Acids, Bases and Indicators

26 Grade 7 Integrated Science questions on Acids, Bases and Indicators, with worked marking schemes. 26 questions are available on this topic in full.

1. Question 1

   1 mark

   When an acidic solution is added drop by drop to a base until it becomes neutral, the process is called:

   A. Neutralisation

   B. Evaporation

   C. Filtration

   D. Condensation

   (1 mark)

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2. Question 2

   1 mark

   Litmus paper turns red in

   A. a basic solution

   B. a neutral solution

   C. an acidic solution

   D. pure water

   (1 mark)

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3. Question 3

   1 mark

   Which of the following substances is an acid?

   A. Soap

   B. Toothpaste

   C. Baking soda

   D. Lemon juice

   (1 mark)

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4. Question 4

   1 mark

   A farmer's soil is too acidic for crops to grow well. Which substance is commonly added to reduce the acidity of the soil?

   A. Common salt

   B. Lemon juice

   C. Vinegar

   D. Agricultural lime

   (1 mark)

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5. Question 5

   1 mark

   Which of the following household substances is a base?

   A. Bar soap solution

   B. Vinegar

   C. Lemon juice

   D. Tomato juice

   (1 mark)

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6. Question 6

   1 mark

   Which of the following is a base?

   A. Hydrochloric acid

   B. Vinegar

   C. Lemon juice

   D. Sodium hydroxide

   (1 mark)

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7. Question 7

   4 marks

   (a) State what is meant by an indicator. (1 mark)

   (b) A student dips blue and red litmus paper into three solutions. State whether each solution is an acid or a base. (3 marks) Solution X: turns blue litmus red Solution Y: turns red litmus blue Solution Z: lemon juice

   (4 marks)

   Marking scheme

   (a) An indicator is a substance that changes colour to show whether a solution is an acid or a base. (1 mark) (b) Solution X: acid (acids turn blue litmus red); Solution Y: base/alkali (bases turn red litmus blue); Solution Z (lemon juice): acid. (3 marks)

8. Question 8

   5 marks

   A learner is given three unlabelled solutions: dilute hydrochloric acid, sodium hydroxide (a base) and distilled water. Describe how the learner can use litmus paper to identify each solution.

   (5 marks)

   Marking scheme

   Test each solution with both red and blue litmus paper. Acid (HCl): turns blue litmus red, no effect on red (2 marks). Base (NaOH): turns red litmus blue, no effect on blue (2 marks). Distilled water (neutral): no colour change to either litmus paper (1 mark).

9. Question 9

   4 marks

   Indicators are used to test whether a solution is acidic or basic.

   (a) State the colour change of blue litmus paper when dipped in:

   (i) An acid (1 mark)

   (ii) A base (1 mark)

   (b) Give one example of a common acid and one example of a common base found at home. (2 marks)

   (4 marks)

   Marking scheme

   (a) (i) Blue litmus paper turns red in an acid. (1 mark) (ii) Blue litmus paper remains blue in a base (no change). (1 mark) (b) Acid (any one): lemon juice / vinegar / juice of oranges (citrus fruits). Base (any one): soap / baking soda (bicarbonate of soda) / wood ash / toothpaste / cleaning detergents. (1 mark each)

10. Question 10

    4 marks

    Neutralisation reactions are useful in everyday life. Give two real-life examples where neutralisation is applied and briefly explain what happens in each.

    (4 marks)

    Marking scheme

    Any two examples (2 marks each): treating indigestion/acidity in the stomach where antacids (bases) neutralise excess stomach acid; adding lime to acidic soil where the base neutralises the acid so crops grow well; using toothpaste (basic) to neutralise acids from food that damage teeth. Each: correct example (1) + correct explanation of an acid and base neutralising each other (1).

11. Question 11

    3 marks

    A farmer realised that crops were not growing well on his farm. Soil testing showed the soil was acidic. Explain how the farmer can improve the soil using the knowledge of acids and bases.

    (3 marks)

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12. Question 12

    1 mark

    Which of the following household substances is basic?

    A. Vinegar

    B. Lemon juice

    C. Baking soda

    D. Orange juice

    (1 mark)

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13. Question 13

    4 marks

    Study these results: Solution P turns blue litmus red; Solution Q turns red litmus blue; Solution R does not change either litmus paper.

    (a) Classify each solution as acidic, basic or neutral.

    (b) Give one everyday example of a substance that behaves like Solution R.

    (4 marks)

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14. Question 14

    1 mark

    Which taste is characteristic of acids such as those found in fruits like oranges and lemons?

    A. Sour

    B. Bitter

    C. Salty

    D. Soapy

    (1 mark)

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15. Question 15

    1 mark

    A substance with a pH of 7 is

    A. neutral

    B. basic

    C. acidic

    D. strongly acidic

    (1 mark)

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16. Question 16

    4 marks

    (a) State the colour change observed when blue litmus paper is dipped in:

    (i) an acidic solution.

    (ii) a basic (alkaline) solution. (2 marks)

    (b) Give one example of an acid and one example of a base found in the home. (2 marks)

    (4 marks)

    Marking scheme

    (a) (i) Blue litmus paper turns red in an acidic solution. (1 mark) (ii) Blue litmus paper remains blue in a basic (alkaline) solution. (1 mark) (b) Acid example (any one): lemon juice / vinegar / orange juice / tomato juice. Base example (any one): baking soda (sodium bicarbonate) / soap / toothpaste / wood ash solution. (2 marks)

17. Question 17

    1 mark

    A farmer adds lime (calcium hydroxide) to acidic soil to

    A. increase the acidity

    B. neutralise the acidity

    C. remove water from the soil

    D. add nitrogen to the soil

    (1 mark)

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18. Question 18

    4 marks

    (a) State the colour change observed when blue litmus paper is dipped in an acid. (1 mark)

    (b) Give one example of a common acid and one example of a common base found at home. (2 marks)

    (c) What is an indicator? (1 mark)

    (4 marks)

    Marking scheme

    (a) Blue litmus paper turns red in an acid. (1 mark) (b) Acid example: lemon juice / vinegar / orange juice (any acidic food). Base example: baking soda / soap / wood ash (any one each). (2 marks) (c) An indicator is a substance that changes colour to show whether a solution is an acid or a base. (1 mark)

19. Question 19

    1 mark

    A learner dips a piece of blue litmus paper into a solution of lemon juice. What colour change is observed?

    A. It turns colourless

    B. It turns green

    C. It remains blue

    D. It turns red

    (1 mark)

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20. Question 20

    1 mark

    An indicator is a substance that

    A. changes colour in acids and bases

    B. produces heat when mixed

    C. is always colourless

    D. dissolves in water only

    (1 mark)

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21. Question 21

    2 marks

    Explain what happens when blue litmus paper is dipped into: (a) an acidic solution, (b) a basic solution.

    (2 marks)

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22. Question 22

    1 mark

    When an acid reacts with a base, the process is called

    A. evaporation

    B. filtration

    C. neutralisation

    D. combustion

    (1 mark)

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23. Question 23

    4 marks

    A learner prepared a natural indicator from purple cabbage.

    (a) Describe briefly how the indicator can be extracted from the cabbage.

    (b) State the colour the indicator would show in (i) an acidic solution and (ii) a basic solution.

    (4 marks)

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24. Question 24

    1 mark

    Universal indicator is added to a solution and it turns green. This shows that the solution is:

    A. Flammable

    B. Strongly basic

    C. Strongly acidic

    D. Neutral

    (1 mark)

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25. Question 25

    1 mark

    A learner tests an unknown solution with red litmus paper and it turns blue. The solution is most likely:

    A. Pure water

    B. A neutral salt

    C. An acid

    D. A base

    (1 mark)

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26. Question 26

    1 mark

    Which locally available plant material can be used to prepare a natural acid-base indicator in the laboratory?

    A. Dry maize grains

    B. Purple/red flowers such as hibiscus

    C. Charcoal powder

    D. Common cooking salt

    (1 mark)

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