Purity and Mixtures

Question 1

Purity

Answer 1

In advertising ‘pure’ means has had nothing added to it or it is in its natural state.

A chemist would define a pure substance as is a single element or compound, not mixed with any other substance.
You can have a pure compound such as carbon dioxide but not a
pure mixture such as sea water which is water with salt dissolved in it.

Question 2

Pure Compounds

Answer 2

Carbon Dioxide
Distilled Water
Hydrochloric Acid
Sodium Hydroxide

Question 3

Impure Compounds

Answer 3

Rocks
Air
Toothpaste
Seawater

Question 4

How to Tell if Something is Pure

Answer 4

We can use melting and boiling points to identify pure substances.
The melting and boiling point of pure substances will be fixed. E.g. water has a boiling point of 100oC and a melting point of 0oC.
How do you think this might be different for a mixture/impure substance?
How can we tell if something is pure?
A mixture has a melting point that is over a range. This is because impurities will change the melting point. The more impure a substance is, the more impurities there must be.

Question 5

A Pure Substance or Mixture?

Answer 5

A pure substance has the same fixed composition in all its parts and cannot be separated into other substances using physical methods i.e. filtering or picking bits out. It can be an element or a compound.
A mixture contains elements and/or compounds that are not chemically joined together. You can use physical methods to separate i.e. you can filter out the sand from a beaker of water that has sand suspended in it.

Question 6

Heating Curves

Answer 6

A heating curve shows the temperature at which a given substance melts or boils.
A pure substance will have a clearly defined or fixed melting and boiling point (blue line on the graph) – its physical properties will always be the same.
A mixture will have melting and boiling points that are over a range of temperatures