Pure And Impure Substances.

Question 1

Define “pure”.

Answer 1

Consists of just only one element or compound.

Question 2

Define “impure”.

Answer 2

Consist of multiple elements or compounds.

Question 3

How can purity affect the melting point?

Answer 3

If impure, the melting point will be reduced and it will melt over a range of temperatures. Pure substances only have one specific temperature.

Question 4

Why do impure substances melt over a range?

Answer 4

Because the more impure the solid is, the more its structure is disrupted and the greater the variation in intermolecular forces in different areas, lowering the chances of melting at the exact same temperature.

(Different components in mixtures melt at different temperatures giving a temperature range. Pure substances are chemically uniform so melts at the same time).

Question 5

Give three example of pure substances.

Answer 5

Hydrogen/Oxygen/Water.

Question 6

Give three examples of impure substances.

Answer 6

Air/paint/juice.

Question 7

Define “alloys”.

Answer 7

A mixture of a metal with one or more other elements.

Question 8

Why is “pure orange juice” impure?

Answer 8

Pure in everyday life refers to “clean”/“not contaminated”/“natural substances that have not been processed or changed”.

However, since orange juice is a mixture, it is scientifically impure.

Question 9

Define “melting point”.

Answer 9

The temperature at which a substance changes from the solid state to the liquid state.

Question 10

What is a mixture?

Answer 10

Multiple elements mixed together in the same space but are NOT chemically bonded.

Question 11

What is melting point determine by?

Answer 11

Its temperature.
This can be by recording the temperature it melts at or by recording the temperature at regular intervals and plotting a graph.
It is important to:
• Heat the substance slowly - to allow the temperature of the whole substance to increase.
Stir the substance as it melts - to ensure the whole sample is at the right temperature.