WJEC AS Biology - Biological Molecules - Water




Water is a polar molecule and has an no overall charge. The oxygen end of the molecule has a slight negative charge and the hydrogen end of the molecules has a slight positive charge. When two water molecules are in close contact their opposing charges attract each other forming a hydrogen bond. Individually hydrogen bonds are weak but because there are many of them they stick together in a strong lattice frame work. This sticking together of water molecules is called cohesion. This means that tall columns of water can be drawn up xylem vessels in tall trees.


At ordinary temperatures water has the highest surface tension of any liquid except mercury. In a pond cohesion between water molecules produces surface tension so that the body of an insect such as the pond skater is supported.


 Water is a solvent. Because water is a polar molecule it will attract other charged particles, such as ions, and other


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