Hard and soft water considers minerals in the water. Predominantly Calcium and Magnesium.
Water in different parts of the UK varies in the number of dissolved mineral ions it contains. This determines whether it is hard or soft water.
There are advantages and disadvantages to each. The damage that can be caused to water pipes and appliances by hard water means that water may need to be softened.
Rainwater is, naturally, slightly acidic because it contains carbonic acid, formed by the reaction between water and carbon dioxide in the atmosphere. When the rain falls, it flows over rocks or soaks into the ground and then passes through layers of rock. Compounds from the rocks dissolve into the water. Hard water contains dissolved compounds, usually calcium or magnesium. The presence of calcium ions and magnesium ions in the water makes it hard.
The types of rocks found in different regions determine how hard or soft the water will be. The water in some parts of the country is soft because it has low levels of dissolved calcium and magnesium compounds, while the water in other parts of the country is hard because it has higher levels of dissolved calcium and magnesium compounds.