Thermal Properties of Matter

Why does a metal spoon feel hot in tea while the handle of a wooden one stays cool? Why are gaps left in railway tracks? The answers lie in the thermal properties of matter: how substances respond to heat. This topic separates two ideas often confused (heat and temperature), and explains expansion and the ways heat travels.

These two are not the same:

• Temperature is a measure of how hot or cold a body is, reflecting the average energy of its particles. It is measured in degrees Celsius or in kelvin (K).
• Heat is a form of energy that flows from a hotter body to a colder one because of the temperature difference. It is measured in joules.

So a cup of boiling water has a higher temperature than a swimming pool, but the pool, being far larger, contains far more total heat energy. Heat always flows from high temperature to low.

Most substances expand when heated and contract when cooled. Their particles move faster and take up more space. This is thermal expansion.

Though small, it matters greatly in engineering:

• gaps are left between railway tracks and in bridges so they can expand in summer without buckling,
• overhead wires are left to sag, and
• a tight metal lid loosens when run under hot water.

Water is a famous exception: it expands when it freezes, which is why ice floats and pipes burst in winter.

Water breaks the usual rule even before it freezes. Most liquids contract steadily as they cool. Water contracts only down to 4°C. Below that, it expands as it cools further. So water is densest at 4°C.

This odd behaviour decides what happens in a frozen lake:

• At the surface: as winter air chills the lake, surface water cools and sinks, until the whole lake reaches 4°C.
• Below 4°C: further cooling makes the surface water lighter, not heavier. It stays on top, cools to 0°C, and freezes there.
• At the bottom: the densest 4°C water settles at the lake bed and stays liquid all winter.

That is why fish survive severe winters. The ice sheet forms only at the top, while the water beneath remains around 4°C. The deciding fact is the density maximum at 4°C, not merely that ice floats or that ice conducts heat poorly.

Different substances need different amounts of heat to warm up:

• Specific heat capacity is the heat needed to raise the temperature of 1 kg of a substance by 1°C. Water has a very high specific heat. It heats and cools slowly, so it is used as a coolant. Coastal climates are mild for the same reason.
• Latent heat is the heat absorbed or released during a change of state (melting or boiling). It changes the state, not the temperature. So the temperature stays constant during the change.

The boiling point of a liquid is not fixed. It depends on the pressure above the liquid. The boiling point of water rises when pressure rises and falls when pressure falls. That is why water boils below 100°C on high mountains, where the air is thin, and why food cooks slowly there.

A pressure cooker uses this fact in reverse:

• Trapped steam: the sealed pot does not let steam escape freely, so steam builds up and raises the pressure inside.
• Higher boiling point: under this higher pressure, water boils well above 100°C, so food cooks faster.
• What sets the pressure: the weight of the whistle (the vent weight) pressing on the small steam-vent hole. Steam escapes only when its pressure can lift that weight. So the inside pressure, and hence the cooking temperature, is fixed by the weight of the lid whistle and the area of the vent hole.
• What the flame does: a stronger flame only supplies heat faster. It does not raise the cooking temperature, because extra steam simply lifts the whistle and escapes.

Heat travels from hot to cold by three methods:

• Conduction: through direct contact, mainly in solids (a metal spoon heats along its length). Metals are good conductors; wood and air are poor (insulators).
• Convection: through the movement of a heated fluid. Hot air or water rises, and cooler fluid sinks to replace it. This drives boiling water and sea breezes.
• Radiation: by electromagnetic waves, needing no medium at all. This is how the Sun's heat reaches the Earth across empty space.