In science, we explain the states of matter by imagining how particles move and interact. In a solid, particles vibrate around fixed positions and strong attractions give solids a definite shape and volume. In a liquid, attractions still matter, but particles can slide past one another, so liquids keep a definite volume while taking the shape of their container. In a gas, particles spread far apart and move quickly, so gases have neither a fixed shape nor a fixed volume. The differences we see—rigid blocks, pouring water, or expanding air—come from changes in particle motion and spacing.
Comprehension Check
Why does a liquid have a definite volume but no fixed shape?
Which statement best describes a gas?
Energy, Phase Changes, and Latent Heat
Heating and cooling change particle motion and can cause matter to change state. At sea level, liquid water freezes at 0 °C and boils at 100 °C, but its temperature stays nearly constant during melting or boiling. That is because the added energy is used to break or form particle attractions instead of raising temperature; scientists call this latent heat. Some substances skip the liquid phase entirely. Solid carbon dioxide, or dry ice, sublimates at −78.5 °C, turning straight into gas. Phase changes are energy trades between particle motion and the forces that hold particles together.
Comprehension Check
Why does temperature stay nearly constant while a pure substance boils?
Which change of state skips the liquid phase?
Gases: Temperature, Pressure, and Volume
In a gas, particle collisions with container walls create pressure. If you heat a sealed container, particles move faster and strike the walls more often and with more force, so pressure rises. If the gas can expand instead, its volume increases when temperature increases. Cooling has the opposite effect. These relationships are summarized by gas laws that scientists test in labs: holding temperature constant links pressure and volume, while holding pressure constant links volume and temperature. The particle model helps us predict what will happen when we pump air into a ball, heat a can, or cool a balloon on ice.
Comprehension Check
What happens to gas pressure in a sealed container when temperature increases?
When a gas can expand at constant pressure, how does its volume change as temperature increases?
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