One gallon of water weighs 8 pounds. This weight is due to the weight of the water molecules and the density of water. Ice is less dense than water, however, so if that gallon of water were frozen, it would take up more than a gallon of volume.
Although an entire gallon of water weighs 8 pounds, a single molecule of water weighs 2.992 x 10^-23 grams. An entire mole of water is needed to weigh just 18 grams.
Salt water weighs more than fresh water, and the variance depends on how salty the water is. Sea water is very salty. However, water becomes saline after only a thousandth of the weight of the water is salt.
Another type of water that is heavier than pure water is hard water. Hard water is a result of high levels of calcium and magnesium ions in the water. The addition of these ions in the water makes it less useful. Hard water is generally not dangerous to drink, as calcium is a necessary mineral. However, hard water cannot always be used in machines as the build up of calcium or magnesium may damage the equipment. Hard water also makes it difficult to bathe because soap is unable to form a lather.