pH Scale & Indicators
pH scale & indicators: acidic/neutral/alkaline ranges, key colour changes (litmus/universal indicator), and how to answer exam-style questions.
On this page
Keep the chemical conclusion separate from the observation: first state the indicator colour change, then decide whether the solution is acidic, neutral or alkaline.
1. Definition
A. pH
pH is a number that shows how acidic or alkaline an aqueous solution is. It depends on the concentration of hydrogen ions, H + (aq).
B. Indicator
An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline (i.e. depending on pH).
2. Key Ideas
- At O-Level, the pH scale is treated as running from 0 to 14.
- In the usual school model near room temperature, pH < 7 is acidic, pH = 7 is neutral and pH > 7 is alkaline. The general definition of neutrality is [H⁺] = [OH⁻]; the neutral pH can change with temperature.
- Lower pH means higher H + (aq) concentration (more acidic). Higher pH means lower H + (aq) concentration (more alkaline).
- Litmus gives a broad acid/alkali indication. Universal Indicator estimates pH by comparison with its colour chart; a pH meter gives a numerical measurement.
3. Detailed Explanations
- pH < 7 acidic, pH = 7 neutral, pH > 7 alkaline.
- Lower pH = higher [H + (aq)] (more acidic).
- Litmus tells acid vs alkali; universal indicator estimates a pH value by colour.
A. The pH scale (what it shows)
The pH scale classifies aqueous solutions as acidic, neutral, or alkaline.
Data table
| Substance | pH |
|---|---|
| Stomach acid | 1 |
| Lemon juice | 2 |
| Vinegar | 3 |
| Pure water | 7 |
| Seawater | 8 |
| Ammonia(aq) | 11 |
| Bleach | 13 |
B. What “acidic/neutral/alkaline” means (ions)
In aqueous solution near room temperature:
| Solution type | pH | Ion comparison (what to say) |
|---|---|---|
| acidic | pH < 7 | [H + (aq)] is greater than [OH⁻(aq)] |
| neutral | pH = 7 | [H + (aq)] equals [OH⁻(aq)] |
| alkaline | pH > 7 | [H + (aq)] is less than [OH⁻(aq)] |
In real chemistry, pH can be below 0 or above 14 for very concentrated
solutions.
For O-Level exam answers, treat pH as 0–14 unless the question explicitly
says otherwise.
Both hydrogen and hydroxide ions are present in acidic, neutral and alkaline solutions. “Neutral” means their concentrations are equal, not that both ions are absent. Square brackets, such as [H⁺], mean concentration.
C. pH depends on concentration (not just “strong/weak”)
pH depends on hydrogen-ion concentration, not the total number of hydrogen ions in the whole container. Two different volumes of the same solution have the same pH.
Diluting an ordinary acidic solution with pure water lowers its hydrogen-ion concentration and raises its pH towards the neutral value. Diluting an alkaline solution moves its pH down towards neutral. Dilution does not turn an acid into an alkali.
A low pH does not by itself prove that an acid is strong: strength describes the extent of ionisation. Compare acid concentration as well as acid identity.
D. Indicator evidence
| Indicator | Acidic result | Neutral result | Alkaline result |
|---|---|---|---|
| red litmus paper | stays red | stays red | turns blue |
| blue litmus paper | turns red | stays blue | stays blue |
| litmus solution | red | purple | blue |
| universal indicator | red / orange / yellow | green | blue / violet / purple |
When asked about indicators, write reagent + observation + conclusion.
Example: “Universal indicator turns red → solution is acidic (low pH).”
Testing with only one colour of litmus can leave two possibilities. Blue paper staying blue could indicate a neutral or alkaline solution; red paper staying red could indicate a neutral or acidic solution. Use both papers or another indicator. Near neutral pH, litmus colours may be less distinct.
E. Universal indicator (pH indicator)
Universal indicator gives a range of colours across different pH values, so you can estimate pH.
| Approximate pH | Universal indicator colour |
|---|---|
| 0–2 | red |
| 3–4 | orange |
| 5–6 | yellow |
| 7 | green |
| 8–10 | blue |
| 11–14 | violet / purple |
These colour ranges are approximate: use the chart supplied with the indicator or question. Report the written colour name and an estimated pH; do not claim more precision than the colour comparison allows.
F. Soil pH (agriculture)
Soil pH affects how readily roots can obtain nutrients. A suitable pH range depends on the crop and soil: many crops prefer slightly acidic to near-neutral soil, but not every plant needs pH 7. Use a supplied target range when deciding whether treatment is needed.
- Soil can become too acidic due to acid rain or overuse of some fertilisers.
- Excess acidity can be treated with slaked lime, Ca(OH)₂ (calcium hydroxide), which neutralises acid and raises pH.
- Test before and after treatment; adding too much base can make the soil too alkaline.
Ca(OH)₂(s) + 2H + (aq) → Ca²⁺(aq) + 2H₂O(l)
The treatment supplies a base, not simply a calcium-containing substance: calcium chloride would not neutralise the acid in this way.
Neutralisation is the key idea: acid + base → salt (and often water).
If you need the ionic equation, revise:
Properties of Alkalis
.
4. Common Mistakes
- Writing “pH > 7 is acidic” (wrong direction).
- Saying litmus “becomes red” without stating the starting colour (write “blue litmus turns red” / “red litmus turns blue”).
- Treating litmus as a pH meter; it classifies a solution but does not estimate a pH value.
- Mixing up strength and concentration (a strong acid can be dilute).
5. Exam Tips
- pH < 7 acidic; pH = 7 neutral; pH > 7 alkaline.
- Universal indicator gives an estimated pH value by colour.
- If the question wants “more acidic/more alkaline”, compare the pH numbers directly (lower = more acidic, higher = more alkaline).
- Use Universal Indicator for an approximate pH from its chart and a pH meter for a numerical measurement; litmus does not give a pH number.
6. Worked Examples
Modelled example 1
Classify from pH
Problem
Study the worked solution
Compare with neutral pH
Method
Compare 3 with 7.Reason
At this level, pH below 7 is acidic, 7 neutral and above 7 alkaline.Working
3 < 7.Classify the solution
Working
The solution is acidic.
Guided practice 2
Compare acidity
Problem
Use pH direction
Hints
Hint 1: direction
View solution step by step
Compare the values
Method
Select the lower pH, 2.Reason
Lower pH corresponds to higher H + (aq) concentration.Working
2 < 5.State the comparison
Working
The pH 2 solution is more acidic.
Common misconception 3
Litmus direction
Evidence check
Use the full colour change
View solution step by step
Read the direction
Method
Use blue-to-red, not the final colour alone.Reason
Indicator evidence is defined by the starting and ending colours.Working
Blue litmus → red.Correct the conclusion
Working
The solution is acidic.
Examiner practice 4
Universal indicator evidence
Examination question
Give value and classification
View solution step by step
Estimate pH
1 markMethod
Match green to approximately pH 7.Reason
Universal indicator is green at the neutral point.Working
Approximate pH = 7.Classify the solution
1 markWorking
The solution is neutral.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark approximate pH and nature.
Challenge 5
Agriculture choice
Agricultural transfer
Connect pH, treatment and mechanism
Hints
Hint 1: classify the soil
Hint 2: choose a base
View solution step by step
Assess the soil evidence
Method
Classify pH 4.5 as too acidic.Reason
It is below this crop’s supplied target range of pH 6–7.Working
pH 4.5 < 6: below the lower end of the target range.Choose and explain treatment
Method
Add slaked lime, Ca(OH)₂.Reason
It neutralises excess acid and raises pH towards neutral.Working
Slaked lime treatment improves the acidic soil condition.
7. Mind Stretchers
Mind stretcher 1: Fix the student’s sentenceExtension
Question: A student writes: “Red litmus turns blue in an acid.” Correct the sentence.
Show answer
Blue litmus turns red in an acid (red litmus shows no change in an acid).
Mind stretcher 2: Same pH, same ion concentrationExtension
Question: Near room temperature, 20 cm³ of solution A and 200 cm³ of solution B both have pH 7. What can you say about their hydrogen-ion concentrations? Do they contain the same total amount of hydrogen ions?
Show answer
They have the same hydrogen-ion concentration in this school model. B has ten times the solution volume, so it contains ten times the amount of hydrogen ions. Equal pH means equal concentration, not equal total amount.
Try independently: A solution leaves blue litmus paper unchanged. A learner calls it neutral. What further test would distinguish the remaining possibilities, and what would each result mean?
Show answer and reasoning
Test with red litmus paper. Red paper turning blue supports an alkaline solution; red paper also remaining unchanged supports a neutral solution in the school test. Universal Indicator with its chart or a pH meter can give more information, particularly near neutral pH.
8. Quiz
Use the Acid–Base Chemistry topic check to practise and check your understanding.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
Last reviewed: