C8

Question 1

What is a pure substance in chemistry?

Answer 1

In chemistry, a pure substance is a single element or compound, not mixed with any other substance.

Question 2

What are the characteristics of pure elements and compounds

Answer 2

Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.

Question 3

What can a pure substance mean?

Answer 3

In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.

Question 4

Note:

Answer 4

AQA says that students should be able to use melting point and boiling point data to distinguish pure from impure substances.

Question 5

What is a formulation?

Answer 5

A formulation is a mixture that has been designed as a useful product. Many products are complex mixtures in which each chemical has a particular purpose.

Question 6

How are formulations made?

Answer 6

Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties.

Question 7

What are examples of formulations?

Answer 7

Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Question 8

Note:

Answer 8

AQA says that students should be able to identify formulations given appropriate information.

Question 9

Note:

Answer 9

AQA says that students do not need to know the names of components in proprietary products.

Question 10

What can chromatography be used for?

Answer 10

Chromatography can be used to separate mixtures and can give information to help identify substances.

Question 11

What does chromatography involve?

Answer 11

Chromatography involves a stationary phase and a mobile phase.

Question 12

What does separation depend on?

Answer 12

Separation depends on the distribution of substances between the phases.

Question 13

What is the Rf value?

Answer 13

The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent

Question 14

How do you find the ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent

Answer 14

Rf = distance moved by substance / distance moved by solvent

Question 15

What helps us identify different compounds?

Answer 15

Different compounds have different R values in different solvents, which can be used to help identify the compounds.

Question 16

What may happen with compounds in a mixture?

Answer 16

The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

Question 17

Explain how paper chromatography separates mixtures

Answer 17

The paper is then placed upright in a suitable solvent , such as water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.

Question 18

Suggest how chromatographic methods can be used for distinguishing pure substances from impure substances

Answer 18

A chromatogram produced by paper chromatography or thin layer chromatography (TLC) can be used to distinguish between pure and impure substances: a pure substance produces one spot on the chromatogram. an impure substance produces two or more spots.

Question 19

Note:

Answer 19

AQA says that students should be able to:

• interpret chromatograms and determine R values from chromatograms
• provide answers to an appropriate number of significant figures.

Question 20

Practical 12:

Answer 20

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Question 21

What is used for the test of hydrogen?

Answer 21

The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.

Question 22

What is used for the test of oxygen?

Answer 22

The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.

Question 23

What is used for the test of carbon dioxide?

Answer 23

The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).

Question 24

What is used for the test of chlorine?

Answer 24

The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.