Water


Water is the medium through which all reactions in living organisms takes place which is why it is essential for nearly all life on earth.

Properties of Water

Specification Reference

"Understand the importance of the dipole nature of water leading to hydrogen bonding and the significance of the following to organisms: high specific heat capacity, polar solvent, surface tension, incompressibility and maximum density at 4 °C."
Water is a dipole and this underpins many of its properties. In a molecule of H2O, the electrons in the covalent bonds are held closer to the large and positive charge of the oxygen atom. This results in:
  • Oxygen having a partial negative charge
  • Hydrogen having a partial positive charge
Because of this water molecules can form weak forces of attraction between each other. These are known as hydrogen bonds and can be used to explain some of waters anomalous properties.
  • Polar solvent - the dipole nature makes water a good solvent. The oppositely charged atoms attract each other allowing molecules of water to surround a solute.
  • High specific heat capacity - lots of hydrogen bonds mean lots of energy is required to overcome them. This results in water providing a stable environment for organisms.
  • Surface tension - molecules of water at the surface are pulled to the sides and down but not up. This uneven distribution results in a surface tension allowing some animals to "walk" on the water.
  • Density - when water freezes each molecule forms 4 hydrogen bonds in an open lattice. This reduces the density of ice allowing it to float on water meaning that aquatic life can survive even when the surface freezes.
  • Incompressibility - water cannot be compressed, allowing it to act as a structural support in plants and animals.