Separating and Analysing

Question 1

What is fractional distillation used to separate?

Answer 1

Miscible liquids e.g. alcohol and water

Question 2

What does fractional distillation make use of?

Answer 2

The different liquids having different boiling points

Question 3

Describe the process of fractional distillation

Answer 3

Both liquids boil but by careful heating you control the temperature of the column so that the water condenses in the column and trickles back into the flask. Only the alcohol remains as a vapour in the condenser so it condenses as pure alcohol.

Question 4

What equipment is required for fractional distillation?

Answer 4

• a bunsen burner/tripod/gauze/heatproof mat
• a flask containing the mixture of miscible liquids
• a fractional column
• a thermometer
• a condenser
• deliver tubes attached to the condenser sending water in and out

Question 5

How can you separate immiscible liquids?

Answer 5

Using a separating funnel to run the liquids off into different beakers

Question 6

Describe the process of chromatography:

Answer 6

• Place a spot of ink on some chromatography paper and hang this in a boiling tube containing a solvent, for instance water.
• The solvent gradually soaks up into the paper. Having this in a sealed tube prevents the solvent from evaporating.
• The dyes that make up the ink will appear on the paper in different positions

Question 7

How do the dyes that make up the ink differ in chromatography?

Answer 7

They will differ in:
• how strongly they stick to the paper
• how soluble they are in the solvent

Question 8

How can you tell the solubility of the separate inks and how well they stick to the paper?

Answer 8

If a dye has not moved very far from the original spot it means either it is rather insoluble in the solvent or it sticks more strongly to the paper (or both). If the dye is far up the chromatography paper it means it is very soluble in the solvent and is not very well attached to the paper.

Question 9

What is the pattern you get in the process of chromatography called?

Answer 9

A chromatogram

Question 10

how can you tell how many dyes were contained in the ink?

Answer 10

It is at least the number of dots you can see on the paper. It is possible however that one dot is made up of two dyes that have moved the same distance.

Question 11

How should you collect hydrogen and what is the test to prove it?

Answer 11

less dense than air and insoluble in water so collect it upwards into a test tube or over water.
Hydrogen pops when a lit splint is held to the mouth of a test tube as it combines explosively to the oxygen in the air.

Question 12

How should you collect oxygen and what is the test to prove it?

Answer 12

over water, relights glowing splint

Question 13

How should you collect carbon dioxide and what is the test to prove it?

Answer 13

downwards in a test tube, and it turns lime water milky or chalky

Question 14

How should you collect chlorine and what is the test to prove it?

Answer 14

denser than air so collected downwards in test tube. It is green so you can see when test tube is full.
Proof: it is a green gas and it bleaches damp litmus paper.

Question 15

How should you collect ammonia and what is the test to prove it?

Answer 15

can only be collected upwards in a test tube or gas jar as it is less dense than air but extremely soluble in water.
It turns red litmus paper blue as it is an alkaline gas

Question 16

How can you test for water? How is this possible?

Answer 16

Water turns white anhydrous copper sulfate blue.
This is because anhydrous copper sulfate lacks water of crystallisation and so dropping water onto it replaces the water of crystallisation and turns in blue.

Or alternatively water turns dry, blue cobalt chloride paper pink. Again, it doesnt have to be pure.

Question 17

Describe the process of using flame tests to show the presence of certain metal ions in a compound

Answer 17

A platinum or nichrome wire is cleaned by dipping it into concentrated hydrochloric acid and then holding it into a hot bunsen flame. This is repeated until the wire doesn’t give any colour to the flame. Dip it back into the acid and then into a tiny sample of the solid you are testing and back into the flame.

Question 18

What colours show what metal ions when in a flame?

Answer 18

Red shows lithium ions
Orange shows sodium ions
Lilac shows potassium ions
Orange-red (brick red) shows calcium ions

Question 19

What does it show when a blue precipitate is formed from a reaction with sodium hydroxide?

Answer 19

Copper (II) ions are present, the precipitate is copper hydroxide

Question 20

What does it show when a orange-brown precipitate is formed from a reaction with sodium hydroxide?

Answer 20

Iron (III) ions are present, the precipitate is ion (III) oxide

Question 21

What does it show when a green precipitate is formed from a reaction with sodium hydroxide?

Answer 21

Iron (II) ions are present, the precipitate is iron (II) hydroxide

Question 22

What does it show when no precipitate is formed from a reaction with sodium hydroxide but there is the smell of ammonia?

Answer 22

This shows the presence of an ammonia salt. Warm it for more gas to be given off. It should turn red litmus blue

Question 23

How do you test for a carbonate?

Answer 23

All acids react with carbonates to produce carbon dioxide so add dilute nitric acid to carbonate compound and look for fizzing and test the gas with limewater, if it is CO2 it will turn cloudy

Question 24

How do you test for a sulfate?

Why do you do this?

Answer 24

Add enough hydrochloric acid to make the solution acidic and then add barium chloride solution. A sulfate will produce a white precipitate of barium sulfate.

You acidify the solution to get rid of other compounds which could also produce false results of a white precipitate. e.g. a carbonate.

Question 25

How do you test for halides?

Answer 25

Make a solution of your suspected chloride, bromide or iodide and add enough nitric acid to make it acidic and add some silver nitrate solution.
• A white precipitate (of silver chloride) shows the presence of chloride ions.
• A pale cream precipitate (of silver bromide) shows the presence of bromide ions.
• A yellow precipitate (of silver iodide) shows the presence of iodide ions.

Question 26

How do you test for pure water?

Answer 26

Pure water boils at exactly 100 and freezes at exactly 0