Thermal Expansion

When a substance is heated, it usually gets a little bigger. This is called thermal expansion. Heating gives particles more energy. They move faster and spread out slightly, so the material takes up more space.

Particle Picture

Solids: Particles are packed closely in a fixed arrangement. When heated, they vibrate more and push a little farther apart, so the solid expands a small amount.

Liquids: Particles are close but can slide past each other. Heating makes them move faster and spread more than in solids, so liquids expand more than solids.

Gases: Particles are far apart and move freely. At constant pressure, heating makes them move much faster and occupy much more space, so gases expand the most.

In order of how much they expand (least to greatest):

$$\text{solids} < \text{liquids} < \text{gases}$$

Everyday Applications and Consequences

• Gaps in bridges and rails: Expansion joints are left so metal can expand on hot days without buckling.
• Overhead cables: Wires sag more in summer because they get longer when warm.
• Jar lids: Warming a metal lid makes it expand slightly, loosening it.
• Thermometers: The liquid (alcohol or mercury) expands and rises as temperature increases.
• Hot-air balloons: Heating air makes it expand and become less dense, helping the balloon rise.
• Glassware: Uneven heating can cause stress and cracks because different parts expand by different amounts.

Simple Demonstrations

• Ball and ring: A metal ball passes through a ring when cool, but after heating the ball expands and no longer fits until it cools.
• Bottle and balloon: A balloon on a bottle inflates slightly when the air inside the bottle is warmed, showing gas expansion at constant pressure.
• Bimetal strip: Two metals bonded together bend when heated because they expand by different amounts.

Common Misconceptions

• Expansion is not melting. The material stays the same state; it just changes size.
• Solids expand in all directions, not only in length.
• A hole in a heated metal sheet gets bigger, because the whole sheet expands outward.