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Exam questions · Biology · Material Cycles

The Water Cycle and Potable Water

  • 8 exam questions
  • 24 marks
  • 8 quick checks
  1. 1 Name [2 marks]

    Name two processes by which water enters the air as water vapour.

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    Model answer

    Evaporation and transpiration.

    Mark scheme

    • Evaporation — 1 mark
    • Transpiration — 1 mark
  2. 2 Describe [3 marks]

    Describe how rain forms from the water in the sea.

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    Model answer

    The Sun's energy makes water at the surface of the sea evaporate into water vapour, which rises. In the cold air higher up, the water vapour cools and condenses into droplets, which form clouds. When the droplets become big and heavy enough, they fall as rain, which is precipitation.

    Mark scheme

    • Water evaporates from the sea (using the Sun's energy) — 1 mark
    • The vapour cools and condenses to form clouds — 1 mark
    • Droplets get heavy and fall as precipitation — 1 mark
  3. 3 Explain [3 marks]

    Explain why the water cycle is important to living things.

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    Model answer

    The water cycle supplies fresh water, because evaporation leaves the salt in the sea, so rain is fresh water for plants and animals to use. Organisms need water for the reactions in their cells and for photosynthesis. The cycle also moves materials, such as mineral ions, between the land and the sea.

    Mark scheme

    • It provides a supply of fresh water — 1 mark
    • Organisms need water, for example for reactions in cells or photosynthesis — 1 mark
    • It moves materials between the abiotic and biotic parts of ecosystems — 1 mark
  4. 4 Explain [2 marks]

    Tap water in the UK is described as potable but it is not pure. Explain the difference between potable water and pure water.

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    Model answer

    Potable water is safe to drink and can contain low levels of dissolved substances, such as minerals. Pure water contains only water molecules and nothing dissolved in it.

    Mark scheme

    • Potable water is safe to drink and can contain dissolved substances — 1 mark
    • Pure water contains only water molecules — 1 mark
  5. 5 Describe [3 marks]

    Describe how potable water is produced from water in a reservoir.

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    Model answer

    A suitable source of fresh water with low levels of dissolved substances is chosen. The water is passed through screens and filter beds of sand and gravel to remove solid particles. It is then sterilised, using chlorine, ozone or ultraviolet light, to kill any microorganisms.

    Mark scheme

    • Choose a source with low levels of dissolved substances — 1 mark
    • Filter it to remove solids — 1 mark
    • Sterilise it to kill microorganisms — 1 mark
  6. 6 Explain [3 marks]

    Explain how distillation can be used to make potable water from sea water.

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    Model answer

    The sea water is heated until it boils, so the water evaporates as steam and the salt is left behind. The steam is cooled in a condenser, where it condenses back into liquid water. This water has no salt in it, so it is safe to drink.

    Mark scheme

    • Sea water is boiled so the water evaporates (as steam) — 1 mark
    • The salt is left behind — 1 mark
    • The steam is condensed to liquid water — 1 mark
  7. 7 Calculate [2 marks]

    A desalination plant treats 250 000 litres of sea water and makes 100 000 litres of potable water. Calculate the percentage of the sea water that becomes potable water.

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    Model answer

    \(100\,000 \div 250\,000 \times 100 = 40\%\)

    Mark scheme

    • \(100\,000 \div 250\,000\) (\(\times 100\)) — 1 mark
    • 40% — 1 mark
  8. 8 Evaluate [6 marks]

    A country with very little rain is thinking of building a desalination plant on its coast to supply potable water. Evaluate this idea.

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    Model answer

    A desalination plant would give the country a reliable supply of potable water that does not depend on the rain, because the sea will not run out. The plant could use reverse osmosis or distillation to take the salt out of the sea water. However, both methods need a large amount of energy, which makes the water expensive, and if the energy comes from fossil fuels it will release carbon dioxide and add to global warming. The plant also leaves a very salty brine that could harm sea life if it is returned to the sea. Overall, desalination is probably a good idea for a dry country on the coast that can afford the energy, ideally from renewable sources, because there are few other sources of water, but it should be combined with saving and recycling water.

    Mark scheme

    • Reliable supply of potable water, independent of rainfall — 1 mark
    • The sea is an unlimited source — 1 mark
    • Needs a lot of energy, so it is expensive — 1 mark
    • Burning fossil fuels for the energy releases carbon dioxide — 1 mark
    • Waste brine may harm sea life — 1 mark
    • A reasoned conclusion that weighs the advantages and disadvantages — 1 mark

Quick check

  1. 1

    Which process turns liquid water from the sea into water vapour?

    1. ACondensation
    2. BEvaporation
    3. CPrecipitation
    4. DRun-off
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    B: Evaporation

    Evaporation, driven by energy from the Sun, changes liquid water to vapour.

  2. 2

    What is transpiration?

    1. AThe taking in of water by the roots
    2. BThe movement of sugar in the phloem
    3. CThe falling of rain onto leaves
    4. DThe loss of water vapour from leaves
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    D: The loss of water vapour from leaves

    Transpiration is the loss of water vapour from the leaves of a plant, mainly through the stomata.

  3. 3

    Which process forms clouds?

    1. ACondensation
    2. BEvaporation
    3. CRun-off
    4. DTranspiration
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    A: Condensation

    Water vapour cools and condenses into tiny droplets, which form clouds.

  4. 4

    What does potable water mean?

    1. AWater that contains only water molecules
    2. BWater that has been boiled
    3. CWater that is safe to drink
    4. DWater that comes from the sea
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    C: Water that is safe to drink

    Potable water is safe to drink, but it can still contain low levels of dissolved substances.

  5. 5

    Which is used to kill microorganisms when making potable water?

    1. ASand
    2. BChlorine
    3. CSalt
    4. DA wire screen
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    B: Chlorine

    Chlorine, ozone and ultraviolet light sterilise the water.

  6. 6

    In reverse osmosis, what happens to the salt in sea water?

    1. AIt stays behind the membrane
    2. BIt passes through the membrane
    3. CIt turns into steam
    4. DIt is filtered out by sand
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    A: It stays behind the membrane

    The salt ions cannot pass through the partially permeable membrane, so they are left behind.

  7. 7

    What is a main disadvantage of desalination?

    1. AIt produces no potable water
    2. BIt needs no energy
    3. CIt cannot be used near the sea
    4. DIt needs a lot of energy
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    D: It needs a lot of energy

    Heating or pressurising sea water needs a large amount of energy, which makes the process expensive.

  8. 8

    Why is rain fresh water although it evaporates from the sea?

    1. AThe salt is filtered out by clouds
    2. BThe salt turns into gas
    3. CThe salts are left behind
    4. DRain is made from river water only
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    C: The salts are left behind

    When water evaporates, the dissolved salts stay behind in the sea.