Sublimation and deposition

Philip is hanging up clothes he just washed. He has one more sweater to hang up, but there's no more space! - Why don’t you hang it outside? - Are you kidding? It’s freezing cold out there, my sweater will freeze and never dry! - Hmm, is that true? Normally, Philip would place the sweater somewhere warm to dry it — like on a radiator or in a warm room. The heat allows water molecules to escape, so that liquid water that’s on the sweater turns into water vapour.

The water evaporates and the sweater dries out. But this time, try hanging the sweater outside, Philip, and see what happens! The temperature outside is -5°C, so it is below the freezing point of water. When Philip hangs the wet sweater in such low temperatures, liquid water on it turns into a solid state — ice. The water on the sweater freezes.

As water turns into ice, the water molecules organise themselves into a solid structure. That’s why the frozen sweater feels so stiff now. But the molecules are trapped in the structure, so how can the sweater possibly dry? Despite the cold temperatures, today is a dry and sunny day. In conditions like this, the molecules in ice can absorb enough energy to break off the solid ice and float away!

So ice turns directly into water vapour, without turning into water first! If Philip’s sweater hangs outside for long enough, all the ice will be gone, and the sweater will dry out! Water vapour is transparent, so it can be hard to see this process of ice turning into water vapour. But you can sometimes notice how ice or snow disappear slowly, even if temperatures are below 0°C. They don’t actually disappear but turn into water vapour — they sublime!

Sublimation is a process in which a solid turns directly into a gas, skipping the liquid phase. Sublimation is a phase transition, just like freezing, melting, condensation and vaporisation. All other phase transitions can be reversed: freezing is the opposite of melting; vaporisation is the opposite of condensation. Is there a phase transition that is the opposite of sublimation? Yes, there is!

Just the way ice can turn into water vapour, water vapour can turn into ice too, without turning into water in between! This phase transition, when a gas turns directly into a solid, without going through a liquid phase is called deposition. If you have ever wondered where frost in a freezer comes from — the answer is deposition! The inside of your freezer, and everything you keep in there, is well below 0°C — so below the temperature at which water freezes. As soon as molecules of water vapour touch a surface this cold, they turn into ice!

The same thing can happen on other cold surfaces, especially if the air is humid. For example, in winter, frost can build up on windows or leaves and branches. In extremely cold weather, water vapour from your breath and from the air can deposit even on your hair and eyelashes! Sublimation and deposition are not as common as melting, freezing, condensation, or vaporisation, because they need certain conditions to occur. But they are still an important part of how water circulates on our planet — the water cycle. - Cool!

My sweater isn’t frozen anymore and it’s dry! I better bring it inside before water vapour deposits on it again!