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Summary OCR A/AS-Level Biology 2.2.2 Water and Its Properties
Revision summary of key knowledge from OCR A/AS-Level Biology 2.2.2 Water and Its Properties.
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Water and its PropertiesStructure and Polarity
Water is made of two hydrogen atoms covalently bonded to one oxygen atom. The oxygen
atom has a slightly greater amount of protons in its nucleus so exerts a greater attraction for
electrons. This makes water a polar molecule since the oxygen has a slightly negative charge
(δ˗) and the hydrogen atoms have a slightly positive charge (δ˖).
Properties
Separate water molecules join together by hydrogen bonds formed by the attraction between
slightly positive hydrogen atoms and slightly negative oxygen atoms. These hydrogen bonds
give water 3 properties:
Cohesiveness – the ability of water molecules to ‘stick’ together
Adhesiveness – the ability of water molecules to ‘stick’ to other polar molecules
Good solvent – water molecules surround polar molecules to break them down easily
Water is unique since it is less dense as a solid than as a liquid and it expands upon freezing.
This is because as water the hydrogen bonds are constantly braking and re-forming enabling a
more dense spacing, whereas as ice, the maximum amount of hydrogen bonds are formed
causing the molecules to be further apart
Heat Properties
Water has the highest heat capacity of all liquids, so it requires a lot of energy to change
temperature and hence heats up and cools down very slowly. This allows large bodies of water
maintain a relatively stable temperature and makes water a good temperature regulator.
Water also has a high latent heat of vaporisation meaning it requires a lot of energy to
evaporate. This gives water an excellent cooling effect as it draws this energy as thermal
energy from the surface it is on. Many organisms utilise this through sweating.
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