Skipping the Liquid

Why it matters

Leave moth balls in a cupboard and after some weeks they simply vanish, without ever leaving a puddle behind. The solid turned straight into vapour, skipping the liquid stage.

Predict first

You gently warm purple iodine crystals in a covered jar. What do you see?

Iodine sublimes: the solid changes directly into vapour without becoming a liquid. On touching the cool lid, the vapour turns back into solid crystals.

• The crystals melt into a purple liquid pool first
• The solid turns straight into purple vapour, with no liquid in between — correct
• The crystals turn into water
• Nothing happens even on strong heating

What you do

Sort each solid by what it does when heated: does it sublime, or does it melt normally into a liquid first?

Check yourself

What exactly is sublimation?

Sublimation is the change of a solid directly into vapour on heating, without passing through the liquid state. Camphor, naphthalene and iodine all sublime.

A mixture of common salt and ammonium chloride is heated in a dish. How does sublimation help separate them?

Ammonium chloride sublimes on heating and rises as vapour, while common salt does not. The vapour is collected away from the salt, separating the two solids.

After ammonium chloride vapour rises and touches the cool part of the funnel, what happens to it?

When the vapour reaches a cold surface it loses heat and changes straight back into solid ammonium chloride. This lets us collect the pure sublimed solid.

• A solid changing directly into vapour without becoming a liquid — correct
• A solid melting into a liquid on heating
• A liquid boiling into a gas
• A gas cooling into a liquid
• Ammonium chloride sublimes and rises as vapour, leaving the salt behind — correct
• The salt sublimes away and the ammonium chloride is left behind
• Both substances melt into one liquid that cannot be separated
• Water is added to dissolve everything
• It cools and deposits back as solid ammonium chloride — correct
• It stays as a vapour forever and cannot be collected
• It turns into common salt
• It disappears completely