Key Concepts: The pH scale runs 0–14. Acids produce H⁺ ions; alkalis produce OH⁻ ions. Indicators change colour with pH. Universal indicator gives a range of colours across the full pH scale. Neutralisation: H⁺ + OH⁻ → H₂O.
Section A — Indicators and Colours
1. Complete the table showing indicator colours in acidic and alkaline solutions. [6]
| Indicator |
In acid |
In alkali |
| Litmus |
|
|
| Phenolphthalein |
|
|
| Methyl orange |
|
|
2. State one advantage that universal indicator has over litmus. [1]
Section B — The pH Scale
3. Complete the table. [4]
| pH value |
Universal indicator colour |
Acidic / Neutral / Alkaline |
| 1 | | |
| 5 | | |
| 7 | | |
| 10 | | |
| 13 | | |
4. Which is more acidic, pH 2 or pH 5? Explain. [2]
5. Give one example each of: a strong acid, a weak acid, a strong alkali and a weak alkali. [4]
6. Match each substance to a likely pH value from the list: stomach acid, distilled water, bleach, vinegar, sodium hydroxide solution. pH values: 1, 4, 7, 10, 14 [5]
Section C — Acids, Alkalis and Ions
7. Define an acid and an alkali in terms of ions. [2]
8. Write the ionic equation for neutralisation. [2]
Section D — Neutralisation Reactions
9. Complete the word equations. [4]
a) hydrochloric acid + sodium hydroxide → __________ + __________
b) sulfuric acid + potassium hydroxide → __________ + __________
c) nitric acid + calcium hydroxide → __________ + __________
d) hydrochloric acid + ammonia solution → __________
10. Write balanced symbol equations for reactions (a) and (b) in Question 9. [3]
11. Describe what happens to the pH of a solution as sodium hydroxide is slowly added to excess hydrochloric acid. [2]
Total marks: 35
Mark Scheme
1. Litmus: acid red, alkali blue; Phenolphthalein: acid colourless, alkali pink; Methyl orange: acid red, alkali yellow [6]
2. Universal indicator gives an approximate pH value rather than just acid/neutral/alkali [1]
3. pH 1: red, acidic; pH 5: orange, acidic; pH 7: green, neutral; pH 10: blue, alkaline; pH 13: purple/violet, alkaline [4 — accept reasonable colours]
4. pH 2 is more acidic; lower pH means higher concentration of H⁺ ions / more acidic [2]
5. Strong acid: HCl or H₂SO₄; weak acid: ethanoic acid/carbonic acid; strong alkali: NaOH/KOH; weak alkali: ammonia solution [4]
6. Stomach acid pH 1; vinegar pH 4; distilled water pH 7; bleach pH 10; sodium hydroxide solution pH 14 [5]
7. Acid: a substance that produces H⁺ ions in aqueous solution; alkali: a substance that produces OH⁻ ions in aqueous solution [2]
8. H⁺ + OH⁻ → H₂O [2]
9. a) sodium chloride + water; b) potassium sulfate + water; c) calcium nitrate + water; d) ammonium chloride [4]
10. a) HCl + NaOH → NaCl + H₂O; b) H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O [3]
11. pH starts low (very acidic); increases as NaOH neutralises the acid; reaches 7 at the equivalence point; continues to rise above 7 if NaOH is in excess [2]