Experimental Techniques

Question 0

Characteristics/principles of chromatography:

Answer 0

• different substances have different solubilities in the same solvent
• the more soluble substances will dissolve faster in the solvent
• the most soluble substance will get carried along faster and further by the solvent than the less soluble substances

Question 1

Identifying (criteria of) purity of a substance?

Answer 1

1. Substance must have sharp melting/boiling point.

2. Through chromatography.

Question 2

Define locating agent and give an example.

Answer 2

A reagent used to locate normally invisible spots. It reacts with the components to form colored compounds.
Example: ninhydrin

Question 3

Define Rf value.

Answer 3

Rf = distance moved by compound / distance moved by solvent

Each substance has its own characteristic Rf value.

Question 4

What is filtration used for?

Answer 4

To separate an insoluble solid from a liquid.

E.g. can be used to separate sand from water.

Question 5

What is crystallisation used for?

Answer 5

To separate a soluble solid from its solution. It’s based on the different solubilities of solids in water. The solubilities of most solids increase rapidly as temperature rises.
E.g. can be used to obtain copper sulphate crystals from its solution.

Question 6

What is dissolution used for?

Answer 6

To separate a mixture of soluble and insoluble solids by the use of a suitable solvent. Basically a mixture of filtration and crystallisation.
E.g. can be used to separate a mixture of salt and sand in water, by first pouring them in a funnel with filter paper (removing sand), then crystallising (removing water).

Question 7

What is sublimation used for?

Answer 7

To separate a mixture of two solids, one of which can sublime.
E.g. can separate a mixture of ammonium chloride (which sublimes) and sodium chloride (which doesn’t).

Question 8

What is simple distillation used for?

Answer 8

To separate a volatile (easily evaporates at normal temp.) liquid from a non-volatile solute.
On heating, the volatile liquid boils and turns to vapour, which is then condensed back into liquid and exits through the condenser tube.
E.g. can separate pure water from seawater

Question 9

What is fractional distillation used for?

Answer 9

To separate 2 or more miscible (easily mixed) liquids. The liquids are removed in order of boiling points, from lowest to highest.
E.g. can be used to separate ethanol and methanol.

Question 10

Industrial applications of fractional distillation:

Answer 10

• to separate petroleum into kerosene, petrol, diesel, etc.
• to obtain nitrogen from liquid air for use in Heber’s Process (manufacture of ammonia).
• to obtain wine, beer, whiskey, and other liquids from fermented liquid