Water is the only common substance that is naturally found as a solid, liquid or gas.

* **Solid water** is called ice. It has a fixed shape and volume.

* **Liquid water** is what we typically think of as water. It has a defined volume, but can change shape.

* **Gaseous water** is called water vapor. It has neither a defined shape nor volume.

The state of water depends on its temperature and pressure. At room temperature and pressure, water is a liquid. However, if the temperature is lowered, water will freeze and become a solid. If the temperature is raised, water will boil and become a gas.

Water’s ability to exist in three states of matter is due to its molecular structure. Water molecules are polar, which means that they have a positive and negative end. This polarity allows water molecules to interact with each other through hydrogen bonding. Hydrogen bonding is a strong intermolecular force that helps to hold water molecules together.

The strength of hydrogen bonding is affected by temperature and pressure. At low temperatures, hydrogen bonding is stronger, which is why water freezes into a solid. At high temperatures, hydrogen bonding is weaker, which is why water boils into a gas.

Water’s ability to exist in three states of matter is important for life on Earth. Water is essential for all living organisms, and its ability to change states allows it to circulate through the Earth’s biosphere.