Define thermal expansion of liquids

Thermal expansion of liquids means a liquid increases in volume when it is heated and decreases in volume when it is cooled. When you heat a liquid, its particles move faster and push each other farther apart. Because of this, the same amount of liquid now takes up more space. This increase in space (volume) with rise in temperature is called thermal expansion. When the liquid cools down, particles move less and come closer, so the volume shrinks.

Key terms you can remember

• Real expansion: The actual increase in volume of the liquid itself when heated.
• Apparent expansion: The observed increase when the liquid is heated in a container that also expands (like a glass or metal flask). Because the container expands too, the liquid appears to expand a bit less than its real expansion.
• Coefficient of volume expansion (β): The fractional change in volume per degree rise in temperature. Higher β means the liquid expands more for the same temperature change.

Simple formula

If a liquid of volume V₀ at temperature T₀ is heated by ΔT, its new volume is approximately:
V = V₀(1 + βΔT)

Everyday examples

• Fuel tanks: Petrol/diesel expand on hot days; tanks leave space to avoid overflow.
• Thermometers: Liquid (mercury or colored alcohol) rises in the thin tube as temperature increases.
• Cooking: Liquids rise in pots when heated due to expansion and steam formation.

Factors affecting thermal expansion

• Type of liquid: Alcohols generally expand more than water; mercury expands less.
• Temperature range: Expansion is small for tiny temperature changes and larger for big changes.
• Container material: Glass and metals also expand; this affects the apparent expansion you see.

Special note about water

Water behaves unusually between 0 °C and 4 °C. As it warms from 0 °C to 4 °C, water contracts (its volume decreases) and reaches maximum density at 4 °C. Above 4 °C, it behaves normally and expands with heat. This is why lakes don’t freeze solid from the bottom up—dense 4 °C water sinks, helping aquatic life survive winter.