Specific latent heat

Cool packs are used to treat injuries by cooling the injured area.

Cool packs are stored in a freezer. The chemical they contain is solid in the freezer, but it melts on contact with warm skin.

When the cool pack melts, the density of the chemical inside decreases.

Explain why the density decreases.

Use ideas about particles.

The cool pack contains 0.125 kg of a chemical with a specific latent heat of 425 000 J / kg.

Calculate the energy needed to completely melt the chemical in the cool pack.

Give your answer to 3 significant figures.

Use the Equation Sheet.

The table gives data on three chemicals which could be used in a cool pack.

Explain why most cool packs use chemical C.

A teacher wants to measure the specific latent heat of ice.

The diagram shows the equipment the teacher uses.

Here are some steps in the method for this experiment. The steps are not in the correct order.

Write numbers in the boxes next to each step to show the correct order. One is already done for you.

In one set of results 0.006 kg of ice melts.

Calculate the weight of the ice that melts.

Gravitational field strength = 10 N / kg

Use the equation: gravitational force = mass × gravitational field strength

The heater is turned on for 150 seconds.

The power of the heater is 15 W.

Calculate the energy transferred by the heater.

Use the equation: energy transferred = power × time

In another set of results 0.015 kg of ice melts.

The energy transferred by the heater is 5010 J.

Calculate the specific latent heat of ice.

Use the Equation Sheet.

Specific latent heat of ice =........ J / kg

A teacher measures the specific latent heat of ice using this method:

• Pour water at 30 °C into a beaker.

• Add the ice cube to the beaker of water.

• Measure the final temperature of the water after the ice cube melts.

Here are the first set of results from the experiment.

Starting temperature of ice cube = 0 °C Mass of water = 0.2 kg Starting temperature of water = 30 °C Final temperature of water = 14 °C The specific heat capacity of water = 4200 J / kg °C

Calculate the change in thermal energy of the water.

Use the Equation Sheet.

Give your answer to 2 significant figures.

The ice must be dried with a paper towel before starting the experiment.

Suggest why.

Describe how the terms specific latent heat and specific heat capacity are different.

The teacher then measures the density of an ice cube.

This is the method they use:

• Measure the mass of the ice cube.

• Place the ice cube in a measuring cylinder containing water.

• Measure the increase in volume of the water after the water has settled.

• Divide the mass by the increase in volume to measure density.

The teacher’s method does not give an accurate value for the density of ice.

Suggest why.

Suggest a change to the method which would give a more accurate value for the density of ice.

A 200 g bar of chocolate is melted in a saucepan.

The specific latent heat of melting chocolate is 31.2 J / g.

How much energy is needed to melt the bar of chocolate?

Use the equation: thermal energy for a change in state = mass × specific latent heat

A 0.156 J

B 6.24 J

C 156 J

D 6240 J

Place the events in the order they occurred.

Write the numbers in the boxes below. One has been done for you.

The events were peer reviewed and communicated to others.

Suggest why both are important in science.

Table 16.1 gives the density of different materials.

Describe any trends in the data in Table 16.1.

Explain the difference in density between solids and liquids.

You may draw diagrams to help your answer.

Table 16.2 gives some information about ethanol and water.

The specific latent heat of vaporisation is the energy transferred when 1 kg of a substance changes from liquid to gas.

Calculate the amount of energy needed to evaporate 0.2 kg of ethanol.

Use Table 16.2 and an equation from the Data Sheet to help you.

The energy needed to heat 1 kg of water by 1 °C is 4200 J.

The energy needed to evaporate 1 kg of water is 2 256 000 J.

This is much more than 4200 J. Explain why.

Place the events in the order they occurred.

Write the numbers in the boxes below. One has been done for you.

The events were peer reviewed and communicated to others.

Suggest why both are important in science.

Table 11.1 gives the density of different materials.

Describe any trends in the data in Table 11.1.

Explain the difference in density between solids and liquids.

You may draw diagrams to help your answer.

Table 11.2 gives some information about ethanol and water.

The specific latent heat of vaporisation is the energy transferred when 1 kg of a substance changes from liquid to gas.

Calculate the amount of energy needed to evaporate 0.2 kg of ethanol.

Use Table 11.2 and an equation from the Data Sheet to help you.

The energy needed to heat 1 kg of water by 1 °C is 4200 J.

The energy needed to evaporate 1 kg of water is 2 256 000 J.

This is much more than 4200 J. Explain why.

The specific latent heat for melting ice is 334 000 J / kg.

Calculate the thermal energy needed to melt 2.0 kg of ice.

A 167 J

B 668 J

C 167 000 J

D 668 000 J