Revision powerpoint with an in depth summary on the topic. Can be printed with 2-4 slides to an A4 page and then cut out to form a mini revision booklet. Very colourful.
36 slides long
, Water
Water has a polar nature so can form
hydrogen bonds
Polar means that the charge is
not evenly distributed across the
particle.
Water consists of 2 H atoms each
covalently bonded to an O atom. As
the O atom has a greater number of
protons in its nucleus, it exerts a
stronger force of attraction for the
shared electrons therefore becoming
delta negative and the H atoms
becoming delta positive
A weak interaction that can occur wherever
molecules contain a slightly negative atom bonded to a slightly
positively charged hydrogen atom
, Properties of water
Liquid- Water molecules continually make & break hydrogen bonds as they move
around. The hydrogen bonds between water molecules make it more difficult for them
to escape to become a gas. Other less polar, similarly sized molecules e.g. H₂S are
gases at room temp. As water is liquid at room temp it:
1. Provides habitats for living things in rivers, lakes & seas
2.Is a component of the tissues in organisms & an effective transport medium e.g.
blood
3.Provide a reaction medium for chemical reactions
Solvent- The polarity of water makes it a good solvent. The + & − parts of the water
molecules are attracted to the + &− parts of the solute. The water molecules cluster
around the charged parts of the solute molecules/ions separating them & keeping them
apart so they dissolve to form a solvent. As water is a good solvent:
1. Molecules & ions can move around & react together in water
2.Molecules & ions can be transported around living things whilst dissolved in water
Density- The density of water provides an ideal habitat for living things. Water
becomes more dense as it gets cooler until about 4°c. Due to its polar nature, unlike
most liquids, as it goes from 4°c to freezing point the water molecules align themselves
in a structure which is less dense than liquid water. As ice is less dense than water:
1. Aquatic organisms have a stable environment in which to live through winter
2.Ice reduces the rate of heat loss so bodies of water are insulated when an ice layer
forms
, Cohesion & surface tension- Water molecules demonstrate cohesion. A drop of water
on a flat surface does not spread out as H bonding between the molecules pulls them
together. Water molecules at the surface are all H bonded to the molecules beneath
them so more attracted to the molecules beneath than the air above. This means the
surface of the water contracts because the molecules are being pulled inwards, so the
water has an ability to resist force applied to it. This is surface tension. (surface tension
is due to cohesive forces in turn due to hydrogen bonding between water molecules at
the surface) Due to cohesion & surface tension:
1. Columns of water in plant vascular tissue are pulled up the xylem from the roots. This
is a capillary action- the smaller the internal diameter of the plant the greater the
capillary action.
2.Insects like pond skaters can walk on water.
High specific heat capacity- water molecule are held together quite tightly by H bonds
so you have to put in a lot of heat energy to increase their kinetic energy & temp. The
amount of heat required is the SHC. To raise the temp of 1kg of water by 1°c it would
take 4.2kj of energy.
1. Living things need a stable temp for enzyme controlled reactions to happen properly.
High SHC makes it easier to maintain a constant body temp.
2. Aquatic organisms need a stable environment to live
High latent heat of vaporisation- when water evaporates, heat energy known as latent
heat of vaporisation helps the molecules to break away from each other to become a
gas. Due to H bonds a relatively large amount of energy is needed for water to
evaporate. Therefore water can help cool living things & keep there temp stable e.g.
sweat.
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