Cambridge IGCSE Physics 0625
Chapter 2.2
Thermal Properties and Temperature
Detailed Core and Extended notes on thermal expansion, internal energy, specific heat capacity, changes of state, boiling and evaporation.
Syllabus coverage
Syllabus Checklist
Core You should be able to:
2.2.1 Thermal expansion
- Describe qualitatively the thermal expansion of solids, liquids and gases at constant pressure.
- Describe some everyday applications and consequences of thermal expansion.
2.2.2 Specific heat capacity
- Know that a rise in the temperature of an object increases its internal energy.
2.2.3 Melting, boiling and evaporation
- Describe melting and boiling in terms of energy input without a change in temperature.
- Know the melting and boiling temperatures of water at standard atmospheric pressure.
- Describe condensation and solidification in terms of particles.
- Describe evaporation in terms of the escape of more energetic particles from the surface of a liquid.
- Know that evaporation causes cooling of a liquid.
Extended Core plus Supplement:
2.2.1 Thermal expansion
- Explain, using particle motion and arrangement, the relative magnitudes of the expansion of solids, liquids and gases as their temperatures rise.
2.2.2 Specific heat capacity
- Describe a temperature increase as an increase in the average kinetic energies of all the particles in an object.
- Define specific heat capacity as the energy required per unit mass per unit temperature increase, and recall and use its equation.
- Describe experiments to measure the specific heat capacity of a solid and a liquid.
2.2.3 Melting, boiling and evaporation
- Describe the differences between boiling and evaporation.
- Describe how temperature, surface area and air movement over a surface affect evaporation.
- Explain the cooling of an object in contact with an evaporating liquid.
Essential language
Definitions
Thermal expansion
An increase in the dimensions of a material when its temperature rises.
Internal energy
The total kinetic energy and potential energy of all the particles in an object.
Specific heat capacity
The energy required per unit mass per unit temperature increase.
Thermal capacity
The energy required to increase the temperature of a complete object by one degree Celsius or one kelvin.
Melting
The change of state from solid to liquid.
Boiling
Rapid vaporisation throughout a liquid at its boiling temperature.
Evaporation
The escape of more energetic particles from the surface of a liquid.
Condensation
The change of state from gas to liquid.
Solidification
The change of state from liquid to solid. It is also called freezing.
Standard atmospheric pressure
The normal atmospheric pressure used when stating standard melting and boiling temperatures.
Core
Core Notes
2.2.1 Thermal expansion of solids, liquids and gases
Most materials expand when heated and contract when cooled. At constant pressure, heating increases particle motion and usually increases the average separation between particles. The amount of expansion depends on the state of matter and the material.
Expansion of solids
Particles in a solid are held near fixed positions by attractive forces. When the solid is heated, its particles gain kinetic energy and vibrate more vigorously. Their average separation increases slightly, causing the solid to become longer, wider or thicker.
The particles do not leave their fixed arrangement during ordinary thermal expansion. The solid remains a solid and usually expands by only a small amount.
Expansion of liquids
Particles in a liquid are close together but are able to move past one another. When a liquid is heated, its particles move more rapidly and their average separation increases. The volume of the liquid therefore increases.
Liquids usually expand more than solids for the same temperature increase. Different liquids expand by different amounts.
Expansion of gases
Gas particles are widely separated and move freely. When a gas is heated, its particles gain kinetic energy and move more rapidly.
At constant pressure, the gas expands so that the faster particles can remain farther apart. Gases generally expand much more than liquids or solids for the same temperature increase.
Applications of thermal expansion
Liquid-in-glass thermometers
A liquid-in-glass thermometer contains a liquid such as coloured alcohol in a bulb connected to a narrow capillary tube.
When the temperature increases, the liquid expands and rises along the capillary. When the temperature decreases, the liquid contracts and the level falls. A calibrated scale converts the liquid level into a temperature reading.
The capillary is narrow so that a small change in liquid volume produces a clearly visible movement of the liquid column.
Expansion joints in bridges and railway tracks
Bridges and railway tracks become warmer in hot weather and expand. Gaps, sliding joints or flexible sections are included so that the material can expand without producing dangerously large forces.
If expansion is prevented, rails may bend or buckle and bridge sections may become damaged.
Bimetallic strips
A bimetallic strip consists of two different metals bonded together. The two metals expand by different amounts when heated.
The metal that expands more becomes the outside of the curve. The strip therefore bends towards the metal that expands less.
Bimetallic strips are used in thermostats. As the temperature changes, the strip bends and opens or closes an electrical contact. This can switch a heater or another appliance on or off.
Consequences of thermal expansion
- Roads and buildings: repeated expansion and contraction can cause cracks unless expansion gaps are included.
- Overhead cables: cables expand and sag more during hot weather. They contract and become tighter during cold weather.
- Pipes: long pipes may require bends or flexible joints to prevent damage when their temperature changes.
- Glass: rapid or uneven heating can produce different amounts of expansion in different parts of the glass, causing it to crack.
- Containers: a tightly fitted metal lid may become easier to remove when warmed because the metal expands.
2.2.2 Temperature and internal energy
The internal energy of an object is the total kinetic energy and potential energy of all its particles.
- Kinetic energy is associated with particle motion or vibration.
- Potential energy is associated with the positions and separation of particles and the forces acting between them.
When the temperature of an object rises, the average kinetic energy of its particles increases. The object's internal energy therefore increases.
- In a solid, particles vibrate more vigorously about their fixed positions.
- In a liquid, particles move more rapidly while remaining close together.
- In a gas, particles move more rapidly between collisions.
2.2.3 Melting, boiling and evaporation
Melting
Melting is the change of state from solid to liquid. When a solid reaches its melting temperature, continued energy input does not increase its temperature until melting is complete.
The supplied energy is used to overcome some of the attractive forces holding particles in fixed positions. Particle potential energy increases as the arrangement changes. Since the average kinetic energy remains constant during melting, the temperature remains constant.
Boiling
Boiling is the change from liquid to gas throughout the liquid. Gas bubbles form inside the liquid and rise to the surface.
When a liquid reaches its boiling temperature, continued energy input is used to overcome attractive forces and separate the particles. The particle potential energy increases, but the average kinetic energy and temperature remain constant until boiling is complete.
Condensation
Condensation is the change of state from gas to liquid. When a gas cools, its particles lose kinetic energy and move more slowly. Attractive forces pull the particles closer together until a liquid forms.
During condensation, energy is transferred from the substance to the surroundings. Condensation is the reverse of vaporisation.
Solidification
Solidification is the change of state from liquid to solid. As a liquid cools, its particles lose kinetic energy. Attractive forces hold the particles in fixed positions, although they continue to vibrate.
Energy is transferred from the substance to the surroundings during solidification. Solidification is the reverse of melting.
Evaporation
Particles in a liquid have a range of kinetic energies. Some particles move more rapidly than others.
Evaporation occurs when more energetic particles at the surface have enough kinetic energy to overcome attractive forces and escape into the gas state. Evaporation occurs only at the surface and can happen at any temperature.
Why evaporation causes cooling
The particles that escape during evaporation have greater-than-average kinetic energy. The particles remaining in the liquid therefore have a lower average kinetic energy.
Since temperature is related to average kinetic energy, the temperature of the remaining liquid decreases. Energy may then be transferred from the surroundings into the cooler liquid.
Equations and units
Key Equations
Specific heat capacity
c is specific heat capacity in J/(kg °C), ΔE is energy transferred in J, m is mass in kg and Δθ is temperature increase in °C.
Energy transferred
Use the change in temperature, not the final temperature. A temperature change has the same numerical value in °C and K.
Thermal capacity
C is the thermal capacity of the complete object in J/°C or J/K.
Electrical energy supplied
I is current in A, V is potential difference in V and t is heating time in s. A joulemeter can measure the energy directly.
Supplement material
Extended Notes
These notes add the Supplement content. The Core notes above are not repeated.
Relative magnitudes of thermal expansion
The amount by which a substance expands depends on the arrangement, separation and motion of its particles. The usual order of expansion at constant pressure is:
gases > liquids > solids
| State | Particle arrangement and motion | Relative expansion |
|---|---|---|
| Solid | Particles are closely packed and held near fixed positions. Heating increases their vibration, but their average separation increases only slightly. | Smallest |
| Liquid | Particles remain close but can move past one another. Their average separation can increase more than in a solid. | Intermediate |
| Gas | Particles are widely separated and move freely. At constant pressure, faster particle motion produces a large increase in volume. | Largest |
This order is qualitative. The exact amount of expansion also depends on the material, the temperature change and the pressure conditions.
Temperature and average kinetic energy
Temperature is related to the average kinetic energy of all the particles in an object. When temperature increases, the particles have greater average kinetic energy.
- Particles in a solid vibrate more vigorously.
- Particles in a liquid move more rapidly and continue to slide past one another.
- Particles in a gas move more rapidly between collisions.
Particles do not all have exactly the same kinetic energy. Temperature is related to their average kinetic energy.
Two objects at the same temperature can have different internal energies. A larger object contains more particles, and the potential energy associated with particle arrangement may also differ.
Specific heat capacity
Specific heat capacity is the energy required per unit mass per unit temperature increase.
Its unit is joules per kilogram per degree Celsius, written as J/(kg °C). It may also be written as J kg−1 °C−1.
- A substance with a high specific heat capacity requires more energy per kilogram for the same temperature increase.
- A substance with a low specific heat capacity requires less energy per kilogram for the same temperature increase.
- For the same mass and rate of energy transfer, a material with a lower specific heat capacity warms more quickly.
- For the same mass and temperature decrease, a material with a high specific heat capacity transfers more energy to its surroundings.
Thermal capacity
Thermal capacity is the energy required to increase the temperature of a complete object by 1 °C or 1 K.
Thermal capacity depends on both the object's mass and the specific heat capacity of its material:
A large object may have a high thermal capacity even if the specific heat capacity of its material is not particularly high.
Experiment: specific heat capacity of a liquid
This experiment measures the energy supplied to a known mass of liquid and the resulting temperature increase.
- Place an empty beaker on a balance and zero the balance.
- Add the liquid and record its mass. Convert the mass to kilograms before using the equation.
- Place an immersion heater and thermometer into the liquid.
- Ensure the heater is submerged but does not touch the container.
- Record the initial temperature of the liquid.
- Wrap the beaker in insulating material and use a lid where possible to reduce energy transfer to the surroundings.
- Connect the heater to a power supply through a joulemeter. Alternatively, measure current, potential difference and heating time.
- Switch on the heater and stir the liquid gently so that its temperature remains uniform.
- Record the energy supplied by the joulemeter or calculate it using ΔE = IVt.
- Record the final temperature and calculate the temperature increase.
- Calculate specific heat capacity using c = ΔE ÷ (mΔθ).
- Repeat the experiment and calculate a mean value if time permits.
Experiment: specific heat capacity of a solid
A metal block with holes for an electrical heater and thermometer can be used.
- Measure the mass of the solid block in kilograms.
- Insert the electrical heater into one hole and the thermometer or temperature probe into another.
- Add a small amount of oil to the holes to improve thermal contact between the block, heater and thermometer.
- Wrap the block in insulating material to reduce energy transfer to the surroundings.
- Record the initial temperature.
- Switch on the heater and measure the energy supplied using a joulemeter or using ΔE = IVt.
- Record the final temperature and calculate the temperature increase.
- Calculate the specific heat capacity using c = ΔE ÷ (mΔθ).
Differences between boiling and evaporation
| Feature | Boiling | Evaporation |
|---|---|---|
| Temperature | Occurs at the boiling temperature for the stated pressure. | Can occur at any temperature. |
| Location | Occurs throughout the liquid. | Occurs only at the liquid surface. |
| Bubbles | Gas bubbles form throughout the liquid. | Bubbles do not form throughout the liquid. |
| Rate | Usually rapid. | Usually slower. |
| Energy | Requires a continuous energy supply to continue. | Also requires energy, taken from the liquid or surroundings even when there is no heater. |
Factors affecting evaporation
Temperature
At a higher temperature, the particles have greater average kinetic energy. A larger proportion of the surface particles therefore has enough energy to escape. Increasing temperature increases the rate of evaporation.
Surface area
Evaporation occurs at the surface. Increasing the surface area exposes more particles at the same time, so more particles can escape each second. Increasing surface area increases the rate of evaporation.
Air movement
Particles that escape form vapour above the liquid. Some vapour particles may return to the liquid.
Moving air carries vapour away from the surface. This reduces the number of particles returning to the liquid and allows evaporation to continue more rapidly.
Cooling an object using evaporation
Evaporation requires energy. If an evaporating liquid is in contact with an object, energy is transferred from the object to the liquid.
The object loses internal energy and its temperature decreases. This is why sweat cools the body. Energy is transferred from the skin to the sweat as the sweat evaporates.
Moving air increases the rate of evaporation, so a breeze or fan can increase the cooling effect.
Worked example: specific heat capacity
A 0.50 kg aluminium block receives 18 000 J of energy. Its temperature increases by 40 °C. Calculate its specific heat capacity.
Equation:
c = ΔE mΔθ
Substitution:
c = 18 000 0.50 × 40
Calculation: c = 18 000 ÷ 20
Final answer: c = 900 J/(kg °C)
The answer includes the correct specific heat capacity unit.
Exam practice
Practice Questions
Question 1
Explain why gaps are left between sections of a railway track.
- Railway tracks expand when their temperature rises.
- The gaps provide space for this expansion.
- This reduces the risk of the tracks buckling or becoming damaged.
Question 2
Describe what happens to the internal energy of an object when its temperature rises.
- The internal energy increases.
- The particles have greater average kinetic energy and move or vibrate more rapidly.
Question 3
Ice at 0 °C is melting while energy is supplied. Explain why its temperature does not increase.
- The supplied energy is used to overcome attractive forces or change the particle arrangement.
- The potential energy of the particles increases.
- The average kinetic energy does not increase, so the temperature remains constant until melting is complete.
Question 4
A 0.80 kg substance receives 24 000 J of energy and its temperature rises by 25 °C. Calculate its specific heat capacity.
Equation: c = ΔE ÷ (mΔθ)
Substitution: c = 24 000 ÷ (0.80 × 25)
Calculation: c = 24 000 ÷ 20
Final answer: c = 1200 J/(kg °C)
Question 5
Describe how to measure the specific heat capacity of a solid metal block using an electrical heater.
- Measure the mass of the block and its initial temperature.
- Insert the heater and thermometer with good thermal contact.
- Insulate the block to reduce energy transfer to the surroundings.
- Supply and measure electrical energy using a joulemeter or ΔE = IVt.
- Measure the temperature increase and calculate c = ΔE ÷ (mΔθ).
Question 6
Wet clothes dry faster on a warm, windy day. Explain the effects of temperature and air movement.
- At a higher temperature, more water particles have enough kinetic energy to escape.
- Moving air carries water vapour away from the surface.
- This reduces the number of particles returning to the liquid and increases the rate of evaporation.
Tap a card to reveal the answer
