Water is one of the most prevalent molecules in the universe, covering about 71 percent of the Earth and making up about 70 percent of the human brain. It is such an important resource that settlers built towns and cities to be near sources of good water.
Most of the Earth's water is saltwater. Freshwater, the kind that most living things need, makes up only 2.5 percent of the Earth's water, and 90 percent of that is in Antarctica, mostly as ice.
Pure water has a pH of 7, meaning that it is neutral rather than acidic or basic. It is cohesive, so its molecules stick to one another, forming surface tension and allowing it to flow through plant roots and blood vessels. Water's ability to dissolve substances makes it useful in chemical reactions such as making soup, washing clothes and manufacturing steel and paper.
Water conducts electricity when it has free ions in it. About half of U.S. water usage is for producing thermoelectric power.
As important as it is to sustaining life, water causes 80 percent of pathogen-borne diseases. To be safe for drinking and bathing, water must be free of contaminants such as bacteria, pesticides and acid rain.