
Phase transitions in everyday life

Upgrade for more content
True or false? Phase transitions can help in preserving food.
Water, just like nearly everything else in the universe, is made of matter. It comes in different physical states: solid, liquid, and gas. Water changes states easily, and frequently: from ice to liquid, or from liquid to steam — or the other way round. These phase transitions are an essential part of the water cycle on Earth. Water, however, is not the only substance that changes its state — all matter does.
And when matter changes its state, it changes its properties. Let's say you want to make a plastic object, like this bottle. To do so, you need to form the plastic into the shape of a bottle. You can do this by raising the temperature of the plastic, until it melts. Liquid plastic is easier to form — it takes the shape of the mould it’s poured into.
Then, when it cools down again, it becomes solid. Solid plastic is rigid, and will keep its shape. Many other objects are made in a similar way - you melt the material, shape it inside a mold, and let it cool, to become solid again. Let’s take a look at this portable camping stove. It uses alcohol as fuel.
The liquid alcohol itself doesn’t burn, but when alcohol vapours are mixed with the air, they ignite and burn easily. Once you fill up the camping stove with alcohol, the alcohol starts to evaporate from the surface of the fuel tank. Light the stove and the alcohol vapours above the surface of the liquid, start to burn. The rest of the liquid alcohol heats up. More of the alcohol vaporises.
This feeds the flame, so you can heat your food! How about something you probably use everyday - your fridge. Have you ever wondered how it’s always cold on the inside? Built into the back of the fridge, there is a pipe filled with coolant. Depending on the pressure in this pipe, the coolant changes from liquid to gas, or from gas to liquid.
When the coolant vaporises, it absorbs heat from inside the fridge. Then, when the coolant condenses, it releases this heat to the outside. The inside of the fridge remains cold enough to store food, thanks to phase transitions of the coolant! And what about when you store food in the freezer? When water inside food freezes into ice, microorganisms that cause food to spoil, become inactive.
As long as food is frozen, it shouldn’t go bad! Phase transitions can help preserve food! Another way to preserve food with phase transition is to dry it — to dehydrate. Hot air in the oven or in a dehydrator causes water to evaporate from the food, so it can last much longer. You can actually combine freezing and drying.
First, you need to lower the temperature, so that all the water in the food freezes. Then, you need to lower the pressure, for example by sucking out the air in a vacuum chamber. When the pressure is low enough, the ice turns directly into water vapour - it sublimes. This is called lyophilisation, or freeze-drying. Nearly all moisture is removed, which makes freeze-dried food very lightweight, and possible to store without refrigeration, for months or even years!
The food can also be easily brought back to its original form by simply adding water. People often associate freeze-dried food with astronauts flying into space. But instant coffee or milk powder are freeze-dried too and you can get them in a grocery store! Can you think of any more examples of how phase transitions help us in everyday life?