Topics
What are Thermal Properties of Matter?
Temperature and Particle Motion
Thermal Properties and Change of State
Thermal Properties of Different Materials
Choosing Materials for Thermal Applications
Thermal Properties in Everyday Life
Applications of Thermal Properties
What are Thermal Properties of Matter?
Thermal properties describe how matter responds when it is heated, cooled, or when thermal energy is transferred through it.
Heating can increase particle motion, raise temperature, cause expansion, change the state of matter, or alter how thermal energy moves through a material.
Different materials respond differently to heating. Some heat up quickly, while others heat up more slowly. Some conduct heat well, while others act as thermal insulators.
Important thermal properties include:
- thermal expansion
- heat capacity
- specific heat capacity
- thermal conductivity
- melting and boiling behaviour
- latent heat
These properties depend on the structure of the material and the behaviour of its particles.
Temperature and Particle Motion
Temperature is related to the average kinetic energy of the particles in a substance.
- When a substance is heated, its particles generally gain energy and move or vibrate more rapidly.
- When it is cooled, particle motion decreases.
The effect depends on the state of matter.
- In solids, particles mainly vibrate around fixed positions.
- In liquids, particles can move past one another.
- In gases, particles move freely and rapidly through the available space.
Heating and Cooling Matter
When thermal energy is transferred to matter, several things may happen.
The substance may:
- increase in temperature
- expand
- melt
- evaporate or boil
- undergo other physical changes
When thermal energy is removed, the reverse may occur.
A substance may:
- cool
- contract
- condense
- freeze
The exact response depends on the substance and the conditions.
Thermal Expansion
Most materials expand when heated and contract when cooled.
This behaviour is called thermal expansion.
Heating increases particle motion.
Although the particles themselves do not necessarily become larger, their average separation may increase.
As a result, the material expands.
Thermal expansion occurs in solids, liquids and gases.
Thermal Expansion of Solids
Solids usually expand by a relatively small amount when heated.
Their particles vibrate more strongly and their average spacing increases slightly.
Even small expansion can become important in large structures.
Examples include:
- bridges
- railway tracks
- pipelines
- metal roofs
- electrical cables
Engineers must allow space for materials to expand and contract safely.
Linear Expansion
When a long solid object is heated, its length may increase.
This is called linear expansion.
For example, a metal rod becomes slightly longer when heated.
The amount of expansion depends on:
- original length
- temperature change
- material
Different materials expand by different amounts for the same temperature change.
Expansion Joints
Expansion joints are gaps or flexible sections provided in structures to allow thermal expansion.
They are used in:
- bridges
- railway tracks
- concrete roads
- pipelines
- large buildings
Without sufficient allowance for expansion, large stresses can develop and damage the structure.
Thermal Expansion of Liquids
Liquids also expand when heated.
Because liquids do not have a fixed shape, the expansion is usually observed as an increase in volume.
This property is used in some types of thermometers.
As the liquid warms, it expands and rises through a narrow tube.
Thermal Expansion of Gases
Gases generally expand much more than solids and liquids when heated, provided they are free to expand.
When a gas is heated, its particles move faster.
If the pressure remains approximately constant, the particles spread farther apart and the volume increases.
If the gas is confined to a rigid container instead, its pressure may increase.
Unequal Expansion
Different materials expand by different amounts when heated.
This difference can be useful.
A bimetallic strip, for example, is made from two different metals joined together.
When heated, one metal expands more than the other, causing the strip to bend.
Bimetallic strips are used in devices such as thermostats and thermal switches.
Heat Capacity
Different objects require different amounts of energy to produce the same temperature rise.
The heat capacity of an object is the amount of thermal energy required to raise its temperature by a certain amount.
Heat capacity depends on:
- the material
- the mass of the object
A large object generally has a greater heat capacity than a small object made from the same material.
Specific Heat Capacity
Specific heat capacity describes how much thermal energy is needed to raise the temperature of a unit mass of a substance by one degree.
It is commonly represented by the symbol c.
The relationship is:
where:
- Q = thermal energy transferred
- m = mass
- c = specific heat capacity
= change in temperature
The SI unit of specific heat capacity is:
J/(kg·K)
It may also be expressed as Joules per kilogram per degree Celsius for temperature changes.
High Specific Heat Capacity
A substance with a high specific heat capacity requires a relatively large amount of energy to change its temperature.
Water has a high specific heat capacity. This means it can absorb or release a considerable amount of thermal energy while changing temperature relatively slowly.
This property is important in:
- climate
- cooling systems
- heating systems
- the human body
- cooking
Low Specific Heat Capacity
A material with a lower specific heat capacity changes temperature more easily when energy is transferred.
Many metals have lower specific heat capacities than water. This is one reason a metal object can become hot quickly when heated.
Different materials therefore respond differently even when they receive the same amount of energy.
Thermal Conductivity
Thermal conductivity describes how easily thermal energy passes through a material by conduction.
- Materials with high thermal conductivity transfer heat readily.
- Materials with low thermal conductivity transfer heat more slowly.
Thermal conductivity is an important property when selecting materials for particular uses.
Good Conductors of Heat
Metals are generally good conductors of heat.
Examples include:
- copper
- aluminium
- iron
This is why metals are often used where rapid heat transfer is useful.
Examples include:
- cooking utensils
- heat exchangers
- radiators
- cooling systems
Thermal Insulators
Materials that conduct heat poorly are called thermal insulators.
Examples include:
- wood
- plastic
- rubber
- wool
- foam
- trapped air
Thermal insulators are useful when heat transfer needs to be reduced.
They are used in:
- building insulation
- refrigerator walls
- oven gloves
- insulated containers
- clothing
Why Trapped Air is a Good Insulator
Air is a poor conductor of heat.
When air is trapped in small spaces, it cannot circulate easily.
This reduces both conduction and convection.
Materials such as wool, foam and double-glazed windows use trapped air to reduce heat transfer.
Thermal Properties and Change of State
Heating does not always increase temperature.
Sometimes the transferred energy changes the physical state of a substance instead.
Examples include:
- melting
- boiling
- evaporation
Cooling can cause:
- condensation
- freezing
During a change of state, energy is involved in changing particle arrangement rather than simply changing particle speed.
Melting Point
The melting point is the temperature at which a solid changes into a liquid under specified conditions.
Different substances have different melting points.
For example, materials used in high-temperature applications must have melting points high enough to remain solid during operation.
Melting point is therefore an important thermal property.
Boiling Point
The boiling point is the temperature at which a liquid boils under specified pressure.
Boiling point depends on pressure.
At lower atmospheric pressure, water boils at a lower temperature.
This is why boiling behaviour changes at high altitude.
Latent Heat
During a change of state, energy can be absorbed or released without changing the temperature of the substance.
This energy is associated with latent heat.
For example, when ice melts at its melting point, energy is absorbed even though the temperature may remain constant until the melting is complete.
Similarly, energy is required to convert a liquid into a gas.
Specific Latent Heat
Specific latent heat is the amount of energy needed to change the state of a unit mass of a substance without changing its temperature.
The relationship is:
Q = mL
where:
- Q = energy transferred
- m = mass
- L = specific latent heat
Different changes of state require different amounts of energy.
Latent Heat of Fusion
The specific latent heat of fusion is the energy required to change a unit mass of a substance from solid to liquid without changing temperature.
The same amount of energy is released when the substance freezes.
Latent Heat of Vaporisation
The specific latent heat of vaporisation is the energy required to change a unit mass of liquid into gas without changing temperature.
For many substances, vaporisation requires considerably more energy than melting.
This is because particles must separate much more when a liquid becomes a gas.
Thermal Properties of Different Materials
Different materials can have very different thermal properties.
For example:
- copper conducts heat well
- wood conducts heat poorly
- water has a high specific heat capacity
- gases expand greatly when heated
- some ceramics tolerate very high temperatures
The suitability of a material depends on the combination of properties required.
Choosing Materials for Thermal Applications
Material selection is important whenever temperature or heat transfer is involved.
- A cooking pan needs a material that transfers heat efficiently.
- Its handle should usually conduct heat poorly.
- Building insulation should slow the transfer of heat.
- Engine components must tolerate high temperatures and thermal expansion.
The same thermal property may therefore be useful in one part of a system and undesirable in another.
Thermal Properties in Everyday Life
Thermal properties can be observed in many familiar situations.
Examples include:
- metal spoons becoming hot in tea
- wooden handles remaining cooler
- gaps in railway tracks
- liquid thermometers
- water warming slowly
- ice melting
- clothes helping the body retain heat
- hot-air balloons expanding when heated
These everyday observations arise from the thermal behaviour of matter.
Applications of Thermal Properties
Thermal properties are important in:
- construction
- transport
- cooking
- refrigeration
- heating systems
- electronics
- power generation
- manufacturing
- medicine
- clothing
- aerospace engineering
Understanding how materials respond to heat allows systems to be designed more safely and efficiently.
Why Thermal Properties of Matter are Important
Materials constantly experience changes in temperature.
Their response can influence:
- safety
- energy efficiency
- comfort
- durability
- performance
Understanding thermal expansion prevents structural damage.
Understanding specific heat capacity helps us manage heating and cooling.
Understanding thermal conductivity helps us choose suitable conductors and insulators.
Understanding changes of state and latent heat explains many natural and technological processes.
Key Points
- Thermal properties describe how matter responds to heating, cooling and thermal-energy transfer.
- Heating generally increases particle motion.
- Most materials expand when heated and contract when cooled.
- Solids, liquids and gases all undergo thermal expansion.
- Different materials expand by different amounts.
- Specific heat capacity describes how much energy is required to change the temperature of a unit mass.
- Water has a relatively high specific heat capacity.
- Thermal conductivity describes how readily heat passes through a material.
- Metals are generally good thermal conductors.
- Materials such as wood, foam and trapped air can act as thermal insulators.
- Matter can change state when thermal energy is transferred.
- Melting point and boiling point are important thermal properties.
- Latent heat is involved in changes of state without a temperature change.
- Specific latent heat describes the energy required to change the state of a unit mass.
- Thermal properties are important in everyday life, nature, engineering and technology.
Glossary of Key Terms
Recap of the Key Terms in Thermal Properties of Matter
- Bimetallic Strip: A strip made of two different metals joined together that bends when heated because the metals expand by different amounts.
- Boiling Point: The temperature at which a liquid changes into a gas throughout the liquid at a given pressure.
- Conduction: The transfer of thermal energy through a material by particle interactions without bulk movement of the material.
- Heat Capacity: The amount of thermal energy required to raise the temperature of an object by a certain amount.
- Latent Heat: Energy absorbed or released during a change of state without a change in temperature.
- Linear Expansion: The increase in the length of a solid when its temperature rises.
- Melting Point: The temperature at which a solid changes into a liquid under specified conditions.
- Specific Heat Capacity: The amount of thermal energy required to raise the temperature of one kilogram of a substance by one degree Celsius or one kelvin.
- Specific Latent Heat: The amount of energy required to change the state of one kilogram of a substance without changing its temperature.
- Specific Latent Heat of Fusion: The energy required to change one kilogram of a substance from solid to liquid without changing its temperature.
- Specific Latent Heat of Vaporisation: The energy required to change one kilogram of a liquid into gas without changing its temperature.
- Thermal Conductivity: A measure of how easily thermal energy passes through a material.
- Thermal Conductor: A material that allows thermal energy to pass through it easily.
- Thermal Expansion: The increase in the dimensions or volume of a material when its temperature rises.
- Thermal Insulator: A material that slows the transfer of thermal energy.
- Thermal Properties: Properties that describe how matter responds to heating, cooling and the transfer of thermal energy.
Questions and Answers
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What are thermal properties?
Thermal properties describe how matter responds to heating, cooling and the transfer of thermal energy.
Give three examples of thermal properties.
Thermal expansion, specific heat capacity and thermal conductivity are three examples.
What happens to particle motion when a substance is heated?
The particles generally move or vibrate more rapidly.
What happens to particle motion when a substance is cooled?
Particle motion generally decreases.
What is thermal expansion?
Thermal expansion is the increase in the dimensions or volume of a material when its temperature rises.
Why do most materials expand when heated?
Their particles move more vigorously and their average separation may increase.
Do solids, liquids and gases all expand when heated?
Yes. Solids, liquids and gases can all undergo thermal expansion.
Which state of matter generally expands the most when heated?
Gases generally expand much more than solids and liquids.
What is linear expansion?
Linear expansion is the increase in the length of a solid when it is heated.
What factors affect the linear expansion of a solid?
The original length, temperature change and type of material affect linear expansion.
Why are expansion joints used in bridges?
They allow materials to expand and contract without producing damaging stresses.
Why are gaps provided in railway tracks?
They provide room for the rails to expand when their temperature increases.
How is thermal expansion used in some thermometers?
A liquid expands when heated and rises through a narrow tube.
What happens to a gas when heated at approximately constant pressure?
The gas generally expands.
What happens when a gas is heated in a rigid sealed container?
Its pressure generally increases.
What is a bimetallic strip?
It is a strip made from two different metals joined together.
Why does a bimetallic strip bend when heated?
The two metals expand by different amounts.
What is heat capacity?
Heat capacity is the amount of energy required to raise the temperature of an object by a certain amount.
What factors affect the heat capacity of an object?
The mass of the object and the material from which it is made affect its heat capacity.
What is specific heat capacity?
It is the energy required to raise the temperature of one kilogram of a substance by one degree Celsius or one kelvin.
What is the formula involving specific heat capacity?
The relationship is Q = mcΔT.
What does Q represent in the specific heat capacity formula?
Q represents the thermal energy transferred.
What does c represent in the specific heat capacity formula?
c represents the specific heat capacity of the substance.
Why does water heat up relatively slowly?
Water has a relatively high specific heat capacity.
Why do many metals heat up more quickly than water?
Many metals have lower specific heat capacities than water.
What is thermal conductivity?
Thermal conductivity describes how easily thermal energy passes through a material.
Give two examples of good thermal conductors.
Copper and aluminium are good thermal conductors.
Give two examples of thermal insulators.
Wood and foam are thermal insulators.
Why is trapped air a good thermal insulator?
Air is a poor conductor of heat, and when trapped it cannot circulate easily, reducing heat transfer.
Why are metals used in cooking utensils?
They conduct thermal energy efficiently.
Why are saucepan handles often made of plastic or wood?
It is the temperature at which a solid changes into a liquid under specified conditions.
What is a melting point?
It is the temperature at which a liquid boils at a given pressure.
What is a boiling point?
Yes. The boiling point changes when pressure changes.
Does boiling point depend on pressure?
Thermal conductivity describes how easily thermal energy passes through a material.
What is latent heat?
Copper and aluminium are good thermal conductors.
What is specific latent heat?
It is the energy required to change the state of one kilogram of a substance without changing its temperature.
What is the formula for specific latent heat?
The relationship is Q = mL.
What is specific latent heat of fusion?
It is the energy required to change one kilogram of a substance from solid to liquid without changing its temperature.
What is specific latent heat of vaporisation?
It is the energy required to change one kilogram of a liquid into gas without changing its temperature.
Why are thermal properties important?
They help us choose materials and design systems safely and efficiently for heating, cooling, insulation, construction, transport and many other applications.