How is drinking water made safe, and what does it mean for a substance to be soluble?
The treatment of water to make it safe to drink, the testing of water purity, and solubility including the idea of saturated solutions.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on water, covering the stages of water treatment, testing water purity, and solubility including dissolving, saturated solutions and how temperature affects solubility.
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What this dot point is asking
WJEC Double Award Unit 2 wants you to describe how water is treated to make it safe to drink, how water purity is tested, and what solubility means including saturated solutions.
Why water must be treated
Water from rivers and reservoirs contains solid particles and harmful microorganisms that could cause disease. It must be treated before it is safe to drink. Treatment removes the solids and kills the microbes.
The stages of water treatment
Each stage removes a different kind of impurity, and the disinfection step is the one that makes the water microbiologically safe.
Testing water purity
Pure water is a single substance and has fixed physical properties: it boils at exactly 100 degrees Celsius and freezes at exactly 0 degrees Celsius (at normal pressure). If water contains dissolved substances, its boiling point is higher and its freezing point is lower. So measuring the boiling or freezing point can show whether water is pure. "Potable" (drinkable) water is safe to drink but is not chemically pure, because it still contains dissolved substances.
Solubility and saturated solutions
When a soluble substance (solute) dissolves in a solvent such as water, it forms a solution. The solubility of most solids increases with temperature, so hot water dissolves more solid than cold water. This is why heating a saturated solution usually lets more solid dissolve, and why crystals can form again when a hot saturated solution cools.
Solubility curves
The solubility of a solid can be shown on a solubility curve, a graph of solubility (grams of solute per 100 g of water) against temperature. For most solids the curve rises, showing that more dissolves as the temperature increases. You can read off the curve to find the solubility at a given temperature, or to predict how much solid will crystallise out when a solution is cooled (the difference between the solubility at the higher and lower temperatures). Reading and using a solubility curve is a common data-handling question.
Hard and soft water
Water that has flowed over rocks such as limestone picks up dissolved calcium and magnesium compounds, which makes it hard water. Hard water forms scum with soap (wasting soap) and leaves limescale in kettles and pipes when heated. Soft water has fewer of these dissolved ions and lathers easily with soap. Hard water can be made soft by removing the dissolved calcium and magnesium ions. Recognising the signs of hard water (scum and limescale) is a common exam point linked to the chemistry of dissolved substances.
Try this
Q1. Name the stage of water treatment that kills microorganisms. [1 mark]
- Cue. Chlorination (disinfection).
Q2. State the boiling point of pure water at normal pressure. [1 mark]
- Cue. 100 degrees Celsius.
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 style4 marksDescribe the main stages used to treat water to make it safe to drink.Show worked answer →
A Unit 2 describe question worth 4 marks. Reward: screening/filtration to remove large objects and solids (1); sedimentation, where fine particles settle out (sometimes helped by adding a chemical) (1); filtration through sand and gravel to remove smaller particles (1); and chlorination (or another disinfectant) to kill harmful microorganisms (1). Markers credit the stages that remove solids and the disinfection step. A common error is to leave out the killing of microbes, which is essential for safety.
WJEC style3 marksExplain what is meant by a saturated solution and how more solid could be dissolved in it.Show worked answer →
A Unit 2 explain question. A saturated solution is one that contains the maximum amount of solute that will dissolve at that temperature (1); no more will dissolve and any extra stays as solid (1); heating the solution usually allows more solid to dissolve, because solubility increases with temperature (1). Markers credit the maximum amount at a temperature and the effect of heating. A common error is to forget that solubility depends on temperature.
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