C2.1.3 Pure And Impure Substances

Question 1

What is a pure substance in science?

Answer 1

In science, a pure substance consists of just one element or compound.

Question 2

Impure substances?

Answer 2

Mixtures are impure substances, since they contain more than one element or compound.

Question 3

What is an alloy?

Answer 3

An alloy is a mixture of a metal with one or more other elements

Question 4

How do you use melting points to determine purity?

Answer 4

The melting point of a pure substance is a single temperature. If a substance is impure:
• its melting point is less than that of the pure substance
• it often melts over a range of temperatures, not just one temperature.
The greater the difference between the measured melting point for a substance and its melting point, the lower its purity is likely to be.

Question 5

What is the melting point of a substance?

Answer 5

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

Question 6

How do you determine melting point?

Answer 6

By heating it. Then either measure the temperature at which it melts, or measure its temperature at regular time intervals and plot a graph

Question 7

What is it important to do when determining melting point and why

Answer 7

• heating the substance slowly allows the temperature of the whole sample to increase.
• stir the substance as it melts as mixing ensures that the entire sample is at the same temperature.