|I suspect that the amount of water on the earth is constant and therefore cannot be used up. Is this true? If so, would it not follow that we don't need to worry about water conservation so much as water pollution? — AP, Kansas City, MO
While the number of water molecules on earth doesn't change very much, it isn't exactly constant. Water molecules are consumed in some chemical reactions (particularly photosynthesis in plants) and produced in other reactions (particularly the burning of petroleum). However, most of the water on our planet is mixed with salt and is therefore unsuitable for drinking. The amount of fresh water on earth is not constant and it can be used up. That's why both the conservation of fresh water and the control of water pollution are important.