Solid, Liquid, Gas or Plasma?

No doubt you already know that water turns into ice if it gets really cold, and that it turns back into water as it warms up again. Ice is solid water, while the stuff we normally call water should really be called "liquid water". Water can also be turned into a gas by heating it in a kettle: we call this gas steam, and steam is invisible. The stuff that you see rising out of a kettle is not actually steam, but water vapour being carried by rising steam: some of the steam turns back into tiny drops of water, and it is these droplets that you see as a mist. Mist, fog and cloud are all made of water vapour. In addition to solid, liquid and gas, there is a fourth state of matter which you can find when things get really really hot: the electrons actually detatch from their atoms and all molecules fall apart, and material in this state is a plasma. You can find plasmas in flames and in the Sun.
When a liquid cools down and turns into a solid, we say that it freezes, but different materials freeze at different temperatures: most metals melt and freeze at temperatures so high that they would burn us. When solid things turn into liquid, they melt. When liquids get hot and turn into gas, they evaporate, and when gases cool and turn back into liquids, they condense: if you didn't know these last two words, listen out for them and see how often you can hear people using them. Some substances jump straight from being solid to gas: carbon dioxide looks just like ordinary water ice when it is very cold, but when it gets warmer it sublimes into a gas without ever being a liquid in between. Carbon dioxide gas will also sublime back into a solid without going through a liquid phase on the way: the same word is used for both directions of the process. Water ice can also sublime directly into steam without becoming a liquid in between, so frozen clothes on a washing line can dry out at temperatures below zero if they are left out in the wind for long enough.
We will explore electricity later on, but it is worth mentioning now that plasmas (things that are too hot to be a gas) are very good at conducting electricity because the electrons detatch from their atoms and can flow around easily. If you have ever seen those special candles for birthday cakes which play a tune when you light them, they work because a flame (which is a plasma) can conduct electricity: running all the way up through the candle is a strip of material which consists of two metal strips stuck to opposite sides of an insulating layer which normally prevents electricity flowing from one metal strip into the other, but as soon as the flame is lit at the top, it allows the electricity to flow round from one side to the other, thus completing a circuit and causing the tune to play. Most adults know absolutely nothing about plasmas, so you're now way ahead of them here too.
If you heat up a substance to turn it from a solid into a liquid or from a liquid into a gas, a curious thing happens: it actually stops getting warmer for a while while it changes state. During this time it is still taking in heat energy, but without warming up at all: this energy is going directly into driving the change of state of the material, overcoming the natural tendency of atoms and molecules to stick together. If you have a thermometer, you may be able to test this for yourself by measuring the temperature of an ice cube as it melts. Its temperature will rise until it reaches zero degrees, and then it will stop at zero for a while as it melts, even though it is still taking in heat from the surroundings, and only once it has fully melted will the temperature start to rise again.
The reverse process is even more surprising. When you cool a gas down and turn it into a liquid, or cool a liquid down to turn it into a solid, again it will stop cooling while it changes state, even though it is continuing to lose heat energy to the surroundings throughout the process: it can continue to lose heat to the surroundings while staying the same temperature! It's more difficult to do experiments to show this, because it is quite normal for a liquid to cool below its freezing point without turning into a solid: it becomes a super-cooled liquid instead. When it eventually turns solid, it will then give out heat and may warm up a bit while it freezes solid: there are handwarmers you can buy which contain a chemical that does this very thing, actually getting hot while turning from liquid to solid. You can turn it back into a liquid afterwards by heating it to just above its melting point (you just put the whole pack in boiling water for five minutes or so), and then it cools back down without turning solid again, becoming a super-cooled liquid once more, ready to turn solid again when you want it to release all that stored energy. I should point out that when the energy is released and it gets hot, it doesn't get as hot as its melting point, so the heat isn't high enough to stop it turning solid. As for how you trigger it to turn solid, that's done by pressing a little metal thing inside the liquid which clicks in such a way as to create a localised high pressure in the liquid; just enough to cause the liquid at that point to turn solid, and this then leads to a chain reaction as the rest of the liquid turns solid on contact with those parts which have already turned solid, so a slow-motion wave of crystalisation gradually spreads through the rest of the liquid and turns the whole lot solid. It's wonderful to watch.
This extra heat energy that comes out of a material while it is changing state is called latent energy, and this phenomenon is also used to allow fridges and freezers to work more efficiently. A fridge uses a special kind of coolant gas which readily changes into a liquid when compressed. When it turns into a liquid it has to release latent energy in the form of heat, and this heat is released outside the fridge, round the back: there is always a radiator built onto the back of a fridge where this heat is allowed to escape. The liquid is then fed into pipes inside the fridge where it is allowed to return to a lower pressure so that it will become a gas again, and the change of state to a gas sucks in lots of heat from the interior of the fridge, thus cooling it down. The coolant is pumped round and round in circles, turning into liquid, then gas, then liquid, then gas again, and so on it goes: it captures heat while inside the fridge, and then lets it go on the outside, cooling the contents of the fridge and warming the room. Of course, the warmth eventually works its way back into the fridge through its walls and door, so it has to run from time to time to maintain the temperature difference between the inside and outside.

Try to remember the following points:-