Thomas Wilson, of Port Chalmers School, asks :-

How does water conduct electricity?

Keith Hunter of the Chemistry Department, University of Otago, responded.

Firstly, to understand this question it is important to remember that electricity is conducted by electrical charges. In a wire, and inside the circuits of a TV, radio or computer, electricity is conducted mainly by electrons which are negatively charged.

Water that contains a dissolved salt (e.g. river and sea waters) conducts electricity mainly through the ions of the salt. Salts are compounds that when they dissolve, break up into species called ions that are positively and negatively charged. For example, table salt produces positive sodium ions and negative chloride ions. The negative ions move towards the positive electrode and the positive ions move in the opposite direction. The conducting ability of a salt solution depends directly on how much salt is present - the more salt, the greater its ability to conduct electricity. Because salt solutions conduct electricity well, chemists call them electrolyte solutions.

Even absolutely pure water contains some ions - these are called hydrogen ions (positive) and hydroxide ions (negative). They are produced from the breakdown of a very small number of water molecules.

In the real world, even the purest water also contains ions from impurities that are very difficult to keep out of the water. For example, carbon dioxide gas will dissolve from the air to produce bicarbonate ions.