Purity, formulations and chromatography (8.1)

Question 1

In Chemistry, what is a pure substance?

Answer 1

a single element or compound, not mixed with any other substance

Question 2

Pure elements and compounds melt and boil at (…) temperatures

Answer 2

specific

Question 3

What can be used to distinguish pure substances from mixtures?

Answer 3

Melting point and boiling point data

Question 4

In everyday language, what can a pure substance mean?

Answer 4

a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk

Question 5

How can boiling and melting point data be used to distinguish pure from impure substances?

Answer 5

Pure substances have a sharp melting point but mixtures (impure substances) melt over a range of temperatures (depending on proportions/ratio of substances within mixture)

Question 6

What is a formulation?

Answer 6

a mixture that has been designed as a useful product

may have to give additional info found on next two cards

Question 7

How are the useful products (not chemically created products) created from formulations designed for purpose?

Answer 7

Many products are complex mixtures in which each chemical has a particular purpose

Question 8

How are formulations made?

Answer 8

by mixing the components in carefully measured quantities to ensure that the product has the required properties

Question 9

7 needed

What are examples of formulations?

Answer 9

• fuels
• cleaning agents
• paints
• medicines
• alloys
• fertilisers
• foods

Question 10

What are the two uses of chromatography?

Answer 10

separate mixtures (of soluble compounds) and can give information to help identify substances

Question 11

What are the two phases of chromatography?

Answer 11

stationary phase and mobile phase

Question 12

What is the stationary phase of chromatography?

Answer 12

Phase in chromatography that does not move, for instance, the paper in paper chromatography.

Question 13

What is the mobile phase of chromatography?

Answer 13

the solvent that moves through the paper, carrying different substances with it

Question 14

Separation in chromatography depends on what?

Answer 14

the distribution of substances between the phases

Question 15

What is the R_f value?

Answer 15

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

Question 15

How is the R_f value calculated?

Answer 15

cannot be above 1

Question 16

What can be used to help identify the compounds in chromatography?

Answer 16

The fact that different compounds have different Rf values in different solvents

Question 17

Describe the difference in observations on a chromatogram for mixtures and pure substances

Answer 17

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 18

How far the substance travels up the solvent on a chromatogram depends on what?

Answer 18

The distribution between the phases.

If the substances are more attracted to the mobile phase, they will travel up the chromatogram faster and spend more time in the mobile phase, if they are more attracted to the stationary phase, they will travel up slower and spend mroe time in the stationary phase

Question 19

Why should a pencil line, not a pen line, be drawn in paper chromatography?

Answer 19

The ink (of the pen) will mix with the sample and run into the chromatogram, so results cannot be properly observed

Question 20

Why should the mobile phase (solvent) be placed underneath the pencil line in paper chromatography?

Answer 20

The substances (solutes e.g. ink) will diffuse into the water and wash away, so no results are produced on the chromatogram