ScienceAges 16-18

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.

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Worksheet StudioPractice pack · Ages 16-18

Acids, bases & the pH scale

Take your time. Show your thinking.

1. 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
2. What weak organic acid gives vinegar its sour taste: ethanoic (acetic) acid or nitric acid?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
3. What is the difference between a strong acid (like HCl) and a weak acid (like ethanoic acid) in water: a strong acid fully ionises (dissociates), whereas a weak acid only partially ionises, or a strong acid has pH 12?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
4. 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
5. Which gas is produced when a reactive metal (such as magnesium or zinc) reacts with dilute hydrochloric acid: hydrogen or oxygen?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
6. 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
7. 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
8. Does an alkaline (basic) solution (such as sodium hydroxide or soapy water) have a pH greater than 7 or less than 7?
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. What are the two products formed when an acid reacts with an alkali in a neutralisation reaction: a salt and water, or hydrogen and carbon?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
11. What colour does blue litmus paper turn when dipped into an acidic solution: red or blue?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
12. 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

12 questions · Seed 104742 · Age guidance, not curriculum certification

View the answer key

Worksheet StudioAnswer key · Ages 16-18

Acids, bases & the pH scale

Answers and reasoning. Review before teaching.

1. 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.
2. What weak organic acid gives vinegar its sour taste: ethanoic (acetic) acid or nitric acid?
Ethanoic (acetic) acid
Vinegar is a dilute aqueous solution of ethanoic (acetic) acid.
3. What is the difference between a strong acid (like HCl) and a weak acid (like ethanoic acid) in water: a strong acid fully ionises (dissociates), whereas a weak acid only partially ionises, or a strong acid has pH 12?
A strong acid fully ionises (dissociates), whereas a weak acid only partially ionises
Acid strength refers to the degree of ionisation in water.
4. What colour does red litmus paper turn when dipped into an alkaline solution: blue or yellow?
Blue
Alkalis turn red litmus paper blue.
5. Which gas is produced when a reactive metal (such as magnesium or zinc) reacts with dilute hydrochloric acid: hydrogen or oxygen?
Hydrogen
Metal + Acid → Salt + Hydrogen gas (which gives a squeaky pop with a lighted splint).
6. 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.
7. 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.
8. Does an alkaline (basic) solution (such as sodium hydroxide or soapy water) have a pH greater than 7 or less than 7?
Greater than 7
Alkalis (soluble bases) have pH values above 7 (from 7 to 14).
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. What are the two products formed when an acid reacts with an alkali in a neutralisation reaction: a salt and water, or hydrogen and carbon?
A salt and water
Acid + Alkali → Salt + Water (H⁺ + OH⁻ → H₂O).
11. What colour does blue litmus paper turn when dipped into an acidic solution: red or blue?
Red
Acids turn blue litmus paper red.
12. 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⁺].

12 questions · Seed 104742 · Age guidance, not curriculum certification

Worksheet StudioPractice pack · Ages 16-18

Acids, bases & the pH scale

Take your time. Show your thinking.

1. 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
2. What weak organic acid gives vinegar its sour taste: ethanoic (acetic) acid or nitric acid?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
3. What is the difference between a strong acid (like HCl) and a weak acid (like ethanoic acid) in water: a strong acid fully ionises (dissociates), whereas a weak acid only partially ionises, or a strong acid has pH 12?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
4. 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
5. Which gas is produced when a reactive metal (such as magnesium or zinc) reacts with dilute hydrochloric acid: hydrogen or oxygen?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
6. 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
7. 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
8. Does an alkaline (basic) solution (such as sodium hydroxide or soapy water) have a pH greater than 7 or less than 7?
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. What are the two products formed when an acid reacts with an alkali in a neutralisation reaction: a salt and water, or hydrogen and carbon?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
11. What colour does blue litmus paper turn when dipped into an acidic solution: red or blue?
pH Scale: 0 (Acid) → 7 → 14 (Alkali) · Indicator Solution
12. 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

12 questions · Seed 104742 · Age guidance, not curriculum certification

Worksheet StudioAnswer key · Ages 16-18

Acids, bases & the pH scale

Answers and reasoning. Review before teaching.

1. 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.
2. What weak organic acid gives vinegar its sour taste: ethanoic (acetic) acid or nitric acid?
Ethanoic (acetic) acid
Vinegar is a dilute aqueous solution of ethanoic (acetic) acid.
3. What is the difference between a strong acid (like HCl) and a weak acid (like ethanoic acid) in water: a strong acid fully ionises (dissociates), whereas a weak acid only partially ionises, or a strong acid has pH 12?
A strong acid fully ionises (dissociates), whereas a weak acid only partially ionises
Acid strength refers to the degree of ionisation in water.
4. What colour does red litmus paper turn when dipped into an alkaline solution: blue or yellow?
Blue
Alkalis turn red litmus paper blue.
5. Which gas is produced when a reactive metal (such as magnesium or zinc) reacts with dilute hydrochloric acid: hydrogen or oxygen?
Hydrogen
Metal + Acid → Salt + Hydrogen gas (which gives a squeaky pop with a lighted splint).
6. 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.
7. 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.
8. Does an alkaline (basic) solution (such as sodium hydroxide or soapy water) have a pH greater than 7 or less than 7?
Greater than 7
Alkalis (soluble bases) have pH values above 7 (from 7 to 14).
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. What are the two products formed when an acid reacts with an alkali in a neutralisation reaction: a salt and water, or hydrogen and carbon?
A salt and water
Acid + Alkali → Salt + Water (H⁺ + OH⁻ → H₂O).
11. What colour does blue litmus paper turn when dipped into an acidic solution: red or blue?
Red
Acids turn blue litmus paper red.
12. 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⁺].

12 questions · Seed 104742 · Age guidance, not curriculum certification