Water is the only substance on Earth that naturally occurs in three physical states: solid, liquid, and gas (see Figure 4). Depending on temperature and atmospheric pressure, water can change from one state to another, a process called physical phase change. Because of this, some geographical regions of the world experience humidity, rain, snow, or even a combination of all three.
Water molecules in water vapor have few hydrogen bonds and more space between them, making vapor light and less dense than water or ice. While the H₂O molecules are closer together in liquid water than in solid ice, there are fewer hydrogen bonds in liquid water than in the rigid lattice of ice. Therefore, water is fluid whereas ice is solid. This video demonstrates the three states of water molecules.
Hydrogen bonds are again the key. The number of bonds between molecules determines whether water will be a solid, liquid, or gas. In the solid state, water molecules have the maximum number of hydrogen bonds (4 per molecule), giving water the rigid characteristic of ice. In its liquid state, water has fewer hydrogen bonds, which accounts for its less-structured, fluid character.
As water changes from solid to liquid to gas, hydrogen bonds are broken, giving water molecules more freedom of movement. This cycling of water through its states is the basis for the hydrologic cycle that is essential for life on earth, purifying water and distributing it across land masses. This important cycle is discussed in the hydrologic cycle section below.