Changes of State (Edexcel GCSE Physics)

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Changes of State

  • When a substance changes state, the number of molecules in that substance doesn’t change and so neither does its mass
    • The only thing that changes is its energy

  • Unlike chemical changes, changes of state (a type of physical change) are reversible
  • In a solid:
    • The molecules are very close together and arranged in a regular pattern
    • The molecules vibrate about fixed positions

  • In a liquid:
    • The molecules are still close together (no gaps) but are no longer arranged in a regular pattern
    • The molecules are able to slide past each other

  • In a gas:
    • The molecules are widely separated - about 10 times further apart in each direction
    • The molecules move about randomly at high speeds

  • There are six changes of state that can occur between solids, liquids and gases:
    • Melting - When a solid turns into a liquid (e.g. ice to water)
    • Boiling - When a liquid turns into a gas (evaporating)
    • Condensing - When a gas turns into a liquid
    • Freezing - When a liquid turns into a solid
    • Subliming - When a solid turns into a gas

Solids liquids gases, IGCSE & GCSE Physics revision notes

Diagram showing the arrangement and motion of different states of matter 

Worked example

A student measures the mass of a beaker of water twice, leaving 24 hours between the readings. The temperature in the room remained constant between readings, however, they notice a decrease in the mass of the beaker of water.WE Decrease in mass Question image, downloadable IGCSE & GCSE Physics revision notesWhich of the following is not a correct conclusion that can be drawn from the experiment?

A      The difference in mass is equal to the mass of the water that evaporated

B      The total energy within the beaker decreased

C      The density of water in the air increased

D      The total number of water molecules in the air and water decreased

ANSWER:   D

    • A is true because the mass lost from the beaker is due to those water molecules evaporating
    • B is true because evaporation causes the most energetic particles to leave the beaker
      • The total number of particles in the beaker decreased

    • C is true because additional water molecules were added to the air, without a significant change in the volume of the air
    • D is not true because no mass is lost during evaporation - it is only changed from a liquid to gas state

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.