Separation Techniques

Question 1

Define separation techniques

Answer 1

Separation techniques involve physical methods to separate mixtures into their constituents.

Question 2

Describe separation of sand from water

Answer 2

Filter the mixture using a filter funnel to obtain sand as residue and water as filtrate

Question 3

Describe separation of salt from water

Answer 3

Heat the solution in an evaporating dish to evaporate off all the water from the solution to obtain salt.

Question 4

Describe separation of copper(II) sulfate from water

Answer 4

The solution is heated in evaporating dish to evaporate off most of the water to produce hot saturated solution. Allow the hot saturated solution to cool to form crystals of copper(II) sulfate. Filter the mixture to obtain __ crystals as the residue and dry the crystal between pieces of filter paper

Question 5

When to use crystallisation over evaporation to dryness

Answer 5

Impurities will remain in the solution and will not crystallise out during crystallisation
Through ETD some solids decompose(thermally unstable)
Solid produced by ETD may not be pure and will be anhydrous

Question 6

When to use evaporation to dryness over crystallisation

Answer 6

When the solubility of the substance does not change significantly with change in temperature

Question 7

Why are boiling chips added to mixture in simple distillation

Answer 7

To ensure smooth boiling process

Question 8

Where should thermometer be positioned in simple distillation and why

Answer 8

Should be positioned with bulb beside the side-arm leading to condenser, ensuring thermometer measures temperature of vapour distilling over to collect boiling point

Question 9

Why does water enter condenser from bottom and leave from top in simple distillation

Answer 9

To ensure complete condensation of all vapour passing through the condenser

Question 10

How to collect volatile liquid

Answer 10

Volatile liquid can be kept in liquid state by placing the receiver over ice

Question 11

Name all magnetic materials

Answer 11

cobalt, Iron, nickel, steel

Question 12

Name all substances that can sublime

Answer 12

iodine, solid carbon dioxide, naphthalene, ammonium chloride

Question 13

Define immiscible liquids

Answer 13

Liquids that do not dissolve in each other

Question 14

Describe using separating funnel

Answer 14

Mixture of immiscible liquids placed in funnel, mixed well and allowed to stand. Liquids form separate layers with less dense liquid being on top of denser one. Stopper removed and tap opened, allow bottom layer to run off and be collected.

Question 15

Use of fractionating column in fractional distillation

Answer 15

Provides large surface area for condensation of vapours

Question 16

Why are colourless substances sprayed with locating agent during paper chromatography

Answer 16

To ensure visibility of the substances/compounds

Question 17

Why should start line be drawn in pencil and not in ink in paper chromatography

Answer 17

Ink may dissolve in the solvent used and thus interfere with the separation of the mixture

Question 18

Why should start line be above solvent level in paper chromatography

Answer 18

If it is below solvent level, original spot mixture would dissolve in the solvent before it can be separated into its components

Question 19

Why should original spot mixture be small in paper chromatography

Answer 19

To prevent separated components from ‘overlapping’ onto one another

Question 20

Why should chromatography paper be sufficiently long in paper chromatography

Answer 20

To ensure complete separation of mixture into components

Question 21

Why should solvent front reach near the top of chromatography paper in paper chromatography

Answer 21

To ensure complete separation of mixture into components.

Question 22

Why should a lid cover the container in paper chromatography

Answer 22

To prevent evaporation of the solvent, especially when the solvent is volatile

Question 23

How to calculate Rf value

Answer 23

Rf value is ratio between distance travelled by substance and distanced travelled by the solvent

Question 24

What is an advantage of using descending method of chromatography

Answer 24

The solvent flows more quickly as it does not have to move against gravity

Question 25

If a solid is impure, how is melting point affected

Answer 25

Its melting point is lowered and it melts over a range of temperature

Question 26

If liquid is impure, how is boiling point affected

Answer 26

Its boiling point is raised and it boils over a range of temperatures

Question 27

Which common gases are not/very slightly soluble in water

Answer 27

Carbon dioxide, hydrogen, oxygen

Question 28

How to collect hydrogen(name three)

Answer 28

Displacement of water, downward displacement of air, using graduated gas syringe

Question 29

Name 3 acidic gases

Answer 29

Carbon dioxide, sulfur dioxide, nitrogen dioxide

Question 30

Which drying agents are for acidic and neutral gases

Answer 30

Concentrated sulfuric acid and fused calcium chloride

Question 31

Which drying agents are for alkaline gas

Answer 31

Calcium oxide

Question 32

Why cant fused calcium chloride be used to dry ammonia

Answer 32

Calcium chloride will react with ammonia