An object is more than the sum of its parts. A glacier, a cloud and a lake are made of the same molecule: water. However, they seem very different to us. Essentially, this is because of the way those molecules are arranged in each case. In a gas, molecules are free to move and occupy all the space available to them. In a liquid, they are mostly free to move but they are also attached to eachother so they stay together and, although the shape of a liquid changes, the volume it occupies stays constant. Finally, in a solid, molecules can’t move, so they usually have a fix shape.

To describe all the different ways in which molecules of water (or of any other kind) can arrange themselves we say that these are different phases or states of matter. A rough classification is to say that there are 3 states of matter: gas, liquid and solid, but in fact there are many more.

In most solids, the molecules are not only not free to move but they are also organized according to a pattern, in that case we call them crystals. For example, the molecules could be arranged in a cubic lattice or in an hexagonal one like oranges in a fruitshop.

If you look at the solid state of water (ice) under a microscope, you will see such crystal structure. However, depending on where you collected your ice cube you will see a different one. In fact, water can crystalize in many different ways, each of them has different macroscopic properties despite the fact that they are all solids made of the same parts: molecules of water.

Perhaps one of the best examples to illustrate how two crystaline structures made with the same kind of molecues can exhibit very different macroscopic properties is carbon. When carbon is arranged in the hexagonal lattice typical of fruits in a supermarket we call it graphite and it is a very soft material that we use to make pencils. However, it can be arranged in a more complicated lattice, based on the cubic one, and then we call it diamond, which is the hardest known mineral. To distinguish these two situations we say that they are different phases of carbon (even though both are solids).

There are basically two ways to transform matter from one phase to another: by heating/cooling or by applying pressure. Sometimes, to achieve a certain crystal structure, a combination of a given temperature/pressure is required. For example to form diamonds you need very high pressure but low temperature. Also, sometimes it is important how fast/slow the change in temperature/pressure is, or you might need to heat and cool an object several times to different temperatures to make it crystalize in a given way.

Why am I telling you all this? I hear you cry. The reason is that most of the trasformations that food undergoes in a kitchen are either due to chemical reactions or are changes of phase. In the following posts I would like to discuss a few of examples where cooking is just forcing a change of phase. But I don’t want to make this post too long, so for that you’ll have to wait until the next one.

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