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Water is an essential resource across the world. It is used in the home for drinking, washing, cooking and cleaning. In industry it is used as a coolant, solvent and as a raw material in chemical reactions.

The water we use has been purified for us. Water from natural sources such as lakes and rivers contain a number of other substances, some of which can be harmful to our health.

• Dissolved ions – As water runs through and over the ground, it dissolves many chemicals from rocks which then become ions in the water. The most common ions found in natural water are sodium, calcium, magnesium, sulfate, chloride and hydrogencarbonate.
• Dissolved gases – Gases from our atmosphere can dissolve in water. The most important dissolved gases in natural water are oxygen (required for aquatic organisms for respiration) and carbon dioxide (required by plants for photosynthesis).
• Microorganisms – Natural water supplies contain vast numbers of bacteria and other microscopic organisms. Some of these microorganisms can cause disease and so it is potentially dangerous to drink untreated water from rivers and lakes. The harmful microorganisms can be removed effectively during the water treatment process.
• Pollutants – As water travels over or through the land to rivers and lakes, it can dissolve chemical fertilizers and pesticides from farms, as well as animal waste. These form the majority of the pollutants found in our natural water supplies and can be harmful to our health in high concentrations. Sometimes pollutants are deliberately ‘dumped’ into water supplies, although it is illegal to do this in the UK.