Experimental Chemistry: measurement, experimental techniques, separation and purification

Question 1

Radioactive

Answer 1

Avoid being near these substances

Question 2

Toxic or poisonous substances

Answer 2

Do not touch, swallow or breathe in such substances

Question 3

Caution

Answer 3

Handle such substances with care

Question 4

Irritating or harmful substances

Answer 4

Do not breathe in the vapour or touch these substances with bare skin

Question 5

Corrosive substances

Answer 5

Do not have direct contact with these substances. Wear gloves when handling. Rinse with water quickly if it spills onto skin

Question 6

Explosive

Answer 6

Place these substances away from heat sources, sparks, friction and shock

Question 7

Oxidising agent

Answer 7

Do not expose these substances to the air or place them close to reducing agents

Question 8

Flammable substances

Answer 8

Keep substances away from flames or sources of heat

Question 9

Displacement of water

Answer 9

This method is used to collect insoluble or slightly soluble gases. Examples are Hydrogen, Oxygen, Carbon dioxide

Gas released is bubbled into an inverted test tube filled with water. Gas will take up the volume and push down the water level in the gas jar ( since it is unable to dissolve in water )

Question 10

Downward delivery

Answer 10

This method is used to collect gases that are denser than air and soluble in water. Examples are Chlorine, Hydrogen Chloride

The gas collected will sink in the test tube and displace the air. Gas jar must not be sealed in order for the air to be pushed out by the gas so that the gas can enter

Question 11

Upward delivery

Answer 11

This method is used to collect gases that are less dense than air and soluble in water. Example is Ammonia

The gas collected will rise up the gas jar displace the air inside. Similarly, the gas jar must not be sealed in order for the displaced air to exit and the gas to enter

Question 12

Uses of a gas syringe

Answer 12

This method is used to collect any type of gas. Gas produced will be collected in the gas syringe and it pushes the plunger. The volume of gas collected can be measure directly

Question 13

Concentrated sulfuric acid

Answer 13

Used to dry acidic gases such as hydrogen chloride. The tube introducing the gas is immersed in the acid while the exiting tube is not

Question 14

Quicklime ( calcium oxide )

Answer 14

Used to dry alkaline gases such as ammonia

Question 15

Fused calcium chloride ( anhydrous calcium chloride )

Answer 15

Used to dry neutral gases such as hydrogen

Question 16

Mixture

Answer 16

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

Question 17

Pure substance

Answer 17

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

Question 18

liquid-liquid mixture

Answer 18

A liquid mixed with another liquid

Question 19

Solid-solid mixture

Answer 19

A solid mixed with another solid

Question 20

Solid-liquid

Answer 20

A solid mixed with a liquid

Question 21

Test for purity

Answer 21

Melting point of a solid ( impurities lower the melting point ) - mixture melts over a range of temperatures
Boiling point of a solid ( impurities increase the boiling point ) - Pure liquid distills at a fixed temperature
Chromatography

Question 22

Filtration

Answer 22

To separate insoluble solid particles from a liquid

The liquid that passes through the filter paper is called the filtrate
The solid that remains on the filter paper is called the residue

Question 23

Evaporation to dryness

Answer 23

Used to obtain a soluble solid from a solution by heating the solution until all the water has boiled off

Question 24

Decomposition

Answer 24

Break down of a compound into 2 or more substances

Question 25

Crystallisation

Answer 25

Used to obtain a pure solid sample from its solution

Many substances decompose when they are heated strongly
Most crystals, such as copper (II) sulfate crystals, give off water to form powders when heated.

Question 26

Why doesn’t the compound remain dissolved in the water when the saturated solution is cooled?

Answer 26

When the temperature decreases, it lowers the solubility of the compound and crystals appear

Question 27

Using a suitable solvent

Answer 27

To separate mixtures of two or more solids, we need to use suitable solvent. The common solvents are water and ethanol

To separate a mixture of two solids, we use a solvent in which only one solid is soluble while the other solid is insoluble

Question 28

Sublimation

Answer 28

Some substance sublime on heating while other substances like sand do not.

Question 29

Using a magnet

Answer 29

Some metals are magnetic while other substances such as sulfur and aluminium are non-magnetic

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

Question 30

Distillation

Answer 30

Used to separate a pure solvent from a solution. It is a process of boiling a liquid and condensing water vapour

Question 31

Why is the bulb of the thermometer placed beside the side arm of the distillation flask?

Answer 31

This ensures that the thermometer measures the boiling point of the substance

Question 32

Why is cold running water allowed to enter the water jacket from the bottom of the condenser and leave from the top instead of the other way round?

Answer 32

This is to prevent the water from coming out

Question 33

Fractional distillation

Answer 33

Used to separate miscible liquids with different boiling points

A fractionating column is attached to the round-bottomed flash and the condenser. Glass beads in the column provide a large surface area for vapour to condense on
The vapours of the liquids will condense along the fractionating column and fall back into the round-bottomed flask
The liquid with the lowest boiling point will distil over first

Question 34

Uses of fractional distillation

Answer 34

(i) Fractional distillation of liquid air to obtain argon, oxygen and nitrogen
(ii) Fractional distillation of crude oil to obtain useful fractions
(iii) Fractional distillation of fermentation mixture of glucose solution to obtain ethanol to manufacture alcoholic beverages or cosmetics

Question 35

Separating funnel

Answer 35

Used to separate immiscible liquids. When immiscible liquids are shaken, they form an emulsion. The emulsion eventually separates into different layers of liquids

Question 36

Paper chromatography

Answer 36

Used to separate two or more components that dissolve in the same solvent
The components in the food colouring mixture can be separated and identified using a suitable solvent due to the differences in solubilities of the different components in the chosen solvent
The more soluble the substance is in the solvent used, the further it will travel up the chromatogram. Identical components will travel the same distance

Question 37

What can be used to identify colourless substances on the chromatogram?

Answer 37

Locating agent

Question 38

Uses of paper chromatography

Answer 38

(i) Separate the components in a sample
(ii) Identify components in a sample - traces of banned substances
(iii) Identify substances ( poisons, drugs and pesticides )
(iv) Determine whether a sample is pure

Question 39

Why must the start line be drawn in pencil?

Answer 39

Pen ink can be dissolved

Question 40

Rf value ( Retention Factor )

Answer 40

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