Thermal Properties of Matter

Explain how Fig. 4.1 shows that the melting temperature of the wax is $63^\circ\text{C}$.

The mass of the wax in the test-tube is $0.040\ \text{kg}$ and its temperature at time $t = 0$ is $21^\circ\text{C}$. The specific heat capacity of wax is $2100\ \text{J / (kg }^\circ\text{C)}$.

Using Fig. 4.1, determine the increase in the energy in the internal energy store of the wax between $t = 0$ and $t = 220\ \text{s}$. Show your working.

increase in energy in internal energy store = ______ $\text{J}$

Describe what happens to the motion of the molecules of the solid wax as the internal energy of the wax increases.

When the wax reaches its melting temperature, energy continues to be transferred thermally to the wax.

Describe, in terms of the forces between particles, why energy is required to melt the wax.

Explain, using ideas about particles, why liquids expand more than solids for the same temperature rise.

State the boiling point of water on the Kelvin scale of temperature.

boiling point = ______ $\text{K}$

The temperature remains constant as water turns from liquid to gas at the boiling point.

Explain, in terms of particles, why energy must be provided even though the temperature stays constant.

An electric heater is used to heat a sample of metal, as shown in Fig. 5.2. There is no thermal energy transferred from the metal to the surroundings during the heating.

The metal has a mass of 200 g and is initially at $15^\circ\text{C}$.

The heater is switched on for 2.0 minutes and then switched off. The maximum temperature reached by the metal is $40^\circ\text{C}$.

The power of the heater is 20 W.

Calculate the specific heat capacity of the metal.

Show your working.

specific heat capacity = ______ $\text{J / (g}^\circ\text{C)}$

State the lowest possible temperature on the Celsius scale and on the Kelvin scale.

Celsius scale ______ $^\circ\text{C}$ Kelvin scale ______ $\text{K}$

The temperature of the solid increases. The sample remains a solid.

Describe how the motion of the particles changes.

The solid melts.

State what happens to the internal energy and the temperature of the solid as it melts.

internal energy ______
temperature ______

Calculate the change in energy (internal energy) of the water as it warms up.

change in energy = ______ $\text{J}$

Calculate the specific heat capacity of iron.

specific heat capacity = ______ $\text{J / (g }^\circ\text{C)}$

Explain, in terms of the movement of particles, why this decrease in temperature occurs.

Explain why energy is needed when the water evaporates.

State two ways in which evaporation differs from boiling.

1. ______
2. ______

Define latent heat of fusion.

On Fig. 3.1, sketch a graph to show how the temperature of the metal changes with time.

A sample of a different metal Q has a greater latent heat of fusion than the sample of P.

P and Q are metals with the same melting points and the samples have the same heat capacity.

The experiment is repeated with the sample of Q. This sample is supplied with the same amount of energy per second as is supplied to the sample of P.

Explain how the graph of temperature against time for Q differs from the graph in (b)(i).

The steel rod has a mass of $2.0\text{ kg}$ and is initially at a temperature of $500^\circ\text{C}$.
It cools to $50^\circ\text{C}$ when placed in the water.

The specific heat capacity of steel is $460\text{ J / (kg }^\circ\text{C)}$.

Calculate the thermal energy (heat) lost by the steel rod as it cools to $50^\circ\text{C}$.

thermal energy = ______

State one way in which boiling is different from evaporation.

The rate of evaporation decreases as the water cools.

Explain why this happens, using ideas about molecules.

State what is inside each bubble.

The mass of water is measured at two different times, while the water is boiling steadily.

During this time

• the mass of water in the beaker decreases by 20 g
• the energy supplied to the hot-plate is 52 000 J
• the energy lost from the hot-plate and beaker to the atmosphere is 6000 J

Calculate the specific latent heat of vaporisation of water.

specific latent heat = ______

The beaker of water is taken off the hot-plate. The boiling stops but evaporation still continues and the water cools.

Explain, using ideas about molecules, how evaporation causes cooling.

State what is meant by boiling point.

The kettle contains $1.5\text{ kg}$ of water which has a specific heat capacity of $4200\text{ J / (kg }^\circ\text{C)}$.

Using Fig. 4.2, determine the increase in the internal energy of the water between $t = 0$ and $t = 250\text{ s}$.

increase in internal energy = ______

Thermal energy (heat) is transferred to the water at a constant rate but the temperature of the water increases at a rate that is not constant, as shown in Fig. 4.2.

Explain why the temperature increases in this way.

State what happens to the temperature of the water.

Explain, in terms of the molecules, why it is necessary to supply thermal energy in order to keep the water boiling.

State one example where expansion is useful.

State one example where expansion causes a problem.

Explain, using ideas about molecules, why solids expand when heated.

When equal volumes are heated through the same temperature rise, the expansions of solids, liquids and gases are different.

Complete each of the two sentences using one of these expressions:

much larger   slightly larger   much smaller   slightly smaller

1. The expansion of a solid is ______ than the expansion of a liquid.
2. The expansion of a gas is ______ than the expansion of a liquid.

The temperature of the wax changes as it cools to room temperature.

On the axes of Fig. 5.1, sketch a graph to show how the temperature changes with time $t$. Mark on the temperature axis $90^\circ\text{C}$, $58^\circ\text{C}$ and $23^\circ\text{C}$.

On the completed graph of Fig. 5.1, mark a point H, where half of the wax is solid and half is liquid.

The specific latent heat of fusion of the wax is $220\ \text{J / g}$ and the mass of the wax is 45 g.

Calculate the thermal energy released by the wax as it solidifies.

thermal energy = ______