[Author © Dr WP Brown PhD: Doc Brown's chemistry exam revision notes on states of matter - physical properties of gases, liquids and solids, suitable for students of UK GCSE level and international IGCSE/O level chemistry courses, ~US grades 9-10 chemistry notes [page updated Nov 29th 2025]

INDEX of all my notes on the states of matter

You need to be able to predict the state of a substance at different temperatures given appropriate data.

Below its melting point a substance is a solid.

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

(however, note that there will always some vapour above a liquid with an exposed surface)

Above its boiling point, a substance is a gas/vapour (vapor).

		FREEZING MELTING
SUBLIMING - the opposite is deposition or 'reverse sublimation'				BOILING or EVAPORATING
SUMMARY of the CHANGES of STATE between a gas, liquid and solid All mass conserved in these PHYSICAL CHANGES				CONDENSING These are NOT chemical changes !

A change of state means an interconversion between two states of matter,

usually the interchange of states: gas <=> liquid <=> solid

A 'triangular' summary of important state changes is illustrated above.

 

The possible state/phase changes and note variations of descriptor names

(a) solid ==> liquid is melting or fusing - thermal energy absorbing - endothermic

 

(b) liquid ==> gas/vapour (vapor) is boiling, evaporation or vapourisation (spellings! vaporisation, vaporization, vapourisation) - thermal energy absorbing - endothermic

 

and the reverse processes is ...

(c) gas/vapour (vapor) ==> liquid is condensation, liquefaction or liquefying (the latter often done under pressure) - thermal energy releasing - exothermic

 

(d) liquid ==> solid is freezing, solidifying or crystallising - thermal energy releasing - exothermic

 

and there is also the 'special' case of sublimation, where the liquid state is 'bypassed'.

(e) solid ==> gas is sublimation, with no intermediate liquid state (endothermic process).

The reverse process i.e.  gas ==> solid is sometimes referred to as deposition or reverse sublimation

 

We can use the state particle models and diagrams to explain changes of state and the energy changes involved. See sections 9 to 12 from the index

 

REMEMBER: These are NOT chemical changes BUT PHYSICAL CHANGES

e.g. the water molecules H₂O are just the same in ice, liquid water, steam or water vapour.

What is different, is how the water particles are arranged, and how strongly they are held together by intermolecular bonding forces in the solid, liquid and gaseous states.

 

Energy changes for these physical changes of state for a range of substances are dealt with in a section of the Energetics Notes and also here in section 14.

14. Comparison of latent heat changes in physical changes of state for different substances

Learning objectives from the summary of state changes defined and illustrated above.

Be able to predict the state of a substance at different temperatures given appropriate data i.e. below its melting point a substance is a solid, between its melting point and boiling point, the substance is a liquid and above its boiling point a substance is a gas or vapour.

Be able to define and know the physical state changes for melting, boiling, evaporation, condensing, freezing, liquefaction, crystallising (crystallizing) and subliming.

Know and be able to explain that state changes are physical changes and NOT chemical changes, no new substances are formed, the chemical composition of the material is unchanged BUT total mass is conserved i.e. no loss or gain of mass..

Know that the identical particles of a pure substance particles are arranged differently and their physical state depends on how strongly the particles are held together by chemical bonding or the weaker  intermolecular bonding forces in the solid, liquid and gaseous/vapour states.

All my UK GCSE level (~US grade 8-10) school chemistry revision notes