Separation Techniques

Question 1

Methods that separate solid from solid:

Answer 1

Sublimation, Magnetic attraction

Question 2

Methods that separate solid from liquid:

Answer 2

Filtration, Decanting, Crystallisation, Evaporation to dryness

Question 3

Methods that separate liquid from liquid:

Answer 3

Simple distillation, Fractional distillation, Paper chromatography, Separating funnel

Question 4

Methods that only separate substances:

Answer 4

Magnetic attraction, Sublimation, Separating funnel, Filtration, Evaporation until dryness, Decanting

Question 5

Methods that separate and purify substances:

Answer 5

Crystallisation, Simple distillation, Fractional distillation

Question 6

Methods that separate and identify substances:

Answer 6

Paper chromatography, simple distillation, fractional distillation

Question 7

Describe sublimation.

Answer 7

A process that separates a mixture of solids, in which one of the solids sublimes and undergoes a change in state from solid to gas upon heating.

Question 8

State the process of a gas turning into solid state.

Answer 8

Deposition

Question 9

1: State a subliming substance that melts at atmospheric temperature.
2: State a subliming substance that does not melt at atmospheric temperature.

Answer 9

1. iodine, naphthalene
   2: carbon dioxide

Question 10

Describe the process of crystallisation fully.

Answer 10

A solid is dissolved in a liquid to form a solution. Filtration is performed to remove excess solute from the solution. Heat the solution until most of the solvent has evaporated. The solution is then allowed to cool, forming a saturated solution. The solution is no longer able to hold any more solute and thus the dissolved solid appear as crystals, while the impurities remain dissolved in the solution.

Question 11

Why is it necessary to cool the solution?

Answer 11

To decrease the solubility of the solute and thus form crystals.

Question 12

What is the recommended procedure after crystallisation (i.e. after the crystals appear)?

Answer 12

Perform filtration again and obtain the residue which are the crystals. Next, wash the crystals with a bit of cold deionised water before drying them on a filter paper.

Question 13

Describe the process of evaporation until dryness.

Answer 13

A solid is dissolved in a liquid to form a solution. Filtration is performed to remove excess solute from the solution. The solution is then heated until all the solvent has evaporated. The solution is no longer able to hold any more solute and thus the dissolved solid appear as crystals.

Question 14

State 2 disadvantages of evaporation until dryness compared to crystallisation.

Answer 14

1) It may decompose thermally unstable substances such as sugar.
2) When the solvent evaporates, the impurities may remain with the solid.

Question 15

In filtration, name the scientific terms of
1. the solid collected in the filter paper;
2. the liquid/solution passing through the filter paper

Answer 15

1. residue
2. filtrate

Question 16

State two disadvantages of filtration.

Answer 16

1) The solids collected on the filter paper may be contaminated with some fluid and thus they have to be washed thoroughly.
2) The filtrate may contain fine particles if the pore size of the filter paper is not small enough to prevent the particles from flowing through.

Question 17

Describe the process of decanting.

Answer 17

Decanting separates an insoluble solid from a liquid by pouring the liquid from the container.

Question 18

Name and describe the product from distillation.

Answer 18

The product is known as the distillate and it is pure.

Question 19

State the function of simple distillation.

Answer 19

To obtain a pure solvent from a solution of a solute.

Question 20

Describe the process of simple distillation.

Answer 20

The mixture is heated and the liquid in the mixture boils and turn into pure vapour. This pure vapour rises and enters the condenser, before it touches the cool surface and cooled to its liquid state. This liquid, also known as the distillate, is collected.

Question 21

State the physical processes involved in distillation.

Answer 21

Boiling, Condensation

Question 22

Why should the bulb of the thermometer be placed at the entrance to the condenser?

Answer 22

To measure only the vapour that is distilling and not any other vapours inside the mixture.

Question 23

Why should cold water flow in from the bottom of the condenser?

Answer 23

To ensure that the column in the condenser is fully filled with water so as to ensure efficient cooling of the hot vapours to liquid.

Question 24

Explain why a stopper is essential for fractional distillation to take place.

Answer 24

To prevent vapours/gases from escaping.

Question 25

What is the purpose of boiling chips?

Answer 25

To ensure smoother boiling.

Question 26

State the function of fractional distillation.

Answer 26

To separate 2 or more miscible liquids or solutions.

Question 27

State the function of the fractionating column.

Answer 27

To separate liquids in order of their relative boiling points.

Question 28

Why is the fractionating column packed with glass beads?

Answer 28

To provide a large surface area for condensation to occur more easily.

Question 29

State two properties of the fractions (separated from petroleum).

Answer 29

Insoluble in water, burn in air

Question 30

State the properties when the boiling points of fractions increase.

Answer 30

1. Molecules become larger.
2. The liquids flow less easily.
3. The liquids burn less easily.
4. The liquids burn with a more smoky flame.

Question 31

Arrange the fractions in petroleum in ascending boiling points.

Answer 31

Petroleum gases, petrol, naphtha, kerosene, diesel, lubricating oil, bitumen

Question 32

State the function of the separating funnel.

Answer 32

To separate immiscible liquids or solutions.

Question 33

Which liquid flows out first?

Answer 33

The liquid that has a higher density.

Question 34

State the function of chromatography.

Answer 34

To separate and identify a mixture of solutes.

Question 35

What causes the separation of the components?

Answer 35

1. Different solubilities of the solutes in the same solvent
2. Selective absorption of the solutes by the paper

Question 36

Why must the start line and solvent front be drawn in pencil?

Answer 36

Graphite in the pencil does not dissolve when the chromatogram is immersed into the solvent.

Question 37

Suggest what will happen if the start line is below the surface of the solvent.

Answer 37

The solute/spots will dissolve into the solvent and will not be developed on the chromatography paper.

Question 38

Why does the spots on the chromatography paper need to be tiny?

Answer 38

To prevent the spread of the dye components.

Question 39

How to find the retardation factor?

Answer 39

distance moved by the substance / distance moved by the solvent

Question 40

State two factors that will affect the retardation factor value.

Answer 40

1) The type of solvent used for separation.
2) The temperature at which the chromatography is conducted at.

Question 41

In chromatography, two substances are likely to be the same if:

Answer 41

1. They produce the same number of spots, and these match in colours
2. The spots travel the same distance (i.e. same retardation factor value)

Question 42

What is the advantage of using two-way paper chromatography compared to one-way chromatography for a mixture of substances?

Answer 42

It allows for better separation of substances which may have similar Rf values for a particular solvent, but different Rf values for another solvent.

Question 43

Suggest a method to solve the problem of some substances on the chromatography paper being colourless. Explain.

Answer 43

Spray the chromatogram with ninhydrin. It acts as a locating agent which will react with colourless components to form coloured compounds, making the spots visible.