pH Scale & Indicators
pH scale & indicators: acidic/neutral/alkaline ranges, key colour changes (litmus/universal indicator), and how to answer exam-style questions.
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The core idea
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Learning objectives
- describe the meanings of the terms acid and alkali in terms of the ions they produce in aqueous solution and their effects on Universal Indicator
- describe neutrality and relative acidity and alkalinity, in terms of — relative H+ and OH– ion concentrations,
- describe neutrality and relative acidity and alkalinity, in terms of — colour in Universal Indicator, and
- describe neutrality and relative acidity and alkalinity, in terms of — the pH scale (calculation of pH from hydrogen ion concentration is not required)
- describe the importance of controlling the pH in soils and how excess acidity can be treated using calcium hydroxide
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 only tells “acid vs alkali”. Universal indicator estimates an actual pH value (by colour).
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 soln | 11 |
| Bleach | 13 |
B. What “acidic/neutral/alkaline” means (ions)
In aqueous solution:
| 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.
C. pH depends on concentration (not just “strong/weak”)
pH is linked to how much H + (aq) is present in solution, so it depends strongly on concentration.
Strength is degree of ionisation; concentration is amount per volume: Introduction to Acids.
D. Indicator evidence
| Indicator | Acidic result | Neutral result | Alkaline result |
|---|---|---|---|
| litmus | 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).”
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 |
Use the written colour names when reporting observations; do not rely on a coloured diagram alone.
F. Soil pH (agriculture)
Most plants grow best when soil pH is around 6 to 7 (slightly acidic to neutral).
- Soil can become too acidic due to acid rain or overuse of some fertilisers.
- Farmers treat excess acidity using slaked lime, Ca(OH)₂ (calcium hydroxide).
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).
- If the question wants an actual pH, use universal indicator or a pH meter, not litmus.
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 the common plant-growth range near pH 6–7.Working
pH 4.5 < 7.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: Two different solutions both have pH 7. What can you say about [H + (aq)] in the two solutions?
Show Answer
They have the same [H + (aq)] because pH is defined by hydrogen ion concentration.
8. Quiz
Test yourself on pH classification, ion comparisons, and indicator colour changes.
Go to Quiz Page