Pure Substances and Mixtures Flashcards
What is an element?
A substance that consists only of atoms with the same atomic number (number of protons in the nucleus).
What is a compound?
A substance that consists of 2 or more different elements, chemically joined together.
What can elements exist as?
Atoms or molecules
Hydrogen and oxygen are both simple molecules, while carbon exists as giant molecules.
What can compounds exists as?
Molecules or ionic structures
What is a pure substance?
A substance containing only 1 element or compound, e.g.
- pure hydrogen contains only hydrogen molecules
- pure water contains only water molecules
What are mixtures?
Substances that contain different elements or compounds that are not chemically joined together.
Mixtures are impure substances.
Most substances are mixtures.
Air is a mixture of elements and compounds.
Do pure substances have a sharp melting point?
Yes, they have a sharp melting point.
This is represented on a graph by a horizontal line that only crosses one temperature.
Would an impure substance have a horizontal line when it melts?
no
it melts over a temperature range
The label on a bottle describes the contents as ‘Pure mineral water’.
Explain why the water is not pure in the scientific sense.
A pure substance contains only 1 element or compound, but mineral water will be a mixture of water and other substances (e.g. minerals).
When it is heated, 18 carat gold melts between 915 degrees C and 963 degrees C.
Explain why it’s melting point is not a single temperature.
18-carat gold must be a mixture because pure substances have a sharp melting point/mixtures melt over a range of temperatures.
Do mixtures have a sharp melting point?
No, they melt over a range of temperatures.
This is represented on a graph by a sloped line that crosses several temperatures rather than just one
