Water
Senior 1 · Chemistry
Water
Lesson 5 of 5
25 min
The sea is salty, yet the rain that falls on Rwanda is almost pure water. Some of the water you drink today may have been in the Indian Ocean a few weeks ago. How does salty water become fresh without a treatment works? And why, if nature keeps cleaning water, do so many rivers end up dirty?
You have studied the water cycle in Geography as a journey through the landscape. In chemistry it is a chain of changes of state, exactly those you met in Unit 3, Lesson 3, driven by energy from the sun (Figure 5.1).
Figure 5.1: The water cycle as a chain of changes of state.
Definition
Water cycle: the continuous movement of water between the Earth's surface and the air by evaporation, condensation and precipitation, driven by energy from the sun. Example: water evaporates from Lake Kivu, condenses into clouds over the Congo-Nile ridge, and falls as rain that flows back into the lake.
Worked example
Problem. Explain why rain is soft and almost pure, although much of it evaporated from the salty sea. Compare the water cycle with distillation in the laboratory.
Solution. Only water evaporates; the dissolved salts cannot turn into vapour, so they stay behind in the sea, just as salt stays in the flask during distillation. The water vapour then condenses in cool air, as it does in a condenser.
| Step | Laboratory distillation | Water cycle |
|---|---|---|
| Source of heat | Bunsen burner | the sun |
| Evaporation | in the flask | at the surface of the sea and lakes |
| Cooling for condensation | cold water in the condenser | cool air high above the ground |
| Pure water collected | distillate | rain |
| Left behind | salt in the flask | salt in the sea |
Answer: rain is naturally distilled water. It is not perfectly pure, because it dissolves some carbon dioxide and dust from the air as it falls.
Table 5.1: Uses of water and the property behind each use
| Use | Examples | Property of water that makes it useful |
|---|---|---|
| In the home | drinking, cooking, washing, cleaning | dissolves food, dirt and soap; not poisonous |
| Farming | irrigating crops in marshland valleys and hillside gardens; watering animals | carries dissolved nutrients into plants |
| Industry | processing tea and coffee, brewing, making soft drinks, mixing concrete, cooling machines | good solvent; takes in a lot of heat before it warms up |
| Energy | hydroelectric power stations on Rwanda's rivers | a moving liquid that turns turbines |
| Transport and fishing | boats and fishing on Lake Kivu | a large, liquid surface that supports floating objects |
| In the laboratory | making solutions, rinsing apparatus, water baths | dissolves many substances; boils at a fixed temperature |
Nature cleans water by distillation, but people add substances to water faster than the cycle can remove them. A substance does not have to be poisonous to be a pollutant: fertiliser that helps a farm is a pollutant in a lake.
Definition
Water pollution: the presence in water of substances or germs, in amounts that are harmful to people, animals or plants, usually as a result of human activity. A pollutant is any substance present in an amount that is not what it should be. Example: sewage that flows from a leaking pit latrine into a stream.
Table 5.2: Main water pollutants
| Pollutant | Main sources | Effects | Prevention |
|---|---|---|---|
| Sewage and animal dung | leaking pit latrines, open defecation, cattle drinking in streams | germs of cholera, typhoid and diarrhoea; uses up dissolved oxygen | latrines well away from water sources; sewage treatment |
| Fertilisers (nitrates, phosphates) | rain washing them off farms | eutrophication | use the correct amount; keep a strip of grass and trees beside rivers |
| Soil (silt) | erosion of bare hillsides, mining, building sites | high turbidity; costly water treatment; buries fish eggs | terraces, trees and grass on slopes; cover mining waste |
| Detergents | washing clothes and cars in rivers and lakes | foam; their phosphates cause eutrophication | wash away from water; let used water soak into the ground |
| Heavy metals (lead, mercury, cadmium) | mining, old batteries, factory waste | poisonous; build up in fish and in people who eat them | treat factory waste; collect batteries (Unit 11) |
| Oil and fuel | garages, leaking vehicles, boats | floating layer stops oxygen dissolving; poisons animals | collect used oil for recycling |
| Plastics and litter | rubbish thrown into drains and streams | block drains and cause floods; harm animals | reduce, reuse and recycle (Unit 11) |
| Pesticides | spraying crops close to water | poison fish and insects | spray only as directed, away from water |
Waterborne diseases. Water polluted with human or animal faeces carries the germs of cholera, typhoid and diarrhoeal diseases. Diarrhoea is especially dangerous for young children, because they lose water and salts faster than they can replace them. Still water containing certain snails can also spread bilharzia (schistosomiasis).
Eutrophication. Nitrates and phosphates from fertilisers, sewage and detergents are plant nutrients. In a lake they cause a chain of events that can kill every fish in it (Figure 5.2).
Figure 5.2: How eutrophication kills the life in a lake.
Definition
Eutrophication: the over-enrichment of a lake or river with nutrients, mainly nitrates and phosphates, which causes a rapid growth of algae followed by a fall in dissolved oxygen that kills fish and other animals.
Worked example
Problem. Figure 5.3 shows the dissolved oxygen measured along a river below a sewage outlet (illustrative data). (a) Describe the pattern. (b) Explain the fall. (c) Where could fish survive?
Figure 5.3: Dissolved oxygen in a river below a sewage outlet (illustrative data).
Worked example
Solution
Answer: the outlet creates an oxygen "sag", from just below the outlet to about 3 km downstream, in which fish cannot live.
Warning
Health. Do not drink, bathe in or wash vegetables with water from streams, ponds or lakes that may be polluted. Avoid wading in still water where snails live. If a child has diarrhoea, give plenty of safe water and oral rehydration salts, and get medical help quickly.
Tip
Remember the four big water pollutants with S-F-S-P: Sewage, Fertiliser, Soil, Plastic.
FuseSchool: What is water pollution? (6 min). Note each pollutant named in the video and add it to Table 5.2 if it is missing.
Practice
Further reading
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
