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STATES OF MATTER Part 13. The behaviour and properties of gases and liquids (fluids) and solids

13. State changes and heating and cooling curves explained - latent heat and kinetic energy of the particles

 The gas <==> liquid <==> solid sequences in terms of temperature versus time graphs on heating or cooling a substance

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry and also helpful for UK advanced level chemistry students aged ~16-18 and US grades 11-12 K12 honors.

13a. Cooling and Heating Curves and the energy changes for changes of state: gas <=> liquid <=> solid

Below the melting/freezing point, the substance is a liquid.

Between the melting/freezing point and the boiling point, the substance is a liquid.

Above the boiling point, the substance is a gas/vapour.

 (c) doc b)

2f(i) Cooling curve:

What happens to the temperature of a substance if it is cooled from the gaseous state to the solid state?

As thermal energy (heat energy) is removed from the substance (the system) by lowering the temperature, the gas will first condense at temperature Tc and then eventually solidifies at temperature Tf.

As the temperature decreases the average kinetic energy of the particle decreases.

Note the temperature stays constant during the state changes of condensing at temperature Tc, and freezing/solidifying at temperature Tf, at the horizontal sections of the graph.

These are the two points (temperatures) where the latent heat of condensation and the latent heat of freezing are removed/lost from the substance/system.

Remember, numerically (ignoring +/- signs), the latent heat of condensation is just the same as the latent heat of boiling/vapourisation and the latent heat of freezing is just the same as the latent heat of melting/fusion.

This is because all the heat energy removed on cooling at these temperatures (the latent heats or enthalpies of state change), allows the strengthening of the inter–particle forces (intermolecular bonding)  without temperature fall.

The heat loss is compensated by the exothermic increased intermolecular force attraction.

In between the 'horizontal' state change sections of the graph, you can see the energy 'removal' reduces the kinetic energy of the particles, lowering the temperature of the substance. See section 2. for detailed description of the state changes.  

A cooling curve summarises the changes:

gas ==> liquid ==> solid

For each change of state, energy must be removed, known as the 'latent heat'

Actual energy values for these physical changes of state for a range of substances are dealt with in more detail in the Energetics Notes.

(c) doc b)

2f(ii) Heating curve

What happens to the temperature of a substance if it is heated from the solid state to the gaseous state?

As thermal energy (heat energy) is added to the substance (the system) by increasing the temperature, the solid first melts at temperature Tm and then eventually the liquid boils at temperature Tb.

As the temperature increases the average kinetic energy of the particle increases.

Note the temperature stays constant during the state changes of melting at temperature Tm and boiling at temperature Tb, at the horizontal sections of the graph.

These are the two points (temperatures) where the latent heat of melting/fusion and the latent heat of boiling/vapourisation are gained/added to the substance/system.

This is because all the energy absorbed in heating at these temperatures (the latent heats or enthalpies of state change), goes into weakening the inter–particle forces (intermolecular bonding) without temperature rise.

The heat energy gain equals the endothermic thermal energy (heat energy) absorbed energy required to reduce the intermolecular forces.

In between the 'horizontal' state change sections of the graph, you can see the energy input increases the kinetic energy of the particles and raising the temperature of the substance.

See section 2. for detailed description of the state changes.  

A heating curve summarises the changes:

solid ==> liquid ==> gas

For each change of state, energy must be added, known as the 'latent heat'

Actual energy values for these physical changes of state for a range of substances are dealt with in more detail in the Energetics Notes.

comparing heating curve and cooling curve graphs explained state changes for IGCS/GCSE chemistry for AQA Edexcel OCR examinations 

A quick comparison of cooling and heating graph curves.


SPECIFIC LATENT HEATS - refer to diagram below

  • The latent heat for the state changes solid <=> liquid is called the specific latent heat of fusion (for melting or freezing).

  • The latent heat for the state changes liquid <=> gas is called the specific latent heat of vaporisation (for condensing, evaporation or boiling)

  • For more on latent heat see my physics notes on specific latent heat

How a refrigerator works

In a refrigeration the refrigerant gas is compressed to a liquid and the latent heat is released and transferred through heat exchanger tubes/fins at the back of the fridge.

The compressed liquid is pumped around in copper tubing in the inner panelling of the refrigerator where it evaporates, absorbing the latent heat of evaporation.

This completes the cycle of removing thermal (heat) energy from inside the refrigerator to the outside air - that's why you can feel warm air at the back of a refrigerator.


Learning objectives for the state changes graph when heating a solid or cooling a gas/vapour

Be able to interpret the graph for the cooling curve of a substance from gas to liquid to solid.

Be able to interpret the graph for the heating curve of a substance from solid to liquid to gas.

Be able to recognise on the heating curve where the temperature stays constant at the melting point and boiling point.

Be able to recognise on the cooling curve where the temperature stays constant at the condensation point and freezing point.

Be able to describing the state changes in terms of the latent heat thermal energy required to be added or removed to effect the change of state and this causes the temperature to remain constant as long as the state change is taking place.

Understand that increasing the temperature increases the average kinetic energy of the molecules and this decreases the attractive forces between the particles.

Understand that decreasing the temperature decreases the average kinetic energy of the molecules and this increases the attractive forces between the particles.

Be able to explain that at the melting point the interparticle forces are sufficiently weakened to allow free movement of the particles to form a liquid.

Be able to explain that at the boiling point the interparticle forces are sufficiently weakened to allow sufficient free movement of the particles to form a gas because the particles can escape the attractive forces in the liquid.


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INDEX of all my notes on the states of matter

(GCSE level and advanced pre-university level revision notes)

Detailed notes on the states of matter and their properties

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