Sublimation

Transcript

0:00Introduction

Hello everyone! Let us start with something from your own home. Think of the white naphthalene balls kept in a cupboard with clothes. After some months, they get smaller and smaller, and one day they are simply gone. But the clothes are not wet, and there is no puddle anywhere. So where did that solid go? It never melted, right? Today we will see exactly what happened, and how chemists use the very same behaviour to purify a solid. Questions on choosing the right purification method come up often in JEE, so this is worth your time. So, let's begin.

0:35What is sublimation?

So, here is the idea. Some solids, when you heat them, pass directly into vapour, without melting first. No liquid in between, not even for a moment. And that is our naphthalene mystery solved: the balls were turning straight into vapour, little by little, without ever melting. Okay? Now, what happens to that vapour when it meets a cold surface? It turns back into solid, again with no liquid in between. The forward change, solid to vapour, is called sublimation. So let me ask you this: what do we call the reverse change, vapour straight back to solid? It is called deposition, very good! Look at the line on screen: solid, then an arrow with the word sublimation written above it, going to vapour. Then a second arrow, with the word deposition above it, going back to solid. Solid to vapour is sublimation, and vapour to solid is deposition, two direct changes.

1:35Purification by sublimation

Clear? Now, how do we use this to purify a compound? Suppose we have a solid that sublimes, we call it a sublimable solid, mixed with an impurity that does not sublime, a non-sublimable impurity. With me? Step one: we heat the mixture gently. Now tell me, what happens to each part of the mixture? Yes, exactly, the sublimable solid goes off as vapour, and it leaves the mixture. Right? Then the vapour meets a cold surface, deposits there as solid again, and we simply collect it. Okay? And the impurity? It cannot sublime, so it just stays behind. Two parts, now in two different places. That's it, that is the whole method! Got it?

2:21Solids that sublime

So, which solids sublime? The course gives you a list, and you should know it well. First, camphor, naphthalene, and anthracene. Okay? Next, benzoic acid, and iodine. Right? And finally, ammonium chloride, and anhydrous aluminium chloride, written A l C l three on screen. The small three written low counts the chlorine atoms, and anhydrous means without water. Seven names in all. Learn them well, because purification questions often build a mixture around one of them.

2:55Careful: a low melting point is not enough

Clear? Now, a classic trap. Suppose a student says that sublimation purifies any compound with a low melting point. It sounds sensible, doesn't it? With a low melting point, it must be easy to heat into vapour. So tell me, is this claim true or false? It is false, so be careful here. Many solids with a low melting point simply melt when you heat them. They become a liquid, and they do not go straight to vapour. With me? So what does a solid really need? It needs a high vapour pressure below its melting point. Vapour pressure is the pressure of the vapour that the solid itself gives off. If it reaches a useful value before the solid melts, the solid passes directly into vapour. So it is the vapour pressure that matters, not merely a low melting point. Okay? So the correct statement is this: sublimation purifies sublimable solids from non-sublimable impurities. Remember this statement, because statement questions often test exactly this idea. Got it?

4:00Example 1: separating naphthalene from common salt

Now, Example one, an easy one to start. How can naphthalene be separated from common salt? Pause the video and try it first. Done? Okay, let's check. Naphthalene, the same substance as our naphthalene balls, is on our list. So it sublimes when we heat the mixture gently. Right? And common salt is sodium chloride, written N a C l on screen. It does not sublime at all. Clear? So the naphthalene vapour collects as solid on a cold surface, and the salt simply stays behind. One part sublimes, the other does not, a perfect pair for this method. With me? So the answer is sublimation. Nice and easy, and you got it, I am sure. Okay?

4:49Example 2: can naphthalene and camphor be sublimed apart?

Now a slightly harder one, Example two. Can a mixture of naphthalene and camphor be separated by sublimation? Pause the video and try it. Here is a hint: check both names against our list. Done? Okay, let's check. Naphthalene sublimes, we know that from Example one. So we heat the mixture and use sublimation again, and the answer is yes, right? No, no, wait, look at camphor. Camphor is also on our list, so camphor sublimes too. Clear? So when we heat the mixture, both go off as vapour, and both collect together on the cold surface. Nothing has been separated at all. This is the trap: sublimation works only when one part sublimes and the other does not. Got it? So the answer is no. Another method is needed, such as crystallisation. Always check both parts of the mixture before you choose sublimation. Right?

5:47Quick revision: sublimation, deposition, vapour pressure

So, quick revision. Sublimation takes a solid directly to vapour, and deposition brings the vapour back to solid. It separates a sublimable solid from non-sublimable impurities, so check both parts of the mixture. And it needs a high vapour pressure below the melting point, not just a low melting point. Now go and try the three practice problems on this page. You will find them easy. All the best!