Specific latent heat

A teacher heats a block of wax until all the wax melts.

The teacher pours the melted wax into a container and allows the wax to cool.

Figure 7 shows how the temperature of the wax changes during a time of 20 minutes.

Four different points on the graph are labelled P, Q, R and S.

Between which two points is all the wax liquid?

Describe the change in the motion of the wax particles as the wax changes from liquid to solid.

Figure 8 shows the values of some properties of the wax.

The wax has a mass of 3.9 kg.

Using information from Figure 8, calculate the volume of the wax when it is all liquid.

Use the equation

volume =_........ ³

State, using information from Figure 8, how the volume of the wax changes when it becomes a solid.

Explain why, using Figure 8, the time from R to S is much shorter than the time from Q to R on the graph in Figure 7.

A coil of copper wire has a mass of 14.1 g.

Calculate the volume of the copper wire.

Use the equation

volume =_........

Figure 23 gives information about the density of aluminium.

Explain the difference between the density of solid aluminium and the density of liquid aluminium in terms of the arrangement of particles.

A student boils some water.

Calculate the amount of thermal energy needed to change 60.0 g of water to steam at its boiling point.

The specific latent heat of vaporisation of water, L, is 2.26 × 10⁶ J/kg.

Use the equation

amount of thermal energy =_........

*(d) A student is investigating the melting of ice.

The student has some crushed ice in a beaker at a temperature of −20 °C.

The student heats the beaker and its contents for 20 minutes.

Figure 24 is a graph of the student’s results.

Using information from the graph, describe the changes that take place in the 20 minutes shown on the graph.

Your answer should refer to

• data from the graph

• the state (solid, liquid or gas) of the contents of the beaker.

Answer lines for this question start on page 31

A coil of copper wire has a mass of 14.1 g.

Calculate the volume of the copper wire.

Use the equation

volume =_........

Figure 7 gives information about the density of aluminium.

Explain the difference between the density of solid aluminium and the density of liquid aluminium in terms of the arrangement of particles.

A student boils some water.

Calculate the amount of thermal energy needed to change 60.0 g of water to steam at its boiling point.

The specific latent heat of vaporisation of water, L, is 2.26 × 10⁶ J/kg.

Use the equation

amount of thermal energy =_........

Some students measure the volume of a lump of modelling clay using a measuring jug, as shown in Figure 8.

Using Figure 8, estimate the volume of the modelling clay in cm³.

You may assume that 1 litre = 1000 cm³.

volume =_........

Explain the difference between the term ‘specific heat capacity’ and the term ‘specific latent heat’ when applied to heating substances.

Figure 19 shows some apparatus that may be used to determine the specific heat capacity of water.

A student measures the initial temperature of the water.

The power supply is switched on for 10 minutes and then switched off.

Explain how the student should then obtain an accurate reading for the final temperature of the water, to be used in the calculation of the specific heat capacity.

*(c) A container of gas is at room temperature.

The gas is then heated.

The volume of the container remains the same.

By considering changes in velocities of the gas particles, explain how the temperature increase affects

• the average kinetic energy of the particles

• the pressure the particles exert on the walls of the container.

An electric kettle contains 1.41 kg of water at 25 °C.

The kettle is switched on.

After a while, the water reaches boiling point at 100 °C.

The specific heat capacity of water is 4200 J / kg °C.

Calculate the amount of thermal energy supplied to the water by the kettle. Give your answer to the appropriate number of significant figures.

Use an equation selected from the list of equations at the end of the paper.

The kettle is kept switched on and the water continues to boil.

After a while, the mass of the water in the kettle has decreased to 1.21 kg.

The thermal energy supplied to the water during this time was 450 000 J.

Calculate the specific latent heat of vaporisation of water.

Use an equation selected from the list of equations at the end of the paper.

A student measures the density of glass.

The student has

− a bag of marbles, all made from the same type of glass

− a weighing balance

− a plastic measuring cylinder containing water

Describe how the student could find, as accurately as possible, the density of the glass used for the marbles.

A beaker contains 0.25 kg of water at room temperature. The beaker of water is heated until the water reaches boiling point (100 °C). The specific heat capacity of water is 4200 J/kg °C. The total amount of thermal energy supplied to the water is 84 000 J.

Calculate the temperature of the water before it was heated.

Use an equation selected from the list of equations at the end of this paper.

The heating continues until 0.15 kg of the water has turned into steam. The thermal energy needed to turn the boiling water into steam is 0.34 MJ.

Calculate the specific latent heat of vapourisation of water. Use an equation selected from the list of equations at the end of this paper.

The graph in Figure 13 shows how the volume of 1 kg of water changes with temperature.

Describe how the density of water changes with temperature over the range of temperature shown in Figure 13.

Calculations are not required.

A student uses the apparatus in Figure 3 to determine the specific heat capacity of water.

State the measurements needed to calculate the specific heat capacity of water.

State two ways that the apparatus could be adapted to improve the procedure.

The student decides to measure the temperature of the water every minute while it is being heated.

Figure 4 shows a graph of the student’s results.

Predict the temperature of the water if the heating continues up to 8 minutes.

Another student decides to melt some ice.

The student melts 380 g of ice at 0 °C.

The specific latent heat of fusion of ice is 3.34 × 10⁵J/kg.

Calculate the thermal energy needed to melt the ice.

Select an equation from the list of equations at the end of this paper.