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How is drinking water made safe, and how do substances dissolve in water?

The water cycle, treatment of water for drinking, the test for water, and solubility including the difference between dilute and concentrated solutions.

A focused answer to WJEC GCSE Chemistry topic 1.3 on water, covering the water cycle, how water is treated to make it safe to drink, the chemical test for water, and solubility including solutes, solvents and saturated solutions.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. The water cycle
  3. Treating water to make it safe
  4. Testing for water
  5. Solubility
  6. Try this

What this topic is asking

WJEC wants you to describe how water moves through the water cycle, how it is treated to be safe to drink, the chemical test for water, and the basics of solubility. This is part of topic 1.3 Water in Unit 1 of WJEC GCSE Chemistry (3430).

The water cycle

Treating water to make it safe

Treatment removes solids and kills microbes, but it does not remove all dissolved substances, so treated tap water is safe but not chemically pure.

Testing for water

To show a sample is pure water, check its physical properties: pure water boils at exactly 100β€‰βˆ˜C100\,^{\circ}\text{C} and freezes at 0β€‰βˆ˜C0\,^{\circ}\text{C} at normal atmospheric pressure. Dissolved impurities raise the boiling point and lower the freezing point.

Solubility

Try this

Q1. Name the chemical used to kill microbes when treating drinking water. [1 mark]

  • Cue. Chlorine.

Q2. State what is meant by a saturated solution. [1 mark]

  • Cue. A solution that holds the maximum amount of dissolved solute at that temperature, so no more will dissolve.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20184 marksDescribe the main stages used to treat water from a reservoir so that it is safe to drink.
Show worked answer β†’

A topic 1.3 structured question. Stages, in order: filtration through screens and sand beds to remove solid bits and larger particles (1 mark); sedimentation/settling, often after adding a chemical to make fine particles clump and sink (1 mark); and chlorination, adding chlorine to kill microbes/bacteria (1 mark). The result is water that is clear and free of harmful microorganisms, safe to drink (1 mark). Markers reward the correct stages in a sensible order and the purpose of chlorine. A common error is to leave out the disinfection step.

WJEC 20212 marksDescribe a chemical test that shows a liquid contains water, and how you would then check it is pure water.
Show worked answer β†’

A topic 1.3 test question. Add the liquid to anhydrous copper sulfate (white): it turns blue if water is present (1 mark) (blue cobalt chloride paper turning pink is an acceptable alternative). To show the water is pure, check that it boils at exactly 100β€‰βˆ˜C100\,^{\circ}\text{C} (and freezes at 0β€‰βˆ˜C0\,^{\circ}\text{C}) at normal pressure; impurities would change these values (1 mark). Markers reward the chemical test for the presence of water and the boiling point check for purity. A common error is to confuse "contains water" with "is pure water".

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