Naturally occurring substances also affect water quality. Even raindrops are not pure water. As they fall, they pick up tiny dust particles and also harmful substances like acid that are in the atmosphere. When the rainwater comes in contact with soil and rock material, some of that material dissolves in the water. Substances that are dissolved in water are called solutes. The concentration of natural solutes depends mainly on two factors: the composition of the soil and rock material, and how long the water is in contact with that material. Calcium makes water “hard,” although not harmful. Hard water has a noticeable taste, and it can leave deposits inside pipes and tanks. The softest and purest water comes from areas that are far from where humans live, and have rock like granite or quartz sandstone that does not dissolve easily in water.