Atoms, Elements, Compounds & Mixtures

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Atoms, elements and compounds

All substances are made of atoms. An atom is the smallest part of an element that can exist
An element is a substance made of only one type of atom. There are about 100 elements, and each has a chemical symbol: O for oxygen, Na for sodium
- Symbols are one capital letter, or a capital followed by a lower-case letter. Co is cobalt; CO is carbon monoxide
A compound contains two or more elements chemically combined in fixed proportions
- Compounds are made by chemical reactions, and can only be split back into elements by chemical reactions
- A compound's formula shows the atoms in it: $\text{H}_2\text{O}$ is two hydrogen atoms and one oxygen atom
A chemical reaction always makes at least one new substance, and often gives a detectable energy change
Reactions are written as word equations or symbol equations, and a symbol equation must be balanced: the same number of each type of atom on both sides
- Balance by changing the big numbers in front of formulae, never the small numbers inside them
- $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$ is unbalanced; $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$ is balanced
(HT) Half equations show what happens to one species, including electrons: $\text{Cl}^- \rightarrow \tfrac{1}{2}\text{Cl}_2 + \text{e}^-$ . Ionic equations show only the ions that change

A mixture is two or more elements or compounds not chemically combined. Each substance keeps its own chemical properties
Mixtures are separated by physical processes: no chemical reaction happens and no new substance is made

Technique	Separates	How it works
Filtration	an insoluble solid from a liquid	liquid passes through filter paper, solid stays as the residue
Crystallisation	a dissolved solid from its solution	heat to evaporate some solvent, leave to cool so crystals form
Simple distillation	a solvent from a solution	boil, the vapour is cooled in a condenser and collected
Fractional distillation	liquids with different boiling points	vapours rise up a column; the lowest boiling point comes off first
Chromatography	substances dissolved in a solvent	more soluble substances travel further up the paper

Choose the technique from the properties you are told: soluble or insoluble, boiling points, and whether you want the solid or the liquid back
- Salt from rock salt: dissolve, filter off the sand, crystallise the salt
- Pure water from sea water: simple distillation
- Ethanol from a water and ethanol mixture: fractional distillation, because the boiling points are close

Simple distillation apparatus: flask, thermometer at the side-arm, condenser with cold water in at the bottom and out at the top, collecting beaker