Hydrogen Bonds Explain Water's Properties
Water is a very small molecule consisting of two hydrogen atoms covalently bonded to one oxgyen atom. The shared electrons in 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 oxgyen 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 degrees before it boils. Molecules such as hydrogen sulfide (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. Thi 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 the 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 stable even when 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 unususal because its solid form - ice - is less dense than its liquid form. As water cools, its density increases until the temperature drops to 4 degrees, 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 live under the ice.

Hydrogen bonds, cohesion and surface tension
A drop of water on a waxy surface, such as a plant leaf, looks almost spherical. It hardly wets the leaf at all. This is because the hydrogen bonds pull water molecules in at the surface. This property of water molecules sticking to each other is called cohesion. Cohesion also results in the surface tension seen at the surface of a body of water.

Water as a solvent
The solubility of a substance in water depends on whether water molecules can interact with it. Any molecule that is polar will dissolve in water. This happens as follows:

the substance to be dissolved has a slightly negative and slightly positive parts.
These will interact with water molecules
The water molecules cluster around the slightly charged parts of the solute molecule.
This keeps solute molecules apaert, so they are dissolved.
Once in solution, molecules can move around and react with pther molecules. This is the cytoplasm.
Ions are charged particles that also dissolve very easily because water molecules can cluster around them and separate them.

Water as a transport medium
Water remains liquid over a large temperature range and can act as a solvent for many chemicals. This makes it an ideal transport medium in living organisms.

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Undergraduate Biology (Biological Molecules) Notiz am Water - A Vital Biological Molecule, erstellt von siobhan.quirk am 29/05/2013.