ScienceAges 13-15

Acids, bases & the pH scale

Use the 0–14 pH scale, indicators and neutralisation reactions to classify acidic, neutral and alkaline solutions. Review suitability for your learners before teaching.

New variation Try interactive challenge Browse templates

Age band:Ages 13-15 Ages 16-18

Choose “Save as PDF” to keep a copy. Use your browser’s paper setting for A4 or Letter.

Worksheet StudioPractice pack · Ages 13-15

Acids, bases & the pH scale

Take your time. Show your thinking.

1. What colour does Universal Indicator turn in a neutral solution of pH 7: green or red?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
2. Which gas is produced when a metal carbonate (such as calcium carbonate) reacts with an acid: carbon dioxide or nitrogen?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
3. Why do indigestion tablets (antacids) contain mild bases such as magnesium hydroxide or calcium carbonate: to neutralise excess stomach acid, or to add more acid?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
4. Because the pH scale is logarithmic, a decrease of 1 pH unit (e.g. from pH 4 to pH 3) means the H⁺ ion concentration increases by a factor of: 10 or 2?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
5. On the standard pH scale (0 to 14) at 25 °C, what is the pH of a neutral solution such as pure water: 7 or 0?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
6. Does an acidic solution (such as lemon juice or hydrochloric acid) have a pH less than 7 or greater than 7?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
7. Which solution is more strongly acidic: Solution A with pH 1, or Solution B with pH 5?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
8. What colour is phenolphthalein indicator in an acidic solution vs a basic solution: colourless in acid and pink in alkali, or blue in both?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
9. Why do farmers sometimes spread powdered calcium carbonate (limestone) or calcium oxide on fields: to neutralise overly acidic soil, or to make the soil more acidic?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
10. Which type of salt is always produced by reactions of sulfuric acid (H₂SO₄): a sulfate or a nitrate?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
11. What colour does red litmus paper turn when dipped into an alkaline solution: blue or yellow?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
12. Which salt is formed when hydrochloric acid (HCl) is neutralised by sodium hydroxide (NaOH): sodium chloride or sodium sulfate?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution

12 questions · Seed 60 · Age guidance, not curriculum certification

View the answer key

Worksheet StudioAnswer key · Ages 13-15

Acids, bases & the pH scale

Answers and reasoning. Review before teaching.

1. What colour does Universal Indicator turn in a neutral solution of pH 7: green or red?
Green
Universal Indicator is green at neutral pH 7, red in strong acid, and purple in strong alkali.
2. Which gas is produced when a metal carbonate (such as calcium carbonate) reacts with an acid: carbon dioxide or nitrogen?
Carbon dioxide
Acid + Carbonate → Salt + Water + Carbon dioxide (which turns limewater milky).
3. Why do indigestion tablets (antacids) contain mild bases such as magnesium hydroxide or calcium carbonate: to neutralise excess stomach acid, or to add more acid?
To neutralise excess stomach acid
Antacids are weak bases that neutralise excess hydrochloric acid in the stomach.
4. Because the pH scale is logarithmic, a decrease of 1 pH unit (e.g. from pH 4 to pH 3) means the H⁺ ion concentration increases by a factor of: 10 or 2?
10
pH = −log₁₀[H⁺], so each whole unit decrease represents a tenfold increase in [H⁺].
5. On the standard pH scale (0 to 14) at 25 °C, what is the pH of a neutral solution such as pure water: 7 or 0?
7
A pH of 7 is neutral, with equal concentrations of H⁺ and OH⁻ ions.
6. Does an acidic solution (such as lemon juice or hydrochloric acid) have a pH less than 7 or greater than 7?
Less than 7
Acids have pH values below 7; the lower the pH, the stronger or more concentrated the acid.
7. Which solution is more strongly acidic: Solution A with pH 1, or Solution B with pH 5?
Solution A with pH 1
Lower numbers on the pH scale indicate higher hydrogen-ion concentration and stronger acidity.
8. What colour is phenolphthalein indicator in an acidic solution vs a basic solution: colourless in acid and pink in alkali, or blue in both?
Colourless in acid and pink in alkali
Phenolphthalein is colourless below pH 8.2 and turns pink-magenta in alkaline solutions.
9. Why do farmers sometimes spread powdered calcium carbonate (limestone) or calcium oxide on fields: to neutralise overly acidic soil, or to make the soil more acidic?
To neutralise overly acidic soil
Basic calcium compounds neutralise excess soil acidity so crops can grow well.
10. Which type of salt is always produced by reactions of sulfuric acid (H₂SO₄): a sulfate or a nitrate?
Sulfate
Sulfuric acid forms sulfate salts; nitric acid forms nitrates; hydrochloric acid forms chlorides.
11. What colour does red litmus paper turn when dipped into an alkaline solution: blue or yellow?
Blue
Alkalis turn red litmus paper blue.
12. Which salt is formed when hydrochloric acid (HCl) is neutralised by sodium hydroxide (NaOH): sodium chloride or sodium sulfate?
Sodium chloride
HCl + NaOH → NaCl (sodium chloride) + H₂O.

12 questions · Seed 60 · Age guidance, not curriculum certification

Worksheet StudioPractice pack · Ages 13-15

Acids, bases & the pH scale

Take your time. Show your thinking.

1. What colour does Universal Indicator turn in a neutral solution of pH 7: green or red?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
2. Which gas is produced when a metal carbonate (such as calcium carbonate) reacts with an acid: carbon dioxide or nitrogen?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
3. Why do indigestion tablets (antacids) contain mild bases such as magnesium hydroxide or calcium carbonate: to neutralise excess stomach acid, or to add more acid?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
4. Because the pH scale is logarithmic, a decrease of 1 pH unit (e.g. from pH 4 to pH 3) means the H⁺ ion concentration increases by a factor of: 10 or 2?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
5. On the standard pH scale (0 to 14) at 25 °C, what is the pH of a neutral solution such as pure water: 7 or 0?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
6. Does an acidic solution (such as lemon juice or hydrochloric acid) have a pH less than 7 or greater than 7?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
7. Which solution is more strongly acidic: Solution A with pH 1, or Solution B with pH 5?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
8. What colour is phenolphthalein indicator in an acidic solution vs a basic solution: colourless in acid and pink in alkali, or blue in both?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
9. Why do farmers sometimes spread powdered calcium carbonate (limestone) or calcium oxide on fields: to neutralise overly acidic soil, or to make the soil more acidic?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
10. Which type of salt is always produced by reactions of sulfuric acid (H₂SO₄): a sulfate or a nitrate?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
11. What colour does red litmus paper turn when dipped into an alkaline solution: blue or yellow?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
12. Which salt is formed when hydrochloric acid (HCl) is neutralised by sodium hydroxide (NaOH): sodium chloride or sodium sulfate?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution

12 questions · Seed 60 · Age guidance, not curriculum certification

Worksheet StudioAnswer key · Ages 13-15

Acids, bases & the pH scale

Answers and reasoning. Review before teaching.

1. What colour does Universal Indicator turn in a neutral solution of pH 7: green or red?
Green
Universal Indicator is green at neutral pH 7, red in strong acid, and purple in strong alkali.
2. Which gas is produced when a metal carbonate (such as calcium carbonate) reacts with an acid: carbon dioxide or nitrogen?
Carbon dioxide
Acid + Carbonate → Salt + Water + Carbon dioxide (which turns limewater milky).
3. Why do indigestion tablets (antacids) contain mild bases such as magnesium hydroxide or calcium carbonate: to neutralise excess stomach acid, or to add more acid?
To neutralise excess stomach acid
Antacids are weak bases that neutralise excess hydrochloric acid in the stomach.
4. Because the pH scale is logarithmic, a decrease of 1 pH unit (e.g. from pH 4 to pH 3) means the H⁺ ion concentration increases by a factor of: 10 or 2?
10
pH = −log₁₀[H⁺], so each whole unit decrease represents a tenfold increase in [H⁺].
5. On the standard pH scale (0 to 14) at 25 °C, what is the pH of a neutral solution such as pure water: 7 or 0?
7
A pH of 7 is neutral, with equal concentrations of H⁺ and OH⁻ ions.
6. Does an acidic solution (such as lemon juice or hydrochloric acid) have a pH less than 7 or greater than 7?
Less than 7
Acids have pH values below 7; the lower the pH, the stronger or more concentrated the acid.
7. Which solution is more strongly acidic: Solution A with pH 1, or Solution B with pH 5?
Solution A with pH 1
Lower numbers on the pH scale indicate higher hydrogen-ion concentration and stronger acidity.
8. What colour is phenolphthalein indicator in an acidic solution vs a basic solution: colourless in acid and pink in alkali, or blue in both?
Colourless in acid and pink in alkali
Phenolphthalein is colourless below pH 8.2 and turns pink-magenta in alkaline solutions.
9. Why do farmers sometimes spread powdered calcium carbonate (limestone) or calcium oxide on fields: to neutralise overly acidic soil, or to make the soil more acidic?
To neutralise overly acidic soil
Basic calcium compounds neutralise excess soil acidity so crops can grow well.
10. Which type of salt is always produced by reactions of sulfuric acid (H₂SO₄): a sulfate or a nitrate?
Sulfate
Sulfuric acid forms sulfate salts; nitric acid forms nitrates; hydrochloric acid forms chlorides.
11. What colour does red litmus paper turn when dipped into an alkaline solution: blue or yellow?
Blue
Alkalis turn red litmus paper blue.
12. Which salt is formed when hydrochloric acid (HCl) is neutralised by sodium hydroxide (NaOH): sodium chloride or sodium sulfate?
Sodium chloride
HCl + NaOH → NaCl (sodium chloride) + H₂O.

12 questions · Seed 60 · Age guidance, not curriculum certification