AND disadvantage for each method, plus a justified recommendation. The 'coloured solution masks the indicator' point is a sophisticated mark few students give.
Question 6
4WSD0/1C style — extended6 marks
Explain, in terms of ions, why solution A (pH 1) is strongly acidic, why solution B (pH 7) is neutral, and why solution C (pH 14) is strongly alkaline. (6 marks)
Model answer
Solution A — pH 1 (strongly acidic). An acid is a source of H⁺ (hydrogen) ions in aqueous solution. A very low pH means there is a high concentration of H⁺ ions, so the solution is strongly acidic. (2)

Solution B — pH 7 (neutral). A neutral solution contains equal numbers of H⁺ and OH⁻ ions, so neither dominates and the solution is neither acidic nor alkaline. (2)

Solution C — pH 14 (strongly alkaline). An alkali is a source of OH⁻ (hydroxide) ions in aqueous solution. A very high pH means there is a high concentration of OH⁻ ions, so the solution is strongly alkaline. (2)
Why this scores
Each part scores 2: name the correct ion (H⁺ / OH⁻) and link the extreme pH to a high concentration of it. The neutral part must say EQUAL H⁺ and OH⁻ — not 'no ions' or 'just water'.

Key Definitions and Keywords — Using the pH scale

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

pH
Examiner keyword
A number from 0 to 14 that measures how acidic or alkaline a solution is. The lower the pH, the more acidic; the higher the pH, the more alkaline.
pH scale
Examiner keyword
The 0–14 scale on which pH is measured: below 7 is acidic, exactly 7 is neutral and above 7 is alkaline.
Acidic
Examiner keyword
Having a pH less than 7. An acidic solution is a source of hydrogen ions (H⁺) in aqueous solution.
Neutral
Examiner keyword
Having a pH of exactly 7, with equal numbers of H⁺ and OH⁻ ions. Pure water is neutral, but it is not the only neutral solution.
Alkaline
Examiner keyword
Having a pH greater than 7. An alkaline solution is a source of hydroxide ions (OH⁻) in aqueous solution.
Acid
Examiner keyword
A substance that is a source of hydrogen ions (H⁺) when dissolved in water; its solution has a pH below 7.
Alkali
A substance that is a source of hydroxide ions (OH⁻) when dissolved in water; its solution has a pH above 7.
Indicator
A substance that changes colour depending on whether a solution is acidic or alkaline.
Universal indicator
Examiner keyword
A mixture of indicators that turns a range of colours (red→green→purple) to give an approximate pH value when matched to a colour chart.
pH probe / pH meter
Examiner keyword
An electronic instrument that gives an accurate numerical pH value when its electrode is dipped into a solution.

Common Mistakes and Misconceptions — Using the pH scale

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

✕Thinking pH 7 is slightly acidic or 'an acid'
Edexcel Chemistry examiner reports — recurring error
Why it happens
Students remember 'acids have low pH' and forget that 7 is the exact neutral point.
How to avoid it
Learn the three regions sharply: < 7 acidic, = 7 neutral, > 7 alkaline. 7 is neutral, never acidic.
✕Calling a high pH a 'strong acid'
Edexcel Chemistry examiner reports
Why it happens
Students associate any extreme pH with 'strong acid' without checking the direction.
How to avoid it
A high pH (e.g. 13, 14) is strongly ALKALINE, not acidic. Only low pH values are acidic.
✕Saying only pure water can have a pH of 7
Edexcel Chemistry examiner reports
Why it happens
Water is the standard example of a neutral substance, so students assume it is the only one.
How to avoid it
Neutral means equal H⁺ and OH⁻ ions — other solutions (e.g. sodium chloride solution) are neutral too.
✕Mixing up which ion belongs to acids and which to alkalis
Edexcel Chemistry examiner reports
Why it happens
H⁺ and OH⁻ look similar and both involve hydrogen, so they get swapped.
How to avoid it
Memorise: Acid → H⁺ (hydrogen ion); Alkali → OH⁻ (hydroxide ion). 'A' for Acid and the H⁺ comes first.
✕Claiming universal indicator gives an exact/accurate pH
Edexcel Chemistry examiner reports
Why it happens
Students see a number on the colour chart and assume it is precise.
How to avoid it
Universal indicator only gives an estimate/approximate pH (colour judged by eye). For an accurate value use a pH probe/meter.
✕Saying universal indicator is 'colourless' when neutral
Edexcel Chemistry examiner reports
Why it happens
Students confuse universal indicator with phenolphthalein, which is colourless in acid.
How to avoid it
Universal indicator is green at pH 7 (neutral) — never colourless. Red/orange = acid, green = neutral, blue/purple = alkali.

Past paper style quiz

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

Description

Example solution

0–3

Strongly acidic

hydrochloric acid (HCl)

4–6

Weakly acidic

vinegar, lemon juice, rainwater

Neutral

pure water, sodium chloride solution

8–11

Weakly alkaline

soap solution, baking soda solution

12–14

Strongly alkaline

sodium hydroxide (NaOH)

Solution

Main ion released

pH region

Acid

H⁺ (hydrogen ion)

below 7

Neutral

equal H⁺ and OH⁻

exactly 7

Alkali

OH⁻ (hydroxide ion)

above 7

How universal indicator works.

Universal indicator is a mixture of several indicators blended together.
Add a few drops to the solution (or use pH paper, which is paper soaked in universal indicator).
It turns a colour that depends on the pH, then you match the colour to a printed pH colour chart to read off the value.

The colour sequence across the scale is:

Red / orange at low pH → strongly/weakly acidic (pH 0–6).
Green at pH 7 → neutral.
Blue / purple at high pH → weakly/strongly alkaline (pH 8–14).

Universal indicator changes through the rainbow — red (acid) to green (neutral) to purple (alkali).

Method

What you do

What you get

Indicator solution (e.g. litmus)

Add drops, watch colour

Tells you acid OR alkali (less detail)

Universal indicator / pH paper

Add drops or dip paper, match colour to chart

Approximate pH value (a colour band)

pH probe / pH meter

Dip the electrode into the solution, read the digital display

Accurate numerical pH (e.g. 3.7)

Using the pH scale

Detailed Notes

What you’ll learn

How it’s examined

1Classify solutions from their pH (Getting started)

2Predict the universal indicator colour (Getting started)

3Link pH to H⁺ and OH⁻ ions (Building confidence)

4Order solutions by acidity (Building confidence)

5Choose a method to measure pH (Stretch)

6Explain what neutral really means (Stretch)

pH

pH scale

Acidic

Neutral

Alkaline

Acid

Alkali

Indicator

Universal indicator

pH probe / pH meter

✕Thinking pH 7 is slightly acidic or 'an acid'

✕Calling a high pH a 'strong acid'

✕Saying only pure water can have a pH of 7

✕Mixing up which ion belongs to acids and which to alkalis

✕Claiming universal indicator gives an exact/accurate pH

✕Saying universal indicator is 'colourless' when neutral

Past paper style quiz

Using the pH scale

Detailed Notes

What you’ll learn

How it’s examined

1Classify solutions from their pH (Getting started)

2Predict the universal indicator colour (Getting started)

3Link pH to H⁺ and OH⁻ ions (Building confidence)

4Order solutions by acidity (Building confidence)

5Choose a method to measure pH (Stretch)

6Explain what neutral really means (Stretch)

pH

pH scale

Acidic

Neutral

Alkaline

Acid

Alkali

Indicator

Universal indicator

pH probe / pH meter

✕Thinking pH 7 is slightly acidic or 'an acid'

✕Calling a high pH a 'strong acid'

✕Saying only pure water can have a pH of 7

✕Mixing up which ion belongs to acids and which to alkalis

✕Claiming universal indicator gives an exact/accurate pH

✕Saying universal indicator is 'colourless' when neutral

Past paper style quiz