experimental techniques

Question 1

Importance of separation techniques

Answer 1

To obtain pure substances, remove impurities and carry out identification

Question 2

Filtration

Answer 2

Used to separate an insoluble solid from a liquid.
The liquid passes through the filter paper and is collected as the filtrate.
The insoluble solid remains in the filter paper and is collected as the residue.

Question 3

Evaporation to dryness

Answer 3

Used to obtain a dissolved solid (solute) from a solution.
Process: the solution of the dissolved sold is heated in an evaporating dish to evaporate off all of the solvent

Question 4

Crystallization

Answer 4

Used to obtain a dissolute solid (solute) from A solution as pure well-formed crystals.
Crystallization occurs because solubility of most solutes decreases as temperature decreases. The mass of solute which cannot dissolve at a lower temperature will come out as crystals.

Question 5

Evaporation to dryness (disadvantages)

Answer 5

The solid obtained by evaporation to dryness is not always pure. Any soluble impurities will be left together with the solid after heating.
Upon heating and evaporation to dryness, some solids decompose.

Question 6

Crystallization (advantages)

Answer 6

During crystallization, the impunities remain in the solution and will not crystallize out. Thus, pure crystals are obtained.

Question 7

When to use evaporation to dryness

Answer 7

When the solubility of the salt does not change significantly with changes in temperature

Question 8

Simple distillation

Answer 8

Used to obtain a pure liquid (solvent) from a solution

Question 9

Simple distillation- boiling chips

Answer 9

The boiling chips are added to the mixture to ensure a smooth boiling process.

Question 10

Simple distillation- thermometer

Answer 10

The thermometer is positioned with its bulb beside the side-arm leading to the condenser to ensure that the thermometer measure the temperature of the vapor, which indicates the boiling point of the liquid distilling over.

Question 11

Simple distillation- condenser

Answer 11

Water enters the condenser from the bottom and leaves the condenser at the top. This is to ensure complete condensation of all the vapor passing through the condenser.

Question 12

Simple distillation - volatile liquids

Answer 12

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

Question 13

Magnetic attraction

Answer 13

Separate magnetic solids from non-magnetic solids

Question 14

Sublimation

Answer 14

Separate a solid that sublimes from the other solids that do not.
Sublimation- a solid changes directly to gas without going through the liquid state

Question 15

Substances that can sublime

Answer 15

Iodine, solid carbon dioxide (dry ice), naphthalene, ammonium chloride

Question 16

Separating funnel

Answer 16

Used to separate a mixture of immiscible liquids.
Immiscible liquids- liquids that do not dissolve in each other

Question 17

Fractional distillation

Answer 17

Used to separate a mixture of two or more miscible liquids which have different boiling points, with the use of a fractionating column

Question 18

Fractional distillation- boiling points of liquids

Answer 18

The larger the difference in the boiling points of the liquids, the more complete the separation of the liquids

Question 19

Fractional distillation- fractionating column

Answer 19

Glass beads or glass folds in the fractionating column, provide a large surface area for condensation of the vapour.
A good separation of the liquids is also achieved by using a long fractionating column with many glass beads or glass folds.

Question 20

Paper chromatography

Answer 20

Separates a mixture of substances which are soluble in a common solvent

Question 21

Paper chromatography-solvent front

Answer 21

Position reached by the solvent at the end of the process

Question 22

Paper chromatography - locating agent

Answer 22

Enable the visibility of the substances/ components on the paper

Question 23

Paper chromatography - ink or pencil?

Answer 23

The start line should be drawn in pencil and not in ink, because ink might dissolve in the solvent used, and thus interfering with the separation of the mixture

Question 24

Paper chromatography - level of start line

Answer 24

The start line should be above the solvent level, because if it is below the solvent level, then the original spot mixture would dissolve in the solvent even before it can be separated into its components

Question 25

Paper chromatography - spot size

Answer 25

The original spot mixture should not be too large to prevent the separated components from overlapping onto one another

Question 26

Paper chromatography - length of paper

Answer 26

The chromatography paper should be sufficiently long to ensure complete separation of the mixture into its components

Question 27

Paper chromatography - solvent front place(?)

Answer 27

The solvent front should reach near the top of the chromatography paper to ensure complete separation of the mixture into its components

Question 28

Paper chromatography - volatile liquids

Answer 28

A lid should be used to cover the container to prevent evaporation of the solvent, especially when the solvent is volatile

Question 29

Paper chromatography - Rf value

Answer 29

Ratio between the distance travelled by the substance and the distance travelled by the solvent.
Distance moved by substance/ distance moved by solvent (no units)

Question 30

Paper chromatography - descending method

Answer 30

Carried out with the solvent running down the paper.
Solvent flows more quickly as it does not have to move against gravity.

Question 31

Criteria of purity - pure solid

Answer 31

A pure solid has a fixed melting point. It melts at a constant temperature.

Question 32

Criteria of purity - impure solid

Answer 32

If a solid is impure, its melting point is lowered and it melts over a range of temperatures.

Question 33

Criteria of purity - pure liquid

Answer 33

A pure liquid has a fixed boiling point, it boils at a constant temperature.

Question 34

Criteria of purity - impure liquid

Answer 34

If the liquid is impure, its boiling point is raised and it boils over a range of temperatures.

Question 35

Criteria of purity - paper chromatography

Answer 35

A pure substance always gives a single spot on a chromatogram.
An impure substance gives more than one spot on a chromatogram.

Question 36

Criteria of purity - heating curve

Answer 36

If a substance is pure, the heating curve shows a fixed and constant melting point for a solid and a fixed and constant boiling point for a liquid.