Thermal properties and temperature

Key points

2.2.1 Thermal expansion of solids, liquids and gases

• Thermal expansion means that a material gets bigger when its temperature rises. When it cools, it gets smaller again (it contracts).
• Solids, liquids and gases all expand when they are heated at constant pressure.
• The mass does not change when a material expands. Its volume gets bigger, so its density decreases.
• For the same rise in temperature, gases expand the most, liquids expand less, and solids expand the least. Extended
• The particles themselves do not get bigger. When heated, they move faster and move further apart. Extended
• Solids: the particles are held in fixed positions by strong forces. When heated, they vibrate more and push each other only a little further apart. So a solid expands very little. Extended
• Liquids: the forces between particles are weaker than in a solid, and the particles are not in fixed positions. When heated, the particles move faster and can move further apart more easily. So a liquid expands more than a solid. Extended
• Gases: the particles are far apart, and there are almost no forces between them. When heated, they move much faster and nothing holds them together. At constant pressure, the gas spreads out a lot. So a gas expands the most. Extended
• Everyday applications and consequences of thermal expansion:
  • Liquid-in-glass thermometer: the liquid expands more than the glass. The liquid can only go up the narrow tube, so a small expansion gives a large movement along the scale.
  • Gaps in railway lines, and expansion joints in bridges and roads: on a hot day the metal or concrete expands. The gap gives it space to expand. Without the gap, the rails would bend and the concrete could crack.
  • Overhead power cables are hung with some slack (they sag). When they cool, they contract. The slack stops them pulling too tight and snapping.
  • Bimetallic strip (in some thermostats): two different metals are joined. One expands more than the other when heated, so the strip bends. The bending can switch a heater off when it gets hot enough.
  • A tight metal lid on a glass jar: run hot water over the lid. The metal expands more than the glass, so the lid loosens.

2.2.2 Specific heat capacity

• The internal energy of an object is the energy stored in its particles.
• When the temperature of an object rises, its internal energy increases.
• Extra detail: internal energy includes the kinetic energy of the particles and the energy stored by the forces between them.
• A rise in temperature means that the average kinetic energy of all the particles in the object has increased. The particles move faster on average. Extended
• Specific heat capacity, c: the energy needed to raise the temperature of 1 kg of a substance by 1 °C. So it is an amount of energy for each kilogram and for each degree Celsius of temperature rise. Extended
• Equation: c = ΔE / (mΔθ). Rearranged: ΔE = mcΔθ. Extended
• The unit of c is J/(kg °C). Some questions use J/(g °C). Extended
• Different substances have different specific heat capacities. A substance with a high c needs more energy for the same rise in temperature. So for equal masses that receive or lose energy at the same rate, the one with the higher c changes temperature more slowly. Extended
• Water has a high specific heat capacity. This is one reason the sea stays cooler than the beach on a sunny day. Extended
• Experiment: specific heat capacity of a solid Extended
  1. Use a metal block with two holes: one for an electric heater and one for a thermometer. Find the mass m of the block on a balance.
  2. Put a few drops of water or oil in the thermometer hole. This gives good thermal contact between the block and the thermometer.
  3. Wrap the block in insulation. This reduces the thermal energy lost to the surroundings.
  4. Read the starting temperature.
  5. Switch on the heater for a measured time. Find the energy supplied ΔE with a joulemeter, or with E = IVt (see 4.2.5) from an ammeter, a voltmeter and a stop-watch.
  6. Switch off. Keep watching the thermometer and record the highest temperature. The temperature keeps rising for a short time after switching off.
  7. Find Δθ = highest temperature − starting temperature. Then calculate c = ΔE / (mΔθ).
• Experiment: specific heat capacity of a liquid (for example water) Extended
  1. Find the mass of the liquid: weigh the empty beaker, then the beaker with the liquid, and subtract.
  2. Use an insulated container, such as a polystyrene cup with a lid.
  3. Put an immersion heater and a thermometer in the liquid.
  4. Measure the energy supplied (joulemeter, or E = IVt) and the rise in temperature.
  5. Stir the liquid before each reading, so that the temperature is the same all through the liquid.
  6. Calculate c = ΔE / (mΔθ).
• In both experiments, some thermal energy is lost to the surroundings. Some also heats the heater, the thermometer and the container. So the measured ΔE is bigger than the energy that went into the material. This makes the calculated value of c too high. Insulation and a lid reduce this error. Extended

2.2.3 Melting, boiling and evaporation

• During melting and boiling, energy is supplied but the temperature stays the same until the change of state is complete.
• The energy supplied is used to overcome (break) the forces of attraction between the particles. It does not increase their kinetic energy, so the particles do not move faster. So the internal energy increases, but the temperature stays constant.
• Melting: the energy breaks enough of the attractions that hold the particles in fixed positions. The particles can then move past each other, and the solid becomes a liquid.
• Boiling: the energy overcomes the attractions that keep the particles close together. The particles move far apart, and the liquid becomes a gas.
• On a graph of temperature against time for a substance being heated, melting and boiling show as flat (horizontal) sections.
• At standard atmospheric pressure, pure water melts at 0 °C and boils at 100 °C.
• Cooling comes first. When a gas or a liquid cools, it transfers energy to the surroundings. Its temperature falls, so its particles move more slowly.
• Condensation (gas → liquid): the forces of attraction pull the gas particles close together, and they form a liquid.
• Solidification (liquid → solid): the forces of attraction hold the particles in fixed positions in a regular pattern. The particles can now only vibrate.
• During condensation and solidification, energy is transferred to the surroundings, but the temperature stays constant until the change of state is complete. The particles do not slow down during the change of state itself.
• Example: water droplets form on the outside of a cold can of drink, or on the outside of the window of an air-conditioned room. Water vapour particles in the air hit the cold surface, lose energy and condense.
• Evaporation is when particles escape from the surface of a liquid and become a gas.
• The particles in a liquid move at different speeds. Only the more-energetic particles near the surface have enough energy to escape.
• Evaporation happens at any temperature, not only at the boiling point.
• Evaporation cools the liquid that is left behind.
• Why: the particles that escape are the ones with the most kinetic energy. So the average kinetic energy of the particles left behind falls. A lower average kinetic energy means a lower temperature. Extended
• Boiling and evaporation are different: Extended

• Evaporation is faster when: Extended
  • the temperature is higher: more particles have enough energy to escape.
  • the surface area is larger: more particles are at the surface, so more can escape each second.
  • air moves over the surface (a wind or a fan): the moving air carries away the escaped particles. So fewer of them fall back into the liquid.
• Example: washing dries fastest on a hot, windy day when it is spread out on the line. Extended
• Cooling of an object in contact with an evaporating liquid Extended
  1. The liquid evaporates, so its temperature falls.
  2. The liquid is now cooler than the object it touches. So thermal energy transfers from the object to the liquid.
  3. The object cools down.
  4. The liquid keeps evaporating, so this continues while there is liquid left.
• Example: sweat evaporates from your skin and cools your body. Wet hands feel cold in the breeze of a fan. Extended

Common mistakes

• Students write that the temperature rises while ice melts or water boils. / The mark scheme wants: the temperature stays constant; the energy is used to overcome the forces of attraction between the particles, not to increase their kinetic energy.
• Students write that evaporation only happens at 100 °C. / The mark scheme wants: evaporation happens at any temperature, from the surface of the liquid.
• Students write that the particles get bigger when a material expands. / The mark scheme wants: the particles move faster and move further apart; their size does not change. Extended
• Students define specific heat capacity as “the energy to raise the temperature by 1 °C”. / The mark scheme wants the mass as well: energy per unit mass (1 kg) per unit (1 °C) rise in temperature. Extended
• Students write the unit of c as J/kg °C or J/kg. / The mark scheme wants J/(kg °C). Extended
• Students write that energy lost to the surroundings makes the measured c too low. / The mark scheme wants: the measured ΔE includes the lost energy, so the calculated c is too high. Extended

Exam tips

• State the melting point and boiling point of water at standard atmospheric pressure: 0 °C and 100 °C.
• Describe an application of thermal expansion with the reason: say what expands, and what the design does about it (for example, “the gap gives the rail space to expand, so it does not bend”).
• Explain why the temperature stays constant during melting or boiling: the energy supplied overcomes the forces of attraction between the particles; it does not increase their kinetic energy.
• Describe condensation or solidification in two stages. First, cooling: the substance loses energy and its particles slow down. Then the change of state: the forces of attraction pull gas particles close together (condensation), or hold liquid particles in fixed positions (solidification). Energy is released, and the temperature stays constant.
• Explain the order of expansion (gas > liquid > solid) using the forces between particles and how far apart they are. Extended
• Describe a specific heat capacity experiment as numbered steps. Name the measuring instruments (balance, thermometer, joulemeter or ammeter and voltmeter, stop-watch). Say how you reduce energy losses. Extended
• Calculate with c = ΔE / (mΔθ): find Δθ first as a difference of two temperatures. Check that the mass is in kg if c is in J/(kg °C). Extended
• Explain why evaporation cools: “the most energetic particles escape, so the average kinetic energy of the particles left falls”. Extended
• A typical 1-mark answer: “Evaporation is the escape of the more-energetic particles from the surface of a liquid.”
• A typical 2-mark answer to “Why do railway lines have small gaps?”: “The steel expands on hot days (1). The gaps give it room to expand, so the rails do not bend (1).”
• A typical 3-mark answer to “State three differences between boiling and evaporation”: “Boiling happens only at the boiling point; evaporation happens at any temperature (1). Boiling happens all through the liquid; evaporation only at the surface (1). Boiling forms bubbles; evaporation does not (1).” Extended