Purity, and Separation by Solubility

Transcript

0:00Introduction

Hello everyone! Okay, here is a small puzzle to start. You have an unknown solid, and it melts at one hundred and thirty two degrees Celsius. Urea also melts at one hundred and thirty two degrees Celsius. So, is your solid urea? Most of us would say yes. But careful, two different compounds can have the same melting point. So how can a chemist be sure? By the end of this video, you will know a neat test for it. This is part of the JEE syllabus, so let's begin.

0:34Why purify, and how to choose a method

So, first, why purify at all? Any compound made in the laboratory, or taken from a natural source, is almost always mixed with other things: by-products, unreacted reagents, and solvents. This mixture is our crude product. Okay? So which method do we use to purify it? That depends on the nature of the compound, and of the impurity. Right? Now we ask: are they solids or liquids, does the compound sublime, and how do their solubilities and boiling points differ? And finally, is the compound stable at its boiling point? With me?

1:10How do we know a compound is pure?

Now, after purifying, how do we check that a compound is really pure? First, a pure solid has a sharp melting point. And a pure liquid has a fixed boiling point, at a given pressure. On screen, m p is short for melting point, and b p for boiling point. Okay? Second, a pure substance gives just one spot in thin-layer or paper chromatography. Right? Now tell me, if a solid has some impurity in it, what happens to its melting point? Does it go up, or come down? It comes down, yes, exactly! Impurities usually lower the melting point, and they also make the melting range wide. So the solid no longer melts sharply; it melts over several degrees. Got it? And now, the neat test from our puzzle: the mixed melting point test. Mix the sample with an authentic specimen, that is, a sample known to be the real compound, and find the melting point of the mixture. If the melting point stays unchanged, the double arrow gives the conclusion: the two are identical.

2:14Crystallisation, step by step

Clear? Alright, now the main method of today: crystallisation. Step one is to dissolve the impure solid in the minimum amount of a hot solvent. Remember that word, minimum; we will come back to it. Okay? Step two: if the solution has coloured impurities, boil it with activated charcoal. The charcoal adsorbs the coloured impurities on its large surface, and this is done before the hot filtration. With me? Step three: filter the solution while it is still hot. Anything that did not dissolve, the insoluble impurity, stays on the filter paper. Got it? And step four: cool the clear filtrate. The compound is much less soluble cold, so it comes out as pure crystals, and we filter them off. Right? But wait, what about an impurity that is very soluble, even in the cold solvent? Where does that one go? Yes, it simply stays dissolved in the cold solution left behind, which is called the mother liquor. So the crystals come out clean. Clear?

3:18Why the minimum solvent? What makes a good solvent?

Now, a question many students get wrong. More solvent dissolves everything easily, so why not just use plenty of hot solvent? Sounds sensible, right? No, no, careful, because this is exactly the trap. Even the cold solvent keeps a little compound dissolved in the mother liquor, and that part is lost. So more solvent means more compound lost; that is the double arrow on screen. That is why we use the minimum amount. Okay? So what makes a good solvent? It dissolves the compound well when hot and poorly when cold, it does not react with it, and it is easy to remove afterwards. With me? Now, what if the impurity has a solubility close to that of the compound? Then one crystallisation is not enough, and we repeat it. Got it? And finally, how well did it work? We measure the recovery: the mass of pure crystals, over the mass of compound present, times one hundred. Clear?

4:17Fractional crystallisation

Now, a close cousin: fractional crystallisation. Here two solids, with different solubilities, are dissolved in the same solvent. When we cool the solution, which one crystallises first: the more soluble one, or the less soluble one? The less soluble one crystallises first, that is correct! More precisely, the one that is less soluble in the cold solvent. Remember this order, because it is very easy to flip it under exam pressure. Right? For example, potassium nitrate is separated from sodium chloride this way. On screen, KNO three is potassium nitrate, the small three below meaning three oxygen atoms, and N a C l is sodium chloride. Careful here: in hot water, potassium nitrate is actually far more soluble. But its solubility falls steeply on cooling, while that of sodium chloride hardly changes, so potassium nitrate crystallises out first. And para isomers are separated from ortho isomers; the small p means para, and the small o means ortho. Okay?

5:22Example 1: mixed melting point test to identify urea

Now, remember our puzzle from the start? Let's finish it. An unknown solid melts at one hundred and thirty two degrees Celsius; the small circle and C mean degrees Celsius. Mixed with authentic urea, whose melting point is also one hundred and thirty two degrees, it still melts sharply at one hundred and thirty two degrees Celsius. What is it? Pause the video and try it first. Done? Okay, let's check. The mixture still melts sharply at one hundred and thirty two degrees Celsius, so there is no drop and no wide range. Right? Now think about this: what if the unknown solid were some other compound? What would the urea do to its melting point? Yes, exactly, the urea would act as an impurity, so the melting point would usually drop and the range would widen! Here nothing dropped, so the mixed melting point test says the two are identical: the solid is urea. So our puzzle is solved: the mixed melting point settles it. Clear?

6:26Example 2: minimum hot solvent and percentage recovery

Now a harder one, with numbers. The compound is called X, okay? It dissolves to twenty five grams per hundred millilitres of a solvent at eighty degrees Celsius, but only three grams per hundred millilitres at twenty degrees Celsius. The crude sample is twenty grams: nineteen grams of the compound, and one gram of a very soluble impurity. Find the minimum solvent at eighty degrees, and the mass of pure compound obtained on cooling to twenty degrees. Pause the video and try it first. Done? Okay, let's check. Minimum solvent means just enough hot solvent to dissolve all nineteen grams at eighty degrees. Twenty five grams need one hundred millilitres, so nineteen grams need nineteen over twenty five, times one hundred. That gives seventy six millilitres. With me? Now we cool to twenty degrees. How much of the compound stays dissolved? Three grams, because the solubility is three grams, right? No, no, wait, that is the trap here! Three grams per hundred millilitres is zero point zero three grams per millilitre, and we used only seventy six millilitres. So zero point zero three times seventy six gives two point two eight grams, and that stays dissolved and is lost. Got it? The very soluble impurity, all one gram, stays dissolved too, so the crystals are pure. So the pure compound is nineteen minus two point two eight, which equals sixteen point seven two grams. The wavy equals sign means approximately, so about sixteen point seven grams. Okay? And the recovery is sixteen point seven two over nineteen, times one hundred, which gives eighty eight per cent. The rest stayed in the mother liquor, which is exactly why we used the minimum solvent.

8:19Quick revision: purity tests and crystallisation steps

Clear? So, quick revision. Pure means a sharp melting point, a fixed boiling point, and one spot in chromatography. Crystallisation means the minimum hot solvent, filter hot, cool, and filter off the crystals. And in fractional crystallisation, the solid less soluble in the cold crystallises first. Now go and try the practice problems on this page. All the best!