2.1.12 Biology- Water

Hydrogen bonds explain water’s unusual properties:

Water is a very small molecule consisting of two hydrogen atoms covalently bonded to one oxygen atom. The shared electrons in the water are not shared evenly. The oxygen atom is capable of pulling the shared electrons towards it and so away from the hydrogen atoms in the molecule.

This means that water molecules become slightly negatively charged at the oxygen end and slightly positively charged at the hydrogen ends. Water is described as a polar molecule.

Hydrogen bonds in liquid water and ice:

In pure liquid water, the molecules form hydrogen bonds with each other. They form a network that allows the molecules to move around. Continually making and breaking hydrogen bonds as they do so. This makes it much more difficult for water molecules to ‘escape’ the liquid to become gas. It explains why water must be heated to 100°c before it boils. Molecules such as hydrogen sulphide (H₂S), which are similar in size to water, are gases at normal environmental temperatures.

As temperature is reduced, water molecules move less because they have reduced kinetic energy. More hydrogen bonds form, but they do not break so easily. As water becomes solid, the hydrogen bonds formed hold the structure in a semi-crystalline form. This form is less dense than liquid water, so ice forms on the surface of water as it cools.

 Hydrogen bonds and temperature changes:

The hydrogen bonds in liquid water restrict the movement of water molecules, so a relatively large amount of energy is needed to increase the temperature of water. This keeps the temperature of large bodies of water, such as lakes and oceans, stable even when the temperature changes dramatically.

The evaporation of water uses up a relatively large amount of energy. This means that water evaporating from a surface ‘removes’ heat from the surface. So heat energy is used in evaporation.

Density and freezing:

Water is unusual because its solid form- ice- is less dense than its liquid form. As water cools, it density increases until the temperature drops to 4°c, then its density begins to decrease again. This means that ice floats on liquid water, so it insulates the water below. This allows living organisms to survive the winter, and even to…