Thermal Properties and Temperature

Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)

Thermal Expansion of Solids, Liquids and Gases

When most substances are heated, they expand (get bigger); when they are cooled, they contract (get smaller). This is called thermal expansion.

heating a substance gives its particles more kinetic energy, so they move faster
- in a solid, the particles vibrate more strongly about their fixed positions
- in a liquid or gas, the particles move faster and, on average, spread further apart
this increases the average distance between particles, so the substance expands
cooling has the opposite effect - particles lose kinetic energy, move closer together on average, and the substance contracts
at constant pressure, all three states of matter expand on heating, but by very different amounts:
- solids expand the least
- liquids expand more than solids
- gases expand the most

small gaps are left between sections of railway track so the metal can expand in hot weather without buckling
bridges are built with expansion joints, which allow the bridge to expand and contract safely as the temperature changes throughout the year
overhead power cables sag more in summer, when they are longer due to thermal expansion, and become tighter in winter
if a structure is not given room to expand, thermal expansion can cause serious damage - for example, railway tracks can buckle, and pipes or window glass can crack, in very hot weather or if heated too quickly

Extended Only

in a solid, particles are held in fixed positions by strong forces between them and can only vibrate about those positions - heating increases the vibrations only slightly, so the increase in average particle separation, and the expansion, is small
in a liquid, the forces between particles are weaker, and particles are free to move around each other - heating lets them push slightly further apart on average, giving more expansion than in a solid
in a gas, the forces between particles are extremely weak, and the particles are already moving freely and are far apart - heating increases their speed and the space between them by a much greater amount, giving far more expansion than in a solid or liquid
this explains the order of expansion for the same rise in temperature: solid < liquid < gas

evaporation is the change of state from liquid to gas that takes place at the surface of a liquid, at any temperature - not just at the boiling point
particles in a liquid have a range of different kinetic energies
- the most energetic particles near the surface can have enough energy to overcome the attractive forces of neighbouring particles and escape into the air as vapour
because it is the most energetic particles that escape, the particles left behind have a lower average kinetic energy, so the temperature of the remaining liquid decreases
this is why evaporation has a cooling effect - for example, sweat evaporating from the skin cools the skin down

Extended Only

temperature - a higher temperature means more particles have enough energy to escape from the surface, so evaporation is faster
surface area - a larger surface area means more particles are at the surface and able to escape at any one time, so evaporation is faster
air movement over the surface - moving air (e.g. wind or a draught) carries escaped vapour particles away from the surface, stopping them from returning to the liquid, and keeping the air just above the surface less concentrated with vapour, so evaporation is faster

Extended Only

evaporation cools the evaporating liquid itself - but it can also cool an object in contact with that liquid
as the liquid evaporates, it loses its most energetic particles, so it needs a constant supply of energy to keep evaporating
this energy is drawn from anything the liquid is touching, so thermal energy flows out of the object and into the liquid, lowering the object's temperature too
example: wrapping a clay pot of water in a wet cloth and standing it in a breeze - as water evaporates from the wet cloth, it draws thermal energy from the pot and the water inside, keeping the water cool
example: sweat evaporating from the skin does not just cool the sweat itself - it also draws thermal energy from the skin underneath, cooling the body

thermal expansion: heating increases particle motion/separation, so a substance expands; gases expand most, then liquids, then solids (least)
everyday applications include expansion joints in bridges and gaps in railway track; without room to expand, structures can buckle or crack
evaporation happens at the surface of a liquid, at any temperature, as the most energetic particles escape - this cools the liquid
increasing temperature, surface area, or air movement over a liquid increases its rate of evaporation
an evaporating liquid can also cool an object it is in contact with, as thermal energy flows from the object into the liquid

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