Sublimation and deposition: skipping the liquid
Most substances go from solid to liquid to gas. A few can skip the liquid state. In sublimation a solid changes straight to a gas. Dry ice, which is solid carbon dioxide, does this at room temperature, and so does iodine. In deposition a gas changes straight to a solid, which is the reverse of sublimation. Frost is an example: water vapour in cold air turns straight into ice crystals on a surface, without first becoming drops of water. Frost is not frozen drops. Sublimation needs energy to be gained, deposition needs it to be lost. Be careful with dry ice: it is about −78 °C and can burn skin, so only adults handle it. The white mist over dry ice is not carbon dioxide gas, which cannot be seen: it is tiny droplets of water from the air that condensed on the very cold gas. Never try to sublime iodine or mothballs, because they are toxic; we only study them in words and pictures.
| Change | From → to | Heat | Example |
|---|---|---|---|
| sublimation | solid → gas | gained | dry ice, iodine |
| deposition | gas → solid | lost | frost on a cold surface |
Frost in the freezer, with a grown-up. Look at the walls of a freezer or a very cold ice-cube tray: you may see a thin white layer of frost. Describe its look with a magnifier. Then draw an arrow diagram for the water vapour in the air, and label it with the name of the change and with “heat lost”. Do not touch dry ice or ever try to sublime iodine.