Changes of State (Edexcel IGCSE Physics)

A cooling tower is designed to transfer thermal energy away from a power station.

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Thermal energy from the power station warms the air inside the cooling tower. Air enters through holes at the bottom of the cooling tower and leaves through the top.

Explain why the air moves as shown by the arrows.

Hot water from the power station is sprayed into the cooling tower, as shown. As it falls through the air, some of the hot water evaporates. The rest of the water is collected and returned as cold water to the power station.

Explain how evaporation cools the water.

James Dewar was a scientist who investigated liquid oxygen.

He discovered that the boiling point of liquid oxygen is –183 °C.

Convert –183 °C to a temperature on the Kelvin scale.

temperature = ....................... 

Use ideas about particles to describe the changes that happen when a liquid boils to form a gas.

Temperature can be measured using different scales.

Complete the table by inserting the missing temperatures. 

Some students measure the volume of a sample of gas at different temperatures.

The table below shows their results. 

 (i) Draw a graph to show how the volume of gas varies with temperature. 

(ii) Circle the anomalous point on your graph. 

 (iii) Use your graph to find the temperature of the gas when its volume is zero. 

temperature = ..................................... °C

The rate of evaporation of a liquid can be influenced by various factors.

State three factors that affect the rate of evaporation.

A person falls into a lake. When they climb out they feel cold due to the evaporation of the water from their body.

Explain why evaporation makes the person feel cold.

The person makes a cup of tea to get over the shock of falling into the water.

They heat water over a stove for 6 minutes until it boils, and continue to heat it for a further 3 minutes.

Explain how the temperature of the water changes during the 9 minutes of heating.

A lidded beaker contains water vapour.

Describe the movement of the particles in: 

(i) A solid box

(ii) The water vapour

The water vapour is allowed to cool and the temperature is taken at regular intervals.

Sketch a graph to show the change in temperature with time.

Explain what is meant by the specific heat capacity of a substance.

A mass of 170 g of water is needed to make a cup of masala chai tea which is brewed at an optimum temperature of 80 ºC. The water is boiled in a kettle from a temperature of 55 ºC from the tap.

The specific heat capacity of water = 4200 J kg^–1 K^–1.

Calculate the energy needed to boil the water to make a cup of masala chai tea.

energy needed = ....................................... J

These are some observations about samples of ice, water and steam. 

Explain each of the observations in terms of the arrangement and motion of the particles.

Ethyne is a substance that is a gas at room temperature. At a temperature of -81 °C, ethyne can exist as a solid, a liquid or a gas. 

This temperature is called the triple point of ethyne.  

(i) Complete the table by giving the missing temperatures.

(ii) State what happens to the average kinetic energy of the gas molecules as the temperature is lowered from room temperature to the triple point of ethyne. 

 (iii) State what happens to the volume of an ethyne molecule when the gas changes to a solid at the triple point.  

The picture shows a runner. 

As he runs, the runner gets hot. To avoid overheating, his body sweats. As the sweat evaporates, it cools his body. 

Use ideas about particles to explain why evaporation leads to cooling. 

At the end of a long race, runners are given a shiny foil sheet to wear.

This stops them cooling down too quickly. 

 (i) Suggest why a runner might cool down too quickly if he does not wear a foil sheet.

(ii) Explain how the foil sheet reduces heat loss. 

A mass of 250 g of water is needed to make a cup of tea. An input of 85 kJ is needed to boil the water. 

Calculate the initial temperature of the water. 

Specific heat capacity of water = 4200 J kg^–1 K^–1.

 initial temperature = ............................. ºC

Most kettles take between 1 to 4 minutes to boil, depending on the amount of water.

If water had a lower specific heat capacity, state two differences this would make to boiling water in an electric kettle.

This question is about determining the specific heat capacity of aluminium.

A block of aluminium has a mass of 0.32 kg and an initial temperature of 30 °C. The block is heated until it has a temperature of 75 °C. This takes 2.5 minutes.

What is the average thermal power transferred away from the metal block? 

You may take the specific heat capacity of aluminium to be 900 J kg⁻¹ K⁻¹.

A student wants to determine the specific heat capacity of aluminium. They use the apparatus shown in the diagram.

A piece of string is tied to the aluminium block so it can be transferred from the boiling water to the cold water.

Describe how this apparatus could be used to determine the specific heat capacity of aluminium.

Your answer should include:

• any additional pieces of equipment required

  • how to obtain the necessary measurements

  • how the measurements should be used to calculate the specific heat capacity of aluminium

The specific heat capacity of water can be measured using the apparatus in the diagram:

(i) State the variables which need to be measured to calculate the specific heat capacity of water.

(ii) Explain how you would measure one of these variables.

(iii) State three ways the experiment could be improved. 

The temperature of the water is recorded every minute for 6 minutes.

Plot a graph of the results.

(i) Circle the anomalous point on the graph.

(ii) State what the student should do with the anomalous reading.

Predict the temperature at: 

(i) 8 minutes

(ii) 9 minutes