Thermal Properties and Temperature

2026 Syllabus Objectives

By the end of this topic, you should be able to:

2.2.1 Thermal expansion of solids, liquids and gases

  • Core: Describe, qualitatively, the thermal expansion of solids, liquids and gases at constant pressure
  • Core: Describe some of the everyday applications and consequences of thermal expansion
  • Supplement: Explain, in terms of the motion and arrangement of particles, the relative order of magnitudes of the expansion of solids, liquids and gases as their temperatures rise

2.2.2 Specific heat capacity

  • Core: Know that a rise in the temperature of an object increases its internal energy
  • Supplement: Describe an increase in temperature of an object in terms of an increase in the average kinetic energies of all of the particles in the object
  • Supplement: Define specific heat capacity as the energy required per unit mass per unit temperature increase; recall and use the equation c = ΔE / (mΔθ)
  • Supplement: Describe experiments to measure the specific heat capacity of a solid and a liquid

2.2.3 Melting, boiling and evaporation

  • Core: Describe melting and boiling in terms of energy input without a change in temperature
  • Core: Know the melting and boiling temperatures for water at standard atmospheric pressure
  • Core: Describe condensation and solidification in terms of particles
  • Core: Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid
  • Core: Know that evaporation causes cooling of a liquid
  • Supplement: Describe the differences between boiling and evaporation
  • Supplement: Describe how temperature, surface area and air movement over a surface affect evaporation
  • Supplement: Explain the cooling of an object in contact with an evaporating liquid

1. Thermal Expansion

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What is thermal expansion?

Thermal expansion is when a material gets bigger (expands) when it is heated. When you heat a substance at constant pressure (meaning the pressure stays the same), three things happen:

  • Its temperature increases (it gets hotter)
  • Its volume increases (it takes up more space)
  • Its density decreases (it becomes less dense because the same amount of matter now occupies more space)

This happens because the molecules or atoms in the material start to move around faster and vibrate more vigorously when they gain energy from the heat.

Why does thermal expansion happen?

When you heat a material, you give energy to its particles (atoms or molecules). Here's what happens step by step:

  1. The particles gain kinetic energy (energy of movement)
  2. They vibrate or move faster
  3. They collide with each other more often and with more force
  4. They push each other apart
  5. The gaps between the particles increase
  6. The whole material expands (gets bigger)

Important: It is the material that expands, not the individual molecules themselves. The molecules move further apart from each other, making the substance bigger.

Thermal expansion in different states of matter (Extended tier)

Different states of matter expand by different amounts when heated:

  • Solids expand the least (smallest expansion)
  • Liquids expand more than solids (medium expansion)
  • Gases expand the most (largest expansion)

Why does this happen?

The amount of expansion depends on:

  • The distances between particles
  • The strength of forces between particles (intermolecular forces)
State of MatterMagnitude of ExpansionExplanation
SolidExpands slightlyThe particles have low energy and cannot overcome the strong intermolecular forces holding them tightly together
LiquidExpands more than solidsThe particles have enough energy to partially overcome the intermolecular forces, so they can move a bit more freely
GasExpands significantlyThe particles have high energy and can completely overcome the weak intermolecular forces, so they can move very freely

Applications of thermal expansion

Thermal expansion has some useful applications in everyday life:

1. Liquid-in-glass thermometers

A thermometer uses thermal expansion to measure temperature. Here's how it works:

  • The thermometer contains a liquid (such as mercury or colored alcohol) inside a thin glass tube called a capillary tube
  • At the bottom is a glass bulb that holds a large volume of the liquid
  • When the bulb is heated, the liquid expands and moves up the narrow tube
  • A scale along the side shows the temperature based on how far the liquid has risen

2. Temperature-activated switches (bimetallic strips)

A bimetallic strip is made from two different types of metal joined together. The two metals expand at different rates when heated.

When the strip is heated:

  • One metal expands more than the other
  • This causes the strip to bend by a predictable amount
  • The bending can be used to close or open an electrical circuit
  • This is used in thermostats (to control heating systems) and electric irons

Consequences of thermal expansion

Thermal expansion can also cause problems:

Buckling of materials

When solid materials like metal get too hot, they can buckle (bend or warp out of shape). This happens because the material tries to expand but doesn't have enough space.

Examples where buckling can occur:

  • Metal railway tracks - can buckle and bend on hot days
  • Road surfaces - can crack or buckle in summer heat
  • Bridges - metal parts can buckle if they expand too much

How to prevent buckling:

Engineers leave gaps between sections of these structures. The gaps create space for the material to expand into when it gets hot, preventing damage. For example, you can see small gaps between sections of railway track - these close up when the metal expands in hot weather.

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