Sublimation and the effect of pressure

A solid that skips the liquid

Light a piece of camphor on a plate. The camphor does not melt into a liquid first. It goes directly from a solid to a gas, and you smell it. Leave a block of dry ice in the open air. It becomes smaller and smaller, with white fog around it, and no liquid is left.

The change of a solid directly to a gas, without becoming a liquid, is called sublimation. The change of a gas directly to a solid is called deposition.

Substances that show sublimation include:

• camphor, naphthalene (moth balls) and ammonium chloride,
• iodine, which gives a purple gas,
• dry ice (solid carbon dioxide).

You may have seen moth balls in a cupboard become smaller and smaller over some weeks, with no liquid. That is sublimation, and the smell is the gas.

We can use sublimation to separate a mixture of a substance that sublimes and a substance that does not. For example, a mixture of ammonium chloride and sand is heated. The ammonium chloride changes to a gas, and then back to a solid on a cold surface above, and the sand stays behind.

Frost is the reverse. On a very cold night, water vapour in the air turns directly into ice crystals on the grass and on a window. This is deposition.

Dry ice

Solid carbon dioxide is called dry ice. At normal pressure it goes directly to a gas at $195\ \text{K}$ ($-78\ ^\circ\text{C}$). It is used to keep ice cream and other food cold, and it does not leave any water behind, because there is no liquid. The white fog that you see is not the gas. It is tiny drops of water and ice that form when the very cold gas cools the water vapour of the air.

Dry ice can become a liquid, but only at a high pressure: above about $5$ times the normal pressure, and at a temperature near $-57\ ^\circ\text{C}$. At the normal pressure, the liquid cannot exist, so the solid goes straight to gas. This shows that the state of matter depends on the pressure.

The effect of pressure

The state of a substance depends on temperature and on pressure. We usually state the normal pressure of the air at sea level. It is about $101\,000\ \text{Pa}$, and we call it $1$ atmosphere.

We can turn a gas into a liquid in two ways: by cooling it, and by compressing it, that is, by increasing the pressure. When we apply pressure, the particles of the gas are pushed closer. When we also cool the gas, the particles slow down, and the force between them can hold them together as a liquid.

You should find that the pressure stops rising at about $2.4$ atmospheres, and that from then on the squeezing only turns more gas into liquid.

There is a limit. Each gas has a critical temperature. Above this temperature, the particles move so fast that no pressure, however large, can make a liquid. Carbon dioxide has a critical temperature of $31\ ^\circ\text{C}$, so on a hot day above that, it cannot be liquefied by pressure alone. Cooking gas can be liquefied at room temperature, because its critical temperature is higher.

• LPG (liquefied petroleum gas) in a cooking gas cylinder is a gas at the normal pressure. It is compressed, and in the cylinder it is a liquid. When we open the valve, the pressure falls, and the liquid becomes a gas again. A litre of liquid LPG makes about $250$ litres of gas, so a small cylinder holds a lot of fuel.
• CNG (compressed natural gas) is natural gas that is compressed to a small volume, for use as a fuel in vehicles.
• Solid carbon dioxide at normal pressure turns directly to gas, as we saw. At a higher pressure it can be a liquid.
• A gas that is compressed warms up, and a gas that expands cools down. This is why a gas cylinder feels cold on the outside when the gas rushes out.

A summary of the changes

A pattern: every change that goes towards a gas, where the particles are freer, needs heat to go in. Every change in the other direction lets heat come out.

Practise with new numbers

Try it in Python

The program finds how much liquid and how much gas is in three sizes of LPG cylinder.

Common mistakes

• Saying that dry ice melts. At the normal pressure it sublimes: it goes to gas with no liquid.
• Thinking that the white fog of dry ice is carbon dioxide gas. The gas is invisible. The fog is made of small drops.
• Thinking that only cooling can liquefy a gas. Compression helps too, but only below the critical temperature.
• Confusing sublimation with evaporation. Evaporation starts from a liquid. Sublimation starts from a solid.

Remember these

• Sublimation is the change of a solid directly to a gas. Examples: camphor, naphthalene, iodine, dry ice. The reverse is deposition.
• Compression and cooling can turn a gas into a liquid.
• LPG and CNG are gases kept at a high pressure.
• Heat goes in for melting, vaporisation and sublimation, and comes out for the reverse changes.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name two substances that sublime, and say what you would see when each is warmed.

2. A mixture of sand and ammonium chloride is heated under a cold funnel. What collects on the funnel, and what is left behind?

3. Why can a lot of cooking gas be kept in a small cylinder?

4. On a day at $35\ ^\circ\text{C}$, carbon dioxide cannot be liquefied by squeezing alone. Why?