Water cycle Water treatment Screening Settling Filtration Chlorination Fluoridation Electrolysis Electrolyte Electrode
Learning Intentions
In this topic we are learning to: zz
zz zz Examine the properties and roles of water.
Outline how water cycles through the living and non-living parts of the Earth.
Investigate the electrolysis of water.
Water (H2O) is a compound of hydrogen and oxygen atoms. Water covers just over 70 per cent of the Earth’s surface.
A supply of clean water is essential for all humans.
Day to day, we need water for drinking, cooking and washing. In agriculture, water is needed for crops and livestock. Water is also used in industry for cooling machinery and as a raw material for products such as medicines and paint.
Water exists in three states: z z
Solid (ice)
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Liquid (water) Gas (vapour or steam). The Properties of Water z Fig. 22.3.1
Humans need to drink around two litres of water a day
Learn more about the test for water in Biological World 2.1
Water has a number of special properties that make it very useful. zz
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The freezing point of pure water is 0 °C and its boiling point is 100 °C. This means that water typically exists as a liquid on Earth, which is ideal for supporting life.
Water expands as it freezes. Most other compounds contract as they freeze. This is why pipes can burst if the water in them freezes.
The density of water is 1.0 g/cm3. At 0.92 g/cm3, ice is less dense than water. This means that ice floats on water.
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Learn more about transport in plants in Biological World 7.2
Water is an excellent solvent. Many substances dissolve in water. This makes it difficult to find pure water.
Adhesion. Water clings to and climbs up through xylem vessels to deliver water and mineral nutrients from the soil to leaves.
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Surface tension. Molecules of water are attracted to one another so it appears as if there is a thin skin on the surface. This allows some small insects to walk on water and also helps water move through the xylem tissue of a plant.
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Fig. 22.3.2
Surface tension allows some insects to walk on water
