Notes on Water including What it is made from and how are the molecules and bonds organized? and going into the effect the hydrogen bonds have on the latent heat of fusion and vaporisation. Also refers to surface tension and cohesion and all of the points needed to know about for A Level Biologica...
Outline the structure and chemistry of water. What it is made from and how are the molecules and
bonds organized?
The chemical formula of water is H2O as it contains 2 hydrogen atoms and 1 oxygen atom and they
are held together by covalent bonds. Each hydrogen atom is covalently bonded to a central oxygen
molecule via a pair of electrons that are shared between the 2 atoms. Oxygen has 6 electrons in its
outer shell and each hydrogen atom has 1 electron. The oxygen atom needs to have 8 electrons so it
shares an electron with each hydrogen atom, who in turn, also share an electron with oxygen. As
hydrogen only has 1 shell, it only needs 2 electrons which it gets by sharing with the oxygen atom.
Due to the covalent bonds between the hydrogen and the oxygen atoms, the sharing is unequal. The
oxygen atom has a negative charge and the hydrogen atoms have a positive charge.
Explain how hydrogen bonds are formed
The water molecules are held together with a hydrogen bond as the polarity of the water molecules
causes the molecules to be attracted to each other. The partially positive hydrogen atoms are
attracted to the partially negative oxygen atoms of a different molecule.
How do hydrogen bonds have an effect upon:
- The latent heat of fusion and vaporization:
The latent heat is the heat needed to undergo a change in state. Therefore the latent heat of fusion
is the heat that’s required to go from a solid to a liquid. Water requires a lot of energy to form a solid
from a liquid so water tends to stay liquid.
Hydrogen bonds give water a higher latent heat. Due to the hydrogen bonds, a lot of heat energy is
required to evaporate water. This allows organisms to cool themselves to maintain a constant body
temperature. In liquid, the water molecules still have kinetic energy and therefore may be able to
escape from the surface – this is evaporation. As the molecules leave the liquid, this reduces the
average temperature. The energy involved is called the latent heat of vaporization.
- Specific heat capacity:
Specific heat capacity (SHC) is a measure of the energy required to raise 1kg of a substance by 1°C.
Water has a high SHC so a large amount of energy is needed to raise the temperature. This is due to
the hydrogen bonds – it takes a lot of thermal energy to break these bonds as well as a lot of energy
to build them. This means that the temperature of water doesn’t change much.
Explain why the density of water changes as it freezes and why this is an important property of
water for maintaining life on earth:
In liquid water, the molecules are close together but in ice (solid water), the hydrogen bonds create
a structure that contains large holes. These gaps in the structure increase the volume and therefore
decrease the density. In ice the hydrogen bonds hold the molecules in a lattice. As the temperature
of water decreases, there is less kinetic energy and therefore the water molecules slow down. In
liquid water, the molecules are always moving around past each other and so they form fleeting
hydrogen bonds. In ice, because the molecules have slowed down, the maximum amount of
hydrogen bonds can form because the molecules spread out.
Because of this change in density when the state changes, ice floats on water. When the
temperature of the air falls, the water on the surface cools. The water freezes which creates a thin
layer of ice with liquid water underneath. As the surface water gets colder, it gets denser and sinks
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