Topic 2 - Separating Techniques

Question 1

When is simple distillation used?

Answer 1

To separate out a liquid from a solution

Question 2

Step by step process on how to use simple distillation to separate pure water from seawater

Answer 2

1. Pour seawater in distillation flask
2. Set up apparatus; connect bottom end of condenser to cold tap
3. Heat flask. Water evaporates first as it has lowest BP
4. Water cools + condenses from condenser + flows to beaker
5. Eventually just salt will be in flask

Question 3

What is a disadvantage of simple distillation?

Answer 3

Can only use it to separate things with very different boiling points

Question 4

When is fractional distillation used?

Answer 4

If you’ve got mixture of liquids with similar boiling points

Question 5

What is the fractional distillation set up?

Answer 5

1. Put mixture in flask + attach fractionating column + condenser above flask
2. Heat flask, different liquids have different BPs so they evaporate at different temperatures.
3. Liquid with lowest BP evaporates first
4. Liquids with higher BPs evaporate but top of column is cooler so they condense.
5. When first liquid collected, raise temperature

Question 6

When is filtration used?

Answer 6

To separate insoluble solid from liquid

Question 7

Process for filtration

Answer 7

Put filter paper into funnel + pour mixture. Solid residue will be left behind

Question 8

When is crystallisation used?

Answer 8

To separate a soluble solid from a solution

Question 9

When is chromatography used?

Answer 9

To separate a mixture of soluble substances + identify them

Question 10

What is a mobile phase?

Answer 10

Where the molecules can move

Question 11

What is stationary phase?

Answer 11

Where the molecules can’t move

Question 12

In paper chromatography what is the stationary phase?

Answer 12

A piece of filter paper

Question 13

In paper chromatography what is the mobile phase?

Answer 13

Water/solvent

Question 14

How do you find the Rf value?

Answer 14

Distance travelled by solute / distance travelled by solvent. Should always be below 1

Question 15

What is the process of a mixture of simple distillation + chromatography to analyse the composition of an ink?

Answer 15

1. To work out what solvent ink contains do simple distillation
2. The thermometer in distillation set up reads BP of solvent. You can then workout what solvent it is eg if evaporated at 100° likely to be water.
3. Then carry out paper chromatography to see how many dyes are in the ink.

Question 16

What is paper chromatography?

Answer 16

The separation of mixtures of soluble substances by running a solvent through the mixture on the paper causing the substances to move at different rates over the paper.

Question 17

How do you know of a substance is pure or not in chromatography?

Answer 17

A pure substance won’t be separated, it’ll move as on dot. Mixture will give you multiple dots.

Question 18

How do you identify substances by calculation in chromatography?

Answer 18

Run a pure sample of substance that you think is in your mixture alongside a sample of the mixture. If the Rf values on both are the same they’re likely to the the same.