Yr 10 Science
1.1
Water
Life on this planet began in water, wherever you find water you will find life.
Some organisms live on the water that sticks to a grain of sand
Life exists under polar ice caps and in hot springs
Plants will rush through a whole life cycle after a downpour of rain
70% of the Earths surface is covered in water
Plants are 65 – 95% water
Humans are 75% water
It is the medium for most biochemical reactions
So what is so special about it?
Polarisation
Water is composed of two hydrogen atoms and one oxygen atom. Each hydrogen is
attached to the oxygen by a single covalent bond. The molecule has a neutral charge
but it is, however, polar. This is due to oxygen having a greater electron affinity and,
therefore a greater share of the electrons.
This makes the oxygen slightly negative and the hydrogen slightly positive. This
polarisation of the molecule causes hydrogen bonding.
Hydrogen Bonding
When one of the charge regions of the water molecule comes close to an oppositely
charged region of another water molecule, the force of attraction forms a bond
between them, which is known as a hydrogen bond. Hydrogen bonds are very weak.
Hydrogen bonds are vey short lived, each one lasting only a fraction of a second, but
as one is broken another is formed. Every water molecule can establish hydrogen
bonds with four other water molecules. Hydrogen bonds are formed with any
charge particle that dissolves in water, and with any charged surface in contact
with water.
It is the presence of hydrogen bonds that gives water its unique properties
Properties of Water
Liquid at room temperature: This very unusual for such a small molecule it is the hydrogen bonds that hold
the molecule together
Biological significance: provides a habitat for living things and a medium for biochemical reactions
High specific heat capacity: A large amount of energy
is needed to raise the temperature of water. Energy is
needed to break hydrogen bonds between molecules.
Biological significance: aquatic environments are slow to change temperature, chemical reactions only take place
within narrow temperature ranges, provide stable internal temperature.
Latent heat of fusion (energy needed to melt ice): a lot of must be extracted from water in order to
freeze it.
Biological significance: contents of cells and aquatic environments are slow to freeze.
Latent heat of vaporization: The evaporation of water requires a great deal of energy and so has a large
cooling effect.
Biological significance: a small amount of sweat of transpiration causes marked cooling.
Density: Water reaches its maximum density at 4 degrees centigrade. This means that when water freezes it is less
dense than the surrounding water and so floats. This then insulates the water below and stops freezing.
Biological significance: ice formed on the surface of water prevents the water underneath from freezing, life is bale
to survive.
Solvent properties: Water is able to dissolve things than any other known substance, thus it is known as the
universal solvent. In fact it will only dissolve other polar substances and it does this by overcoming the forces of
attraction between the oppositely charged anions and cations.
Biological significance: Transport medium for polar solutes ie movement of minerals to lakes and seas, transport in
the blood, removal of metabolic waste such as urea.
Strong cohesion and adhesion:
Hydrogen bonding causes water molecules to stick together(cohesion) and
to stick to other polar or charged substances. At the surface of the water most hydrogen bonds point inwards
towards other water molecules and so produce a surface tension, as the molecules are attracted to one another.
Biological significance: Water forms droplets which run off, certain animals are able to walk on water (provides a
habitat). Water can be drawn up long thin tubes
Colourless: Water allows light to pass through it.
Biological significance: Water plants are bale to carry out photosynthesis below the surface of the water. Aquatic
organisms can use light to find food.