8a. Chemical Analysis

Question 1

How can you distinguish pure substances from mixtures?

Answer 1

Check the melting and boiling point

Mixtures melt and boil over a range of temperatures (see book for curve)

Question 2

What is a formulation?

Answer 2

A mixture that has been designed as a useful product

In a formulation, every chemical has been added in a carefully measured amount. Each chemical has a specific purpose in the formulation
- e.g. fuels, paints, medicines, cleaning products, alloys, fertilisers and foods

Question 3

How do you test for hydrogen?

Answer 3

Insert a lit splint and if hydrogen is present you will hear a squeaky popping sound

Question 4

How do you test for chlorine?

Answer 4

Insert damp litmus paper and if chlorine is present it will be bleached white

Question 5

How do you test for oxygen?

Answer 5

Insert a glowing splint and if oxygen is present the splint will reignite

Question 6

How do you test for carbon dioxide?

Answer 6

Bubble the gas through limewater and if it is carbon dioxide the limewater will turn cloudy white/milky

Question 7

What is paper chromatography used for?

Answer 7

Paper chromatography is used to separate mixtures of soluble substances and to provide information on the possible identity of the substances present in the mixture. These are often coloured substances such as food colourings, inks, dyes or plant pigments.

Question 8

What are the 2 phases in paper chromatography?

Answer 8

• The mobile phase - the phase that moves, often the solvent that moves through the paper
• The stationary phase - the phase that does not move, for example the paper

Question 9

How can you distinguish between pure and impure substances using chromatography?

Answer 9

A pure substance will produce one spot on the chromatogram while impure substances will produce 2 or more spots

Question 10

How can you use chromatography to tell if 2 substances are the same?

Answer 10

If they produce the same number of spots that match in colour

If the spots travel the same distance up the paper (same Rf value)

Question 11

How do you calculate the Rf value of a spot?

Answer 11

Distance travelled by substance/distance travelled by solvent

Question 12

How are Rf values used?

Answer 12

Rf values can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value is always the same for a particular substance if run in the same solvent system.

Rf values vary from 0 (the substance is not attracted to the mobile phase) to 1 (the substance is not attracted to the stationary phase).