Changes of state: Students should be able to describe how, when substances change state (melt, freeze, boil, evaporate, condense or sublimate), mass is conserved.

The idea

In a change of state, particles gain or lose energy but no particles are created or destroyed, so total mass stays the same. Melting and freezing swap solid and liquid; boiling and evaporating swap liquid and gas; condensing swaps gas to liquid; sublimation goes straight from solid to gas. For example, 18 g of ice melts to give 18 g of water, and 18 g of water boils to give 18 g of steam. In a sealed container the mass before and after is identical. In an open container steam may escape, so the mass of what remains falls, but the mass of the whole system is still conserved.

What earns the marks

• State that during any change of state the number of particles is unchanged, so total mass is unchanged.
• Name the six changes: melting, freezing, boiling, evaporating, condensing and sublimating, and give the direction of each.
• Use a numerical example, such as 18 g ice → 18 g water → 18 g steam, to show mass before equals mass after.
• Explain that in an open system escaped gas can make the measured mass appear to decrease, but the total mass of all products is still conserved.
• Link conservation of mass to the particle model: arrangement and energy change, but particle count and identity stay the same.

Common mistakes and how to put them right

• Thinking mass is lost when a liquid evaporates in an open dish.

  Put it right: The vapour has escaped to the surroundings; the total mass of liquid plus vapour is unchanged.

• Believing boiling destroys particles or creates new ones.

  Put it right: Particles separate and move faster, but each particle still exists, so mass is conserved.

• Confusing sublimation with melting.

  Put it right: Sublimation is solid → gas directly, with no liquid stage, for example dry ice (solid carbon dioxide) forming carbon dioxide gas.

Exam tips

• Always name the starting and finishing state, for example 'liquid → gas', before explaining conservation of mass.
• If a question mentions an open container, comment on gas escaping as well as on conservation of mass.
• Use the phrase 'no particles are created or destroyed' to show the reason mass is conserved, not just the observation.