Particle Motion and States of Matter
Students use particle models to explain the properties of solids, liquids, and gases and predict how temperature and pressure can change a material's state.
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Matter and Its States
Matter is anything that has mass and takes up space. The three common states of matter are solid, liquid, and gas. A solid has a definite shape and a definite volume. For example, an ice cube keeps its shape when moved to another container. A liquid has a definite volume but takes the shape of its container. Water poured from a cup into a bowl changes shape without changing its amount. A gas has neither a definite shape nor a definite volume. It spreads out to fill its container. Air inside a balloon expands throughout the available space. The state of a material depends on how its particles are arranged, how they move, and how strongly they attract one another.
Particle Arrangement
All matter is made of tiny particles, even though they are too small to see directly. In a solid, particles are packed closely in an orderly arrangement and held in nearly fixed positions by strong attractions. This arrangement helps a solid keep its shape. In a liquid, particles are also close together, but their arrangement is less orderly. They can change neighbors and slide past one another, allowing the liquid to flow. In a gas, particles are much farther apart, with mostly empty space between them. Their weak attractions allow them to spread throughout a container. For example, the particles in solid ice form an organized structure, while particles in liquid water remain close but can move around one another.
Particle Motion
Particles in every state of matter are always moving. In a solid, particles vibrate back and forth around fixed positions, but they do not travel freely through the material. In a liquid, particles move around one another, so the liquid can flow and take the shape of its container. In a gas, particles move rapidly in all directions and frequently collide with each other and the container walls. Gas particles spread out because they can move freely across the available space. For example, when perfume is sprayed in one corner of a room, its gas particles move through the air, and the smell eventually reaches other areas. Particle motion differs among states, but the particles themselves do not become larger or smaller when the state changes.
Temperature and Pressure
Temperature measures the average kinetic energy of particles. When a substance is heated, its particles usually move faster. With enough energy, particles can overcome some attractions, causing a solid to melt or a liquid to vaporize. Cooling removes energy, so particles move more slowly and may condense or freeze. Pressure is caused by particle collisions and can also affect state. Increasing pressure pushes gas particles closer together and can help a gas become a liquid, especially when the gas is cooled. Decreasing pressure can make it easier for a liquid to become a gas. For example, water boils at a lower temperature high on a mountain because air pressure is lower. Temperature and pressure work together to determine a substance’s state.
Predicting State Changes
A particle model can be used to predict state changes. Adding thermal energy usually moves matter from solid to liquid to gas. Melting changes a solid into a liquid, and vaporization changes a liquid into a gas. Removing thermal energy usually reverses the sequence. Condensation changes a gas into a liquid, and freezing changes a liquid into a solid. Some substances can also change directly between solid and gas through sublimation or deposition. To make a prediction, identify the starting state, decide whether temperature or pressure increases or decreases, and describe how particle motion and spacing will change. For example, cooling water vapor slows its particles. They move closer together and condense into liquid droplets. Further cooling may arrange them into fixed positions as solid ice.