Chemical analysis

Question 1

What is a pure substance?

Answer 1

• In chemistry, the word pure has a special meaning – a pure substance contains only one type of element or one type of compound. This means that pure substances: melt and solidify at one temperature called the melting point, boil and condense at one temperature called the boiling point.

Question 2

What are impure substances?

Answer 2

• Impure substances are mixtures. They do not melt and boil at one temperature –they change state over a range of temperatures.

Question 3

What is a formulation?

Answer 3

• Formulations are mixtures that have been carefully designed to have specific properties. The components in a formulation are carefully controlled. Examples of formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and food.

Question 4

What is chromatography?

Answer 4

• Chromatography involves: a stationary phase, which does not move and a mobile phase, which does move.
• In paper chromatography: the stationary phase is the absorbent paper and the mobile phase is the solvent, which is often water.
• During chromatography, mixtures are separated into their constituent components.
• The solvent dissolves the samples and carries them up the paper.
• Each components moves a different distance up the paper depending on its attraction for the paper and for the solvent.
• Chromatography can be used to identify artificial colours (e.g. food) by comparing them to the results obtained from known substances.

Question 5

How do you investigate how paper chromatography can be used to separate and tell the difference between coloured substances?

Answer 5

Sample method:
* Draw a ‘start line’, in pencil, on a piece of absorbent paper.
* Put samples of five known food colourings (A, B, C, D and E), and the unknown substance (X) on the ‘start line’.
* Dip the paper into a solvent.
* Wait for the solvent to travel to the top of the paper.
* Identify substance X by comparing horizontal spots with the results of A, B, C, D and E.

Considerations, mistakes and errors:
* Pure substances produce a single spot in all solvents.
* Only ever use pencil to draw you start line, as ink will dissolve and affect your results.

Question 6

What is the retention factor and how do you find it?

Answer 6

• Different components have different Rf value. Providing the same temperature and solvent are used, the Rf value for a particular components is constant and can be used to identify the component.

Question 7

What are the tests for hydrogen, chlorine, oxygen and carbon dioxide?

Answer 7

• Hydrogen: when mixed with air, burns with squeaky pop.
• Chlorine: Turns admp indicator paper white.
• Oxygen: Relights a glowing splint.
• Carbon dioxide: when bubbled through limewater (a solution of calcium hydroxide), turns the limewater cloudy.