Acids and Bases: The pH Scale, Indicators and Neutralisation — Answers
Answer sheet with model answers and success criteria for the Year 9 mixtures & chemical reactions worksheet.
Success TutoringWorksheetsAnswer sheetYear 9 · Science · Mixtures & Chemical ReactionsAcids and Bases: The pH Scale, Indicators and Neutralisation — Answers
Part A: The pH scale
Acids taste sour, can be corrosive, turn blue litmus red and react with many metals and with carbonates. Lemon juice, vinegar and stomach acid contain acids. Bases can be corrosive, feel slippery or soapy and turn red litmus blue. A base that dissolves in water is called an alkali. Soap, baking soda and drain cleaner are basic.
The pH scale measures how acidic or basic a solution is, usually from 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is basic. Each step of one pH unit is a tenfold change: pH 3 is 10 times more acidic than pH 4 and 100 times more acidic than pH 5.
Never taste or touch chemicals in the laboratory. Strong acids and bases cause burns; always wear safety glasses.
Answer: acidic
Answer: neutral
Answer: basic
- ApH 1
- BpH 4
- CpH 7
- DpH 13
Answer: pH 1
The lower the pH, the more acidic the solution. pH 13 is strongly basic.
- A10 times
- B1 time (the same)
- C2 times
- D100 times
Answer: 10 times
Each step of one pH unit is a tenfold (×10) change in acidity.
Answer: 1000 times (5 − 2 = 3 steps, and 10 × 10 × 10 = 1000).
Answer: Stomach acid, lemon juice, pure water, baking soda solution, soapy water, drain cleaner.
Part B: Indicators
An indicator is a substance that changes colour depending on whether a solution is acidic or basic. Universal indicator colours run: red (strong acid), orange and yellow (weak acid), green (neutral), blue (weak base), purple (strong base).
- Blue litmus turns red
- Red litmus turns blue
- Universal indicator turns green
- Universal indicator turns red
- Universal indicator turns purple
- Phenolphthalein turns pink
- The solution is acidic
- The solution is neutral (about pH 7)
- The solution is strongly basic
- The solution is strongly acidic
- The solution is basic
- The solution is basic
Answer: Blue litmus turns red → The solution is acidic; Red litmus turns blue → The solution is basic; Universal indicator turns green → The solution is neutral (about pH 7); Universal indicator turns red → The solution is strongly acidic; Universal indicator turns purple → The solution is strongly basic; Phenolphthalein turns pink → The solution is basic
Answer: True
Phenolphthalein is colourless in acidic and neutral solutions and turns pink in basic solutions (above about pH 8).
Answer: Universal indicator shows a range of colours, so it gives an approximate pH value and shows how strongly acidic or basic the solution is.
Answer: Basic, and only weakly basic (a pH a little above 7). Strongly basic solutions turn it purple.
Part C: Neutralisation and salts
When an acid reacts with a base, they cancel each other out. This is neutralisation: acid + base → salt + water. A salt is a compound made of a metal (from the base) and the non-metal part of the acid.
The acid decides the second part of the salt name: hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
Acids also react with metal carbonates: acid + metal carbonate → salt + water + carbon dioxide, and with many metals: acid + metal → salt + hydrogen.
Answer: sodium chloride + water
Answer: potassium sulfate + water
Answer: sodium nitrate + water
Answer: calcium chloride + water + carbon dioxide
Answer: magnesium chloride + hydrogen
Answer: zinc sulfate + hydrogen
Answer: Acid: sulfuric acid (H₂SO₄). Base: sodium hydroxide (NaOH). Salt: sodium sulfate (Na₂SO₄).
Part D: Gas tests and everyday neutralisation
Gas tests are carried out by the teacher or under teacher supervision in the school laboratory.
- ACarbon dioxide
- BHydrogen
- COxygen
- DNitrogen
Answer: Carbon dioxide
Carbon dioxide reacts with limewater (calcium hydroxide solution) to form tiny particles of calcium carbonate, which make it look milky.
- AHydrogen
- BCarbon dioxide
- COxygen
- DChlorine
Answer: Hydrogen
Hydrogen burns rapidly with a squeaky pop. (A glowing splint relighting is the test for oxygen.)
Answer: They contain a base (such as magnesium hydroxide or calcium carbonate), which neutralises some of the excess hydrochloric acid in the stomach, raising the pH.
Answer: To neutralise acidic soil. Many crops grow poorly if the soil is too acidic, so the lime raises the pH to a better level for the plants.
Answer: The pH rises from a low value towards 7, and the colour changes from red through orange and yellow to green when the solution is neutral.
Part E: Explain it
Model answer: In neutralisation an acid reacts with a base, and they cancel out each other's acidic and basic properties, so the pH moves towards 7. The general word equation is acid + base → salt + water. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water. Everyday uses: antacids neutralise excess stomach acid; farmers add lime to neutralise acidic soil; toothpaste is slightly basic and helps neutralise acids produced by bacteria in the mouth.
Success criteria:
- Describes an acid reacting with a base, pH moving towards 7
- Gives acid + base → salt + water
- A correct named example
- Two correct everyday uses
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
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