Water Hardness

16 Grade 9 Integrated Science questions on Water Hardness, with worked marking schemes. 16 questions are available on this topic in full.

1. Question 1

   4 marks

   Distinguish between temporary hardness and permanent hardness of water. In your answer, name ONE compound responsible for each type.

   (4 marks)

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2. Question 2

   4 marks

   A student is given two beakers, X and Y, containing colourless water. Beaker X lathers readily with soap while beaker Y forms scum and lathers poorly.

   (a) Identify which beaker contains hard water and give a reason.

   (b) State ONE method the student could use to soften the water in the hard-water beaker and briefly explain how it works.

   (4 marks)

   Marking scheme

   (a) Beaker Y contains hard water (1); because it forms scum and lathers poorly with soap, showing it contains dissolved Ca2+/Mg2+ salts (1). (b) Any one valid method with explanation (2): Boiling — heat decomposes calcium hydrogencarbonate to insoluble calcium carbonate, removing the ions (temporary hardness); OR Adding washing soda (sodium carbonate) — carbonate ions react with Ca2+/Mg2+ to form insoluble carbonates that are removed.

3. Question 3

   4 marks

   Explain why boiling removes temporary hardness but does NOT remove permanent hardness from water.

   (4 marks)

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4. Question 4

   4 marks

   Distinguish between temporary hardness and permanent hardness of water. State one method of removing each.

   (4 marks)

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5. Question 5

   1 mark

   Temporary hardness can be removed by

   A. distillation only

   B. adding more soap

   C. filtration

   D. boiling the water

   (1 mark)

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6. Question 6

   5 marks

   (a) Distinguish between hard water and soft water. (2 marks)

   (b) State one disadvantage of using hard water in the home. (1 mark)

   (c) State two methods that can be used to remove hardness from water. (2 marks)

   (5 marks)

   Marking scheme

   (a) Hard water is water that does not easily form a lather (foam) with soap and instead forms scum, because it contains dissolved calcium and magnesium salts; soft water is water that readily forms a lather with soap and contains little or no dissolved calcium or magnesium salts. (b) Any one of: it wastes soap because a lot of soap is needed to form lather; it forms scum that stains clothes; it deposits scale (fur) in kettles, boilers and pipes which blocks them and wastes fuel/heat. (c) Any two of: boiling the water (removes temporary hardness only); adding washing soda (sodium carbonate); distillation; using ion-exchange resins/water softeners.

7. Question 7

   4 marks

   (a) State what is meant by 'hard water'. (2 marks)

   (b) Describe one method that can be used to remove temporary hardness from water. (2 marks)

   (4 marks)

   Marking scheme

   (a) Hard water is water that does not easily form a lather (foam) with soap; instead it forms a scum, because it contains dissolved calcium and/or magnesium salts (e.g. calcium hydrogencarbonate, magnesium hydrogencarbonate, calcium sulphate). (b) Boiling: when temporarily hard water is boiled, the dissolved calcium hydrogencarbonate decomposes to form insoluble calcium carbonate, water and carbon dioxide; the insoluble calcium carbonate settles out (as scale), removing the hardness. (Adding washing soda / sodium carbonate is also acceptable, as it precipitates the calcium and magnesium ions as insoluble carbonates.)

8. Question 8

   1 mark

   Permanent hardness in water is caused by dissolved

   A. sodium chloride

   B. calcium sulphate

   C. calcium hydrogen carbonate

   D. potassium hydroxide

   (1 mark)

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9. Question 9

   5 marks

   In a class activity, learners found that soap formed very little lather with water collected from a borehole, but plenty of lather with rain water. The borehole water could be softened by boiling.

   (a) State the type of hardness present in the borehole water and name one substance that causes it. (2 marks)

   (b) Describe one method, other than boiling, that the learners could use to soften the borehole water. (3 marks)

   (5 marks)

   Marking scheme

   (a) The borehole water shows temporary hardness (it can be removed by boiling). It is caused by dissolved calcium hydrogencarbonate (or magnesium hydrogencarbonate). (Type = temporary hardness, 1 mark; named cause = calcium/magnesium hydrogencarbonate, 1 mark) (2 marks) (b) Adding washing soda (sodium carbonate): the carbonate reacts with the dissolved calcium (or magnesium) ions to form an insoluble precipitate of calcium (or magnesium) carbonate, removing those ions from the water so that the soap is no longer wasted and can form lather. (Method named = 1 mark; explanation of forming insoluble precipitate that removes the ions = 2 marks) (3 marks)

10. Question 10

    1 mark

    Temporary hardness in water is caused by

    A. dissolved potassium nitrate

    B. dissolved calcium sulphate

    C. dissolved sodium chloride

    D. dissolved calcium hydrogen carbonate

    (1 mark)

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11. Question 11

    4 marks

    A learner suspects a water sample is hard.

    (a) Describe a simple experiment the learner could carry out to test whether the water is hard.

    (b) State the expected observation if the water is hard.

    (4 marks)

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12. Question 12

    3 marks

    Hard water causes several problems in homes and industries. State THREE disadvantages of using hard water.

    (3 marks)

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13. Question 13

    1 mark

    Hard water is water that

    A. does not lather easily with soap

    B. contains no dissolved minerals

    C. lathers easily with soap

    D. is dirty and polluted

    (1 mark)

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14. Question 14

    3 marks

    Some water collected from a borehole does not form lather easily with soap and instead produces a white scum.

    (a) State what is meant by hard water. (1 mark)

    (b) Describe how the hard water sample can be made to form lather easily by boiling. (2 marks)

    (3 marks)

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15. Question 15

    1 mark

    One advantage of hard water is that it

    A. is better for industrial boilers

    B. produces more soap lather

    C. cleans clothes better

    D. provides calcium for strong bones and teeth

    (1 mark)

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16. Question 16

    1 mark

    Permanent hardness of water can be removed by

    A. adding salt

    B. adding washing soda (sodium carbonate)

    C. filtering

    D. boiling

    (1 mark)

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