Topic 6 (Separation)

Question 1

How do you test for the purity of a substance?

Answer 1

Check its boiling point and melting point

Question 2

How do impurities affect the melting and boiling point of a substance?

Answer 2

Impurities increase the boiling point and decrease the melting point. They also cause the substance to melt and boil over a range of temperatures.

Question 3

When is filtration used?

Answer 3

To separate an insoluble solid from a liquid

Question 4

What is the residue and filtrate?

Answer 4

Residue in the solid left on filter paper, filtrate is the liquid

Question 5

What are two limitations of filtration?

Answer 5

-The size of pores must be smaller than the solid particles.
-Some of the liquid is absorbed by the filter paper so not all of it can be collected.

Question 6

When is evaporation to dryness used?

Answer 6

To separate a dissolved solute from its solution.

Question 7

What are the limitations of evaporation to dryness?

Answer 7

-Hydrated salts may lose their water of crystallization. —Some substances will decompose when heated strongly.
-If there are multiple impurities in the sample, they will all remain on the evaporating dish.

Question 8

When is crystallisation used?

Answer 8

To separate a dissolved solute from its solution.

Question 9

Describe the procedure for crystallisation.

Answer 9

1. Heat the solution to evaporate most of the solvent until the solution is saturated. (A saturated solution is a solution where no more solute can dissolve.)
2. Cool the hot solution and pour off/filter the solution to obtain the crystals.
3. Dry them between sheets of filter paper.

Question 10

Advantages of crystallisation instead of evaporation?

Answer 10

The remaining solute in the form of crystals will be pure.

Question 11

Limitations of crystallisation?

Answer 11

• Some of the solute remains dissolved in the saturated solution
  -If the substance’s solubility is relatively constant when temperature decreases, very little crystals is produced.

Question 12

How does crystallisation work?

Answer 12

At higher temperatures, solubility increases. When a saturated solution at high temperature is cooled, the maximum amount of solute that can dissolve decreases, so the remaining solute comes out of the solution and forms crystals.

Question 13

What are the uses of chromatography?

Answer 13

Testing the purity of substances, identifying what’s in a mixture

Question 14

How does chromatography work?

Answer 14

Substances are either more attracted to the stationary phase or the moving phase, so they travel at different speeds and separate.

Question 15

How is Rf value calculated?

Answer 15

Distance travelled by substance/distance travelled by solvent. Rf value will always be between 0 and 1.

Question 16

What does it mean when the Rf value is 0?

Answer 16

The component is not soluble in the solvent. The solvent is not suitable for testing the component.

Question 17

How can chromatography be used on colourless substances?

Answer 17

Add a locating agent (e.g. ninhydrin) or use ultraviolet light

Question 18

When is simple distillation used?

Answer 18

To obtain a solvent from the solution

Question 19

In distillation, water enters from which side of the condenser?

Answer 19

It enters from the lower opening and leaves from the higher opening

Question 20

Fractional distillation is used to…

Answer 20

To separate two or more miscible liquids in a solution

Question 21

Which liquid distils first in fractional distillation?

Answer 21

The liquid with the lowest boiling point

Question 22

How can you tell the boiling point of the substances from a temperature graph during fractional distillation?

Answer 22

The temperature is constant when one liquid boils.

Question 23

How can we ensure smooth boiling during distillation?

Answer 23

Add boiling chips in the flask.

Question 24

Why is a fractionating column required?

Answer 24

It provides a large surface area for repeated evaporation and condensation of the liquids.

Question 25

What are immiscible liquids?

Answer 25

Liquids that do not dissolve in each other forming layers.

Question 26

How can we separate immiscible liquids?

Answer 26

By using a separating funnel. The less dense liquid forms the top layer while the denser liquid forms the bottom layer.

Question 27

What are 3 ways to separate a mixture of solids.

Answer 27

Using a magnet, sublimation and using a suitable solvent

Question 28

How does sublimation work?

Answer 28

The subliming substance condenses on the cooler parts of the inverted filter funnel as solid crystals. The remaining mixture that does not sublime remains in the evaporating dish.

Question 29

How can you separate sodium chloride and sand?

Answer 29

Add water to dissolve the sodium chloride, and filter.