1.1 Understand the importance of water as a solvent in transport, including its dipole nature
The water molecule
- The water molecule (H2O) is made up of 1 oxygen atom and 2 hydrogen atoms joined together by a covalent bond.
- Water molecules are polar (i.e. have uneven charge distribution).
- More specifically, water molecules are dipoles (i.e. have slightly positive and slightly negative ends). The hydrogen end is slightly positive (δ+) , the oxygen end is slightly negative (δ-).
Water as an excellent solvent - "like dissolves like"
- Ionic substances (e.g. sodium chloride NaCl) can dissolve easily. The positive ends of water molecules attracts the negative chloride ions while the negative ends of water molecules attracts the positive sodium ions.
- Polar molecules which have charged groups (e.g. -OH in glucose and –NH2 and –COOH in amino acids) can also dissolve easily in water.
The positively and negatively charged ions in sodium chloride are separated due to the dipole nature of water molecules.
- Dipole water molecules are held together by hydrogen bonds.
The significance of hydrogen bonds:
- Allow water to remain as a liquid in room temperature (unlike carbon dioxide CO2, which is a gas)
- Create cohesion, which is important in the transportation of water and other substances in the xylem of plants.
- Give rise to a high specific heat capacity and latent heat of vaporization so water can help regulate temperature in living things
And don't forget to....
1. Edexcel AS Biology Revision Guide. Edexcel.