[Mujo]

Hello everyone, so i got a little problem with someone experiments that we should do.

Here are the questions that I should answer and also what i could answer until now.

Questions:

1. You have a aqueous solution of sucrose. Prove sucrose in the solution!
2. Make glucose out of sucrose!
3. How would you experimentally prove the chirality of glucose solution?
4. You have a mixture of starch and cellulose. Separate the starch from cellulose!
5. You have a mixture of fat and oil. Separate fat from oil!

Now my answers until now:
1. So, in all substances that contain aldehyde group, the group is free , but in sucrose however the aldehyde group is not free. So if we combine it with Potassium (II) Di chromate, which would by a normal aldehyde group lead to a reaction, in this case there will be no reaction so we have a proof that it is sucrose.

2. So, i think i have solved this one. We put water and combine it with the sucrose. Then we add HCl and heat it a little bit. Then at the end combine it with NaOH and then we can see that the sucrose separated itself, but i cannot really explain why.

The others are a mystery for me.

[Borek]

1. There will be no reaction as well in the case of NaCl solution - so lack of reaction is hardly a proof.

2. It is called hydrolysis.

[Dan]

1. So, in all substances that contain aldehyde group, the group is free , but in sucrose however the aldehyde group is not free. So if we combine it with Potassium (II) Di chromate, which would by a normal aldehyde group lead to a reaction, in this case there will be no reaction so we have a proof that it is sucrose.

By potassium (II) dichromate, do you mean K₂Cr₂O₇?

This is called potassium dichromate (VI). Potassium (II) dichormate means that K is in the formal oxidation state +2, which I don't like the sound of.

Next problem, the hemiacetal form of sugars (or "not free" as you have phrased it) is in equilibrium with the open chain ("free") form in aqueous solution, and does show some aldehydic reactivity. You can oxidise it, especially with an oxidising agent as powerful as dichromate, which will also oxidise hydroxyl groups in the molecule.

"Proving" that you have sucrose as opposed to anything else is no easy task, and requires spectroscopic methods, and I expect that the task is not within the scope of your assignment (based on the fact that this is the high school chemistry forum). Is the task literally phrased "prove this is sucrose" or is it more specific?

[UnintentionalChaos]

1. Assuming you already know that there is some form of saccharide in solution, do the following. Use this test: http://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/seli/seli.htm It should be positive and turn red since the sucrose is hydrolyzed to fructose and glucose, the fructose being a ketose. Then test some of the same solution with benedict's reagent which should give a negative by remaining blue. Combine some HCl and the original solution and heat. The repeat the Benedict's solution test, which will now test positive for the presence of glucose and fructose. Just for kicks, you could do an iodine test for amylose, the only semi- soluble polysaccharide.

2.Heat with Some HCl as you did in the first experiment to hydrolyze into sucrose and glucose. Good luck separating them though.

4. Water. Starch is soluble, cellulose is cotton (good thing it isn't soluble).

5. Fat and oil? As in cooking (olive, canola, etc.) oil and fat (lard, tallow, etc?) They're both triglycerides. The only difference is melting point and they are probably soluble in each other to an extent. Or do you mean petroleum oils? Specify please.