Tests and Importance of Carbohydrates

Transcript

0:00Introduction

Hello everyone! Let us start with a small mystery. On the table is a white powder with no label. It could be glucose, it could be sucrose, it could be starch; all three look the same. And in a chemistry lab we never taste anything, right? So how does a chemist find out what it is? The answer is a set of colour tests: a violet ring, a silver mirror, a red precipitate, a blue colour. Each colour answers one question about the sugar. Questions on these tests do appear in JEE, and by the end of this lesson you will solve this mystery yourself. So, let's begin.

0:37Molisch's test: is it a carbohydrate at all?

So, we have our white powder. Before we ask which sugar it is, what is the very first question we should ask about it? Is it a carbohydrate at all? Yes, exactly, that question comes first. For that we use Molisch's test. We add alpha-naphthol to the solution; alpha is the Greek letter you see on screen. Then we pour concentrated sulphuric acid, H two S O four, slowly down the side of the tube. The short word before it simply means concentrated, and the small lowered numbers count the atoms. Okay? And what do we see? We see a violet ring. This test is given by all carbohydrates. So the violet ring tells us only that it is a carbohydrate, and not which one. Clear?

1:22Is it reducing? Tollens' and Fehling's tests

Now the next question: is it a reducing sugar? First, Tollens' reagent. It is ammoniacal silver nitrate, A g N O three on screen; ammoniacal means it is made with ammonia. The arrow means gives, so a reducing sugar gives a shining silver mirror. Okay? Then Fehling's and Benedict's solutions. Each is an alkaline complex of the copper ion, C u two plus; the raised two plus is its charge. With a reducing sugar, this copper is reduced to C u two O, cuprous oxide, which comes down as a red precipitate. Right? Both tests are given only by reducing sugars: all the monosaccharides, and the disaccharides maltose and lactose. Now tell me, sucrose, our cane sugar: will it give a silver mirror with Tollens' reagent? The answer is no, because sucrose is a non-reducing sugar; both its reducing groups are used up in the linkage. So there is no silver mirror, and no red cuprous oxide with Fehling's, while glucose gives both. So two of our white powders, glucose and sucrose, are told apart with one test. Many students say yes here, just because sucrose is a sugar, so be careful.

2:30Barfoed's test: monosaccharide or disaccharide?

Got it? Now a harder question. Glucose and maltose both reduce Fehling's solution. So how can we tell a monosaccharide from a reducing disaccharide? We use Barfoed's test. The reagent is copper acetate in ethanoic acid. So this copper reagent is weakly acidic, not alkaline like Fehling's. With me? In this reagent, monosaccharides give red C u two O within minutes, while reducing disaccharides such as maltose react only slowly. So here the speed is the clue: a quick red means a monosaccharide. Okay?

3:07Seliwanoff's test and the iodine test

Next question: is the sugar a ketose? For that we use Seliwanoff's test, with resorcinol in H C l, which is hydrochloric acid. Ketoses give a red colour. Clear? So fructose, a ketose, gives the red colour. But sucrose also gives it, on warming. Sucrose is not a ketose, so why does it turn red? Yes, exactly, the hydrochloric acid in the reagent hydrolyses sucrose and sets free fructose, which is a ketose. So the red colour really comes from fructose. Right? And the last test is the simplest. Iodine solution gives a blue colour with starch. But glycogen, the animal starch, gives a red-brown colour, not blue, so watch that one. Got it?

3:54Topper's trick: one question, one test

Now let me share a trick that toppers use. Do not learn these tests as a long list; ask one question per test, in order. First Molisch's test: is it a carbohydrate at all? Okay? Then the iodine test: is it starch? And Tollens' or Fehling's: is it reducing? Right? And then Barfoed's: is it a monosaccharide? And Seliwanoff's: is it a ketose? Each test answers just one question, so a long identification problem becomes a few short steps. With me?

4:28Why carbohydrates matter

Now, why do carbohydrates matter so much? First, they are our chief source of energy, as starch and glucose, and honey is an instant source of energy. Plants store their energy as starch. So tell me, what do animals store it as? It is glycogen, the animal starch, yes. So starch is the storage material of plants, and glycogen is the storage material of animals. Okay? Next, structure. Cellulose is the structural material of cell walls. Clear? Then two sugars with five carbon atoms, ribose and two-deoxyribose; the word in brackets, pentoses, means five-carbon sugars. The number two in front is just a carbon number. Both are part of the nucleic acids. Right? And carbohydrates are raw material for industry too: textiles, paper, lacquer and brewing. With me?

5:20Example 1: how many sugars give red Cu2O with Fehling

Now, Example one. Read it with me. How many of these give a red precipitate with Fehling's solution: glucose, fructose, sucrose, maltose, lactose, starch and galactose? Pause the video and try it first. Are you done with it? Okay, let's check. Start with the monosaccharides in the list: glucose, fructose and galactose. But fructose is a ketose, not an aldose. Does it still reduce Fehling's solution? Yes, it does, because all monosaccharides are reducing sugars, aldoses and ketoses alike. So all three count, and that gives three. Okay? Next, maltose and lactose. Both are reducing disaccharides, so they give the red precipitate too. That makes two more. Right? And sucrose and starch are both non-reducing, so there is no red C u two O from them. Three plus two gives five, so five of the seven give the red precipitate. If you left out fructose because it is a ketose, you got four, and that is the trap. Clear?

6:26Example 2: identify a white powder from its test results

Now a harder one, and it is our mystery powder. Read it with me. The white powder gives a violet ring in Molisch's test, a blue colour with iodine, and no reaction with Fehling's solution. After boiling with dilute acid, it reduces Fehling's solution. Identify it. Pause the video and try it first. Are you done with it? Okay, let's check, one question at a time, just like our trick. Violet ring in Molisch's test, so it is a carbohydrate. With me? Now the Fehling's clues. No reaction at first, so it is non-reducing. But after boiling with dilute acid, it does reduce Fehling's. That sounds exactly like sucrose, right? So is it sucrose? It is not sucrose, so be careful here. We skipped one clue, the blue colour with iodine, and only starch gives that. Sucrose does not. So the powder is starch. Never ignore a clue; that is the trap. Okay? Now check the Fehling's clues against starch. Starch is non-reducing, so there is no red at first. Boiling with dilute acid hydrolyses it to glucose, which reduces Fehling's. Every clue fits, and our mystery from the start is solved. See how fast that was?

7:42Quick revision: carbohydrate tests and their importance

So, quick revision. Molisch's test is given by all carbohydrates, while Tollens', Fehling's and Benedict's tests are given by reducing sugars. Barfoed's picks out monosaccharides by their speed, Seliwanoff's picks out ketoses, and iodine turns starch blue. And carbohydrates give us energy, storage, cell walls and parts of the nucleic acids, and they serve industry too. Now go and try the practice problems on this page. All the best!