Why matter changes size
All matter is made of tiny particles that are always moving. When a substance is heated, its particles gain energy and move faster and further apart, so the substance expands (gets bigger). When it is cooled, the particles lose energy and move closer together, so the substance contracts (gets smaller). The number of particles does not change, only the spaces between them.
Key idea
Heating → particles move more → expansion. Cooling → particles move less → contraction. Gases expand most, then liquids, then solids expand the least.
Order of expansion
For the same rise in temperature, gases expand the most because their particles are furthest apart and free to move. Liquids expand more than solids, and solids expand the least because their particles are tightly held.
Uses in daily life
- A thermometer works because the liquid inside expands and rises when heated.
- Gaps are left between railway tracks and in bridges so they can expand on hot days without bending.
- A tight metal lid loosens when warmed because the metal expands.
Example
Telephone and power cables are hung slightly loose. In cold weather they contract and tighten; if hung tight, they might snap.
Remember
- Heating causes expansion; cooling causes contraction.
- Gases expand most, solids least.
- The particles stay the same; only the gaps between them change.