At any given time, all material on Earth is in the form of either a solid, a liquid, or a gas. A solid maintains its shape because its component particles are bonded to one another and cannot move around much. A liquid doesn't have a definite shape; it can flow, be poured, and be spilled. The particles of a liquid can move past each other. And even though when the wind blows you may feel the air -- a collection of gases -- gases go largely unnoticed because so many are invisible and odorless. Charged with energy, the component particles of a gas move freely.

Changes in certain physical factors may result in phase changes. The primary phase changes are as follows: A solid can turn into a liquid when it melts or, less commonly, directly into a gas as a result of sublimation. A liquid can become a gas when it boils or a solid when it freezes. A gas can change into a liquid due to condensation or, less commonly, directly into a solid as a result of disposition. No matter how many times a substance changes from one phase to another, it always remains the same substance.

Temperature is a key determinant of phase. An increase in temperature adds energy that causes molecules to become more active, which breaks the bonds between them. By contrast, a decrease in temperature slows particle movement and allows bonds to reform. Every substance has a critical temperature at which its bonds break or reform and it therefore changes from one phase to another. These temperatures differ, sometimes greatly, from one substance to another. At standard pressure, water changes from a liquid to a solid at 0° C (32°F) and from a liquid to a gas at 100°C (212°F). By comparison, acetic acid, of which household vinegar is a diluted form, freezes at about 17°C (63°F) and boils at 118°C (244°F), and iron freezes at 1535°C (2795°F) and boils at 2750°C (4982°F).

Life in our solar system requires liquid water, which provides the medium in which certain critical biochemical reactions occur. (Particles in a solid medium move far too slowly for such reactions to reliably occur, and in a gas medium they move far too freely.) Because water remains liquid over a temperature range of 100°C (180°F), it provides a relatively consistent fluid medium. Ammonia, which is also abundant on Earth, only exists as a liquid within a narrow range of 30°C (54°F), and it is therefore a less suitable medium to support life as we know it.

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