Water

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Hydrogen bonds

Water consists of two hydrogen atoms covalently bonded to an oxygen atom. The oxygen atom is slightly negative while the hydrogen atoms are slightly positive. This is why water is a polar molecule.

A hydrogen bond is a weak interaction that can occur wherever molecules contain a slightly negatively charged atom bonded to a slightly positively charged hydrogen atom. Water molecules hydrogen-bond with each other extensively but not permanently.

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Reasons for water's properties

Most of water's properties are due to its polar nature. This means that water molecules will form hydrogen bonds with each other.

  • In pure liquid water molecules continually make and break hydrogen bonds which makes it difficult for water molecules to be free enough to be gas. This is why water has a boiling point of 100 degrees. It takes up a lot of energy so heat energy is used in evaporation to 'remove' the heat from the surface of the organism.
  • At low temperatures the water molecules move around less. More hydrogen bonds form which do not break as easily. The bonds hold the structure in a semi-crystalline form which is less dense than liquid. Ice floats on liquid water so it insulates the water below.
  • Hydrogen bonds pull water molecules in. This attraction is called cohesion which also results in surface tension at the surface of a body ofwater.
  • Any polar molecule will dissolve in water. This is because the slightly negative parts of the molecule will be attracted to the slightly positive hydrogen parts of water. The water molecules will cluster around the solute molecules, keeping them apart and therefore dissolving them.
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Importance of water

  • As a solvent it dissolves chemicals. In cytoplasm which is 70-95% water, metabolic reactions take place e.g. respiration and photosynthesis. These all take place when dissolved in water.
  • Water remains liquid over a large temperature range so is the liquid transport medium in organisms. It is in blood in animals and in vascular tissue in plants.
  • Transport of water in the xylem relies on cohesion of water molecules to make the transpiration stream. Cohesion makes long, thin water columns which are strong and difficult to break.
  • As ice, water is less dense and so it insulates the water beneath the surgace allowing aquatic organisms to survive underneath. It also allows organisms such as polar bears to live on the ice.
  • Water provides thermal stability to an organism as many organisms use water as a cooling mechanism by evaporation e.g. sweating.
  • Water molecules are used in reactions such as hydrolysis and photosynthesis.
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