By Kevin Lee | Updated Mar 24, 2022
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Atoms combine into molecules such as water. The strength of intermolecular forces (IMFs) determines a substance’s phase. When IMFs are weak, the substance is typically a gas at 1 atm and 25 °C (77 °F). Strong IMFs generally yield a solid under the same conditions.
In a solid, particle attraction exceeds kinetic energy and particles are tightly packed. Liquids have comparable kinetic energy and attraction, allowing particles to slide past one another. Gases contain widely spaced particles whose kinetic energy dominates over attraction.
Temperature, pressure, and composition govern phase changes. A phase diagram charts these transitions. Common processes include vaporization (liquid → gas), condensation (gas → liquid), sublimation (solid → gas), deposition (gas → solid), freezing (liquid → solid), and melting (solid → liquid).
Ice, liquid water, and water vapor share the same molecules but differ in compressibility, shape, and volume behavior. Solids and liquids resist compression; gases compress readily. Liquids and gases conform to container shapes, while solids retain their own shape. Gases also expand to fill the entire volume of their container.
