CIE A Level Biology (9700) 2019-2021

Revision Notes

2.3.7 Water & the Hydrogen Bond

Water Molecules: Hydrogen Bonds

  • Water is of great biological importance. It is the medium in which all metabolic reactions take place in cells. Between 70% to 95% of the mass of a cell is water
  • As 71% of the Earth’s surface is covered in water it is a major habitat for organisms
  • Water is composed of atoms of hydrogen and oxygen. One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)
  • Although water as a whole is electrically neutral the sharing of the electrons is uneven between the oxygen and hydrogen atoms
    • The oxygen atom attracts the electrons more strongly than the hydrogen atoms, resulting in a weak negatively charged region on the oxygen atom (δ) and a weak positively charged region on the hydrogen atoms(δ+), this also results in the asymmetrical shape
  • This separation of charge due to the electrons in the covalent bonds being unevenly shared is called a dipole. When a molecule has one end that is negatively charged and one end that is positively charged it is also a polar molecule
  • Water is a polar molecule

A water molecule, downloadable AS & A Level Biology revision notes
The covalent bonds of water make it a polar molecule

  • Hydrogen bonds form between water molecules
    • As a result of the polarity of water hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules
  • Hydrogen bonds are weak, when there are few, so they are constantly breaking and reforming. However when there are large numbers present they form a strong structure
  • Hydrogen bonds contribute to the many properties water molecules have that make them so important to living organisms:
    • An excellent solvent – many substances can dissolve in water
    • A relatively high specific heat capacity
    • A relatively high latent heat of vaporisation
    • Water is less dense when a solid
    • Water has high surface tension and cohesion
    • It acts as a reagent

Hydrogen bonds between water molecules, downloadable AS & A Level Biology revision notes
The polarity of water molecules allows hydrogen bonds to form between adjacent water molecules

Exam Tip

It is important to know where the hydrogen bonds form between water molecules (oxygen of one water molecule to the hydrogen atom of another).

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.
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