Separation Techniques

Question 1

Define mixture.

Answer 1

A mixture is made up of two or more substances that are not chemically combined.

Question 2

Define pure substance.

Answer 2

A pure substance is made up of one single element or compound. It is not mixed with any other substance.

Question 3

What are the separation techniques used to separate a solid from a liquid?

Answer 3

• Filtration

- Evaporation to dryness

Question 4

What is filtration used for?

Answer 4

Filtration is used to separate insoluble solid particles from a liquid.

Question 5

What makes filtration possible?

Answer 5

A solid can be separated from a liquid by filtration because the filter paper acts as a sieve. A liquid can pass through the pores (small holes) of the filter paper, but a solid cannot do so.

Question 6

Define residue and filtrate.

Answer 6

• Residue: the solid that remains on the filter paper.

- Filtrate: the liquid or solution that passes through the filter paper

Question 7

What is evaporation to dryness used for?

Answer 7

Evaporation to dryness is used to obtain a soluble solid from a solution by heating the solution until all the water has boiled off.

Question 8

What are the separation techniques used for separating solids?

Answer 8

• Using a suitable solvent
• Sublimation
• Using a magnet

Question 9

How do we use a suitable solvent to separate two solids?

Answer 9

To separate a mixture of two solids, we use a solvent in which only one solid is insoluble. (The other solid is soluble.)

Question 10

What is sublimation used for?

Answer 10

Sublimation is used to separate a solid that sublimes from one that does not.

Question 11

How do we use a magnet to separate two solids?

Answer 11

A magnet can be used to separate a magnetic substance from a non-magnetic substance.

Question 12

What is the separation technique used for separating a liquid from a solution?

Answer 12

Simple distillation

Question 13

What is simple distillation used for? Describe distillation.

Answer 13

• Simple distillation is used to separate a pure solvent (liquid) from a solution.
• Distillation is the process of boiling a liquid and condensing the vapour. Pure water can be obtained from a salt solution by distillation.

Question 14

List the step-by-step process of how simple distillation is carried out (context: salt solution being distilled)

Answer 14

1. In the distillation flask, the solution boils. Boiling chips are added to ensure smooth boiling. Water vapourises, rises and enters the condenser.
2. In the condenser, water vapour is cooled. The vapour condenses and changes back into pure water (the distillate).
3. Pure water is collected as a distillate in the receiver (a conical flask).
4. The salt solution, which remains in the distillation flask, becomes more concentrated as distillation continues. If distillation is allowed to carry on, a solid residue of salt will be left in the flask.

Question 15

Simple distillation: Why should the bulb of the thermometer should be placed beside the side arm of the distillation and not be dipped into the solution?

Answer 15

This ensures that the thermometer measures the boiling point of the substance that is being distilled.

Question 16

Simple distillation: Why should the condenser slope downwards?

Answer 16

This ensures that the pure solvent formed runs downwards into the receiver (a conical flask.)

Question 17

Simple distillation: Why is cold running water allowed to enter the water jacket from the bottom of the condenser and leave from the top?

Answer 17

If water enters from the top of the condenser, it will exit the condenser before the water jacket can be completely filled. Thus, water has to enter the condenser from the bottom to ensure that the entire jacket is always completely filled. This provides a more efficient cooling system.

Question 18

Simple distillation: what should be done if the distillate is volatile, and why?

Answer 18

The receiver can be put into a large container filled with ice. This helps to keep the temperature of the distillate low so that it remains in the liquid state.

Question 19

What are the separation techniques used for separating liquids, and what does the method used for separating a mixture of two liquids depend on?

Answer 19

• Using a separating funnel and fractional distillation

- It depends on whether the liquids are immiscible or miscible.

Question 20

What is a separating funnel used for?

Answer 20

A separating funnel can be used to separate immiscible liquids

Question 21

What is fractional distillation used for?

Answer 21

Fractional distillation is used to separate a mixture of miscible liquids with different boiling points.

Question 22

State and describe the column attached to the round-bottomed flask and the condenser in a fractional distillation setup.

Answer 22

The column is called the fractionating column. Many glass beads in the fractionating column provide a large surface area for vapour to condense on.

Question 23

What is chromatography used for?

Answer 23

Chromatography is the method of separating two or more components that dissolve in the same solvent.

Question 24

What does the distance travelled by the solvent front and substance up the chromatogram depend on?

Answer 24

It depends on how long the experiment is allowed to run.

Question 25

What is the formula for the retention factor of a substance? (Rf value)

Answer 25

Rf value = distance travelled by the substance/ distance travelled by the solvent

Question 26

What happens if we need to run a chromatogram of colourless substances such as amino acids?

Answer 26

We simply spray a locating agent on the chromatogram. This causes the colourless substances to show up as coloured spots.

Question 27

What are the steps taken to separate and analyse colourless substances? (context: amino acids as colourless substances)

Answer 27

1. Separate the mixture of amino acids by chromatography using a suitable solvent.
2. Stop the chromatography before the solvent reaches the top of the paper. Dry the paper.
3. Spray a locating agent onto the paper.
4. The locating agent reacts with each of the amino acids to form coloured spots on the paper. By checking the Rf value of each coloured spot, we can identify the different amino acids.

Question 28

What are the uses of chromatography?

Answer 28

• Separate the components in a sample
• Identify the components present in a sample
• Identify substances
• Determine whether a sample is pure.