WJEC GCE BIOLOGY Properties of Water Water is important to living organisms because it is a medium for: Metabolic processes Transport Aquatic organisms to live in

WJEC GCE BIOLOGY Properties of Water Water consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds. –The shared electrons are not shared equally. The oxygen atom pulls the electrons closer to it –This leaves the water molecule with positively and negative charged ends and is therefore called a polar molecule. Water is a Polar Molecule -has oppositely charged ends

WJEC GCE BIOLOGY Properties of Water slightly positive charge slightly negative charge hydrogen bond between (+) and (-) areas of different water molecules Water molecules form hydrogen bonds

WJEC GCE BIOLOGY Properties of Water Density Surface Tension Cohesion and adhesion Colour State at room temp. Latent heat Specific heat capacity Next

WJEC GCE BIOLOGY Properties of Water Surface Tension Water has the highest surface tension of any liquid except mercury The surface of water can behave like an elastic sheet, this is due to cohesion between water molecules. Those molecules in the surface are not effected by molecules above them, and therefore ‘pull’ together more strongly, effectively resembling a stretched membrane A habitat can therefore be produced on the surface of the water A pond skater can walk across the surface of the water Back

WJEC GCE BIOLOGY Properties of Water Colour – transmission of light Water is colourless and is therefore transparent to light This means that sunlight can reach the cells of water plants so that photosynthesis may occur 53% 10% 13% 9% 32% 8% 7% Reflected by leaves Transmitted through leaves Sun light Light absorbed by leaves (typical values): Near IR 15% Red 82% Green 80% Blue 84% Back

WJEC GCE BIOLOGY Properties of Water Cohesion and adhesion Water molecules show a strong attraction to one another due to hydrogen bonding, this is called cohesion. Water molecules can also show attraction to other polar molecules called adhesion The forces of cohesion and adhesion help water travel up the xylem vessels of plants. Droplets of water form spheres due to cohesion between water molecules Back Adhesion and cohesion help water move up xylem

WJEC GCE BIOLOGY Properties of Water Latent heat While changing state, a substance will take in heat energy (solid  liquid  gas), or expel heat energy (gas  liquid  solid) without a change in temperature. This is termed Latent Heat. Back temperature time SOLID LIQUID GAS melt solidify evaporate condense Water has a large latent heat of vaporisation, and therefore absorbs a large amount of heat energy while changing from water to vapour. Lots of energy is needed to break hydrogen bonds Organisms use this to cool down by the process of sweating.

WJEC GCE BIOLOGY Properties of Water State at room temperature Water is a liquid at room temperature it can therefore be used as a solvent. –This allows chemical reactions to take place in solution –Water can acts as a transport medium carrying dissolved molecules e.g. blood is a transport medium in animals, in plants water transports mineral ions in the xylem. Back Water is an effective solvent as it can form hydrogen bonds with ions. e.g. Na Cl. –The positive end of the water molecule attracts the negative ion and the negative end of the water molecule attracts the positive ion. –The water molecules surround the ions and they therefore dissolve.

WJEC GCE BIOLOGY Properties of Water Specific Heat Capacity The heat needed to raise the temperature of 1kg of water by 1°C is termed the Specific Heat Capacity. Back 1kg of water Water has a large specific heat capacity, and therefore can absorb large amounts of heat energy before its temperature raises a significant amount. Again it takes a lot of energy to break hydrogen bonds for molecules to move around. This prevents large fluctuations in the temperature of water in the environment e.g ensures a stable environment for aquatic organisms thermometer

WJEC GCE BIOLOGY Properties of Water Density of water Unlike other substances, water expands as it freezes. It has it’s maximum density at 4 0 C Back As water expands when it freezes, its density (mass per unit volume) will decrease. This means that ice is less dense than water, and will therefore float on top of it. In aquatic environments, ice forms an insulatory layer and prevents the entire water column from freezing Water molecules are able to approach one another quite closely HH O HH O HH O HH O HH O HH O HH O HH O HH O HH O HH O Water molecules in ice form a rigid structure so that there is more space between them, this is seen as expansion as a whole Water Ice

WJEC GCE BIOLOGY Properties of Water Match the property of water with the corresponding significance for life and correctly fill in the table START

WJEC GCE BIOLOGY Properties of Water Property of water Ice is less dense than water High surface tension Strong cohesive properties and high tensile strength Colourless with a high transmission Liquid at room temperature In order to evaporate it must absorb a large amount of energy (high latent heat of vaporisation) Water can absorb a lot of energy for only a small rise in temperature (high specific heat capacity) Significance for life Light can pass through cells for photosynthesis Can be used for transport and a medium for reactions Can be used for cooling organisms by evaporation of for example sweat Ice forms an insulating layer over water The high surface tension of water means that it can form a habitat on the surface of the water Conditions are stable in cells and aquatic environments Water can be pulled through plants in a column as the water molecules are held together by H-bonds Answers

WJEC GCE BIOLOGY Properties of Water Property of water Ice is less dense than water High surface tension Strong cohesive properties and high tensile strength Colourless with a high transmission Liquid at room temperature In order to evaporate it must absorb a large amount of energy (high latent heat of vaporisation) Water can absorb a lot of energy for only a small rise in temperature (high specific heat capacity) Significance for life Conditions are stable in cells and aquatic environments Can be used for cooling organisms by evaporation of for example sweat The high surface tension of water means that it can form a habitat on the surface of the water Ice forms an insulating layer over water Can be used for transport and a medium for reactions Light can pass through cells for photosynthesis Water can be pulled through plants in a column as the water molecules are held together by H-bonds Next