Because chemists always want to get the chemicals they get as pure as possible. And a good way to purify chemicals is to make crystals of them. When they are in solutions you can have all sort of impunities but when they form crystals the crystals contain much purer compounds than in the solution and the impurities are left in the solution. generally the crystal is a very precise arrangement of molecules all the same fitting together like pieces and some sort of construction toy ,the impurity has a different shape so it doesn’t fit in properly. Occasionally you are unlucky you get an impurity that’s the wrong shape and then you can’t get rid of it so you make some solutions, forms some crystals , filter the amount, redissolve them, form more crystals and so on and each time you crystallize it you get a purer and purer compound. Sometimes in the old days some people crystallized thousands of times to get something really pure. Now the problem is that when you have a solution even if you want to cool it down that is the standard ways of getting crystals to form. the crystals cant form unless you give something small some tiny bit for it to the first crystal to form and once the first one goes the whole lot goes. Sometimes it can just be dirt. There is a famous organic chemist here called William Perkin, but he was famous because he had this huge beard. And it was said he was really successful as a chemist because also some bit felt from his beard into the solution and helped the crystals grow in the solution. There are other examples when people sometimes scratch the side of the beaker, and this forms tiny fragments of glass so that they also can act as a nucleus for the crystals to form. One advantage is that crystals of a substance obtained from solution are usually relatively pure. Another is that crystal size can be controlled. One disadvantage is that the recovery of the materials in relatively pure form is usually...
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