1.7 Water

• Water has mass flow - hydrogen bonding between water molecules allows them to move together, a mass of water moving is called mass flow.
• Water has cohesion - the way water molecules stick together by hydrogen bonding, allowing tall, continuous columns of water. (Seen in transpiration and taps).
• Water has adhesion - the attraction between water molecules and other substances.
• Water has surface tension - water molecules stick together by H bonding on all sides, so that where the water meets the air it behaves as though it were forming a thin 'skin', allowing small organisms to walk on water.
• The Oxygen in water has a negative dipole charge because it attracts negative electrons towards itself.
• The negative dipole on the Oxygen atom in water is attracted to the positive dipole on the Hydrogen atoms, so a Hydrogen bond is formed.
• Water has a high Specific Heat Capacity, meaning it takes a lot of energy to raise the temperature of 1kg of water by 1 degree Celsius.
• Water has a low density at its freezing point compared with most liquids, which means ice floats on liquid water.
• Water maintains aquatic environments by buffering temperature changes because of the high specific heat capacity.
• Some water molecules have more energy than others. Those with more energy move faster and break the H bonds, becoming a gas. (Evaporating).
• Latent heat of vaporisation is the energy lost when a substance changes state from a liquid to a gas. Water has a high latent heat of vaporisation, so a small amount of evaporation has a large cooling effect.
• Water is a metabolite, so it is needed for metabolic reactions such as condensation and hydrolysis.
• Water is a good solvent because it is a polar molecule and can dissolve polar molecules.
• Salts and sugars dissolve in water because they are also polar and are attracted to the dipole charges on water.
• It is important for substances to the be dissolved so they can be absorbed and transported in the blood stream to areas of the body where they are needed for metabolic reactions.
• Water does not change state in boiling, it just bubbles.
• In evaporation, water turns from a liquid to a gas (water vapour). It only occurs on the surface and is slower than boiling.
• Atmospheric pressure is lower at a higher altitude, so water boils more quickly. Because of this lower boiling point, it needs to be boiled for longer to kill all possibly pathogenic microorganisms.
• Water is a good solvent so allows the transport of substances.
• The high specific heat capacity means that water takes a long time to warm up or cool down which helps maintain constant temperature inside living organisms.
• Water has a low viscosity meaning that molecules move freely through it allowing diffusion to occur easily.
• Latent heat of fusion = amount of energy required to convert one kilogram of ice into one kilogram of water
• Water has a high latent heat of fusion and vaporisation which means it absorbs large amounts of energy when changing states without increasing its own temperature.