Vapours of sulphur escaping from a volcano often form a crust on the rocks. The process involved is an example of

The correct answer is C, deposition. Sulphur vapour turns straight into solid sulphur on the cooler rocks without first becoming a liquid, and a direct change from gas to solid is called deposition. Deposition is the reverse of sublimation, in which a solid turns directly into a gas. Hot volcanic gases carry sulphur vapour up through vents called fumaroles; when the vapour meets the cool air and rock around the vent, it settles as yellow sulphur crystals and builds up a crust. Frost forming on a cold window from water vapour in the air is an everyday example of the same change. Option A is wrong because condensation is the change of a gas into a liquid, such as dew forming from water vapour. Option B is wrong because precipitation in chemistry is the separation of a solid from a solution, and in weather it means rain or snow falling. Option D is wrong because evaporation is a liquid changing into a gas, the opposite direction. Exam tip: solid to gas is sublimation; gas to solid is deposition.