What's covered
Key facts
Mixtures and pure substances
Air is a mixture of gases: mainly nitrogen (~78%), oxygen (~21%), argon, and carbon dioxide, not a compound.
Separation techniques
Paper chromatography separates dissolved substances (e.g. food colourings, inks) based on how far they travel up the paper with the solvent.
The periodic table
Group 0 (or Group 18) elements are called the noble gases (e.g. helium, neon, argon); they are very unreactive because their outer electron shell is full.
Mixtures and pure substances
A mixture contains two or more substances that are not chemically combined and can be separated by physical methods.
Separation techniques
In distillation, a condenser cools the vapour back into liquid; the liquid collected is called the distillate.
The periodic table
Group 1 elements are called the alkali metals (e.g. lithium, sodium, potassium); they react vigorously with water to produce hydrogen gas and an alkaline solution.
Mixtures and pure substances
Because a mixture contains different particles, it melts and boils over a range of temperatures rather than at a single sharp point.
Separation techniques
Distillation separates a soluble solid from a liquid, or two liquids with different boiling points, by boiling and condensing the vapour.
The periodic table
Group 7 elements are called the halogens (e.g. fluorine, chlorine, bromine, iodine); they are reactive non-metals that form salts when they react with metals.
Mixtures and pure substances
A pure substance boils at a single fixed temperature because all its particles are identical and need the same amount of energy to break apart.
Practice questions with answers
A taste of the 44 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
What is an atom?
- •A chemical reaction product
- •A mixture of two elements
- ✓Smallest unit of an element
- •The smallest cell in the body
What is a mixture?
- •Always a pure substance with no mix
- •Same element in different states of matter
- •Two atoms chemically bonded together tightly
- ✓Two or more substances physically combined
Which method separates a soluble solid from a solvent?
- •Chromatography on paper
- ✓Evaporation or crystallisation
- •Filtration through paper
- •Magnetism attraction
How is the periodic table arranged?
- •Alphabetical element name order
- •Date of element discovery
- •Element mass from heaviest to lightest
- ✓Increasing atomic number across rows
What is an element?
- ✓A substance made of one type of atom
- •Any mixture of atoms
- •Anything found in a periodic table
- •Atoms in a compound only
What is a pure substance?
- •A liquid with nothing dissolved in it
- •Anything from a chemistry lab
- •Mixtures with sharp boundaries
- ✓Only one type of element or compound
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