Elements, Compounds and Mixtures

1. Elements

An element is a pure substance that cannot be broken down into simpler substances by chemical methods.

Each element is represented by a unique symbol (one or two letters). The first letter is always capital; the second letter is always small.

Atomic No.	Element	Symbol	Atomic No.	Element	Symbol
1	Hydrogen	H	11	Sodium	Na
2	Helium	He	12	Magnesium	Mg
3	Lithium	Li	13	Aluminium	Al
4	Beryllium	Be	14	Silicon	Si
5	Boron	B	15	Phosphorus	P
6	Carbon	C	16	Sulphur	S
7	Nitrogen	N	17	Chlorine	Cl
8	Oxygen	O	18	Argon	Ar
9	Fluorine	F	19	Potassium	K
10	Neon	Ne	20	Calcium	Ca

Common Mistake: Writing 'CO' for cobalt. 'CO' means carbon monoxide (compound). Cobalt is 'Co' (capital C, small o).

Type	Properties	Examples
Metals	Shiny, good conductors, malleable, ductile	Iron (Fe), Copper (Cu), Aluminium (Al)
Non-metals	Dull, poor conductors, brittle	Oxygen (O), Carbon (C), Sulphur (S)
Metalloids	Properties of both	Silicon (Si), Germanium (Ge)

A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio.

Example: Water (H2O) is a compound of hydrogen and oxygen in a 2:1 ratio by volume.

Worked Example: List the elements present in calcium carbonate (CaCO3) and their count.

Calcium (Ca): 1 atom. Carbon (C): 1 atom. Oxygen (O): 3 atoms.

Exam Focus (3 marks): 'Differentiate between an element and a compound.'

Element	Compound
Contains one kind of atom	Contains two or more kinds of atoms
Cannot be broken down	Can be broken down into elements
Has one symbol	Has a formula
Example: Oxygen (O2)	Example: Water (H2O)

A mixture is formed by physically mixing two or more substances without any chemical reaction.

Type	Definition	Example
Homogeneous	Uniform composition throughout	Salt solution, air, sugar solution
Heterogeneous	Non-uniform composition	Sand and water, oil and water, salad

Common Mistake: Thinking a mixture is always heterogeneous. Some mixtures like air and salt solution are homogeneous (uniform composition).

Feature	Compound	Mixture
Formation	Chemical reaction	Physical mixing
Composition	Fixed ratio	Variable ratio
Properties	Different from constituents	Retains properties of constituents
Separation	Difficult (chemical methods)	Easy (physical methods)
Examples	Water, salt, CO2	Air, sand and salt, milk

Separating larger impurities by hand.
Example: Removing stones from rice.

Separating particles of different sizes using a sieve.
Example: Separating sand from gravel.

Separating lighter and heavier components using wind.
Example: Separating husk from grains.

Separating insoluble solids from liquids using a filter paper.
Example: Separating sand from water.

Heating a solution to remove the liquid, leaving behind the solid.
Example: Obtaining salt from seawater.

Letting heavier particles settle, then pouring out the liquid.
Example: Purifying muddy water.

Using a magnet to separate magnetic materials.
Example: Separating iron filings from sand.

Heating to evaporate and then condensing the vapour.
Example: Obtaining pure water from saltwater.

Define an element. Write the symbols for: Sodium, Potassium, Iron, Calcium.
Write the formulae for: (a) Water (b) Carbon dioxide (c) Common salt.
Differentiate between a compound and a mixture.
Name the method used to separate: (a) Sand from water (b) Husk from grain (c) Iron filings from sand.
Classify as homogeneous or heterogeneous: (a) Air (b) Oil and water (c) Salt solution.
State two differences between metals and non-metals.
List the elements present in H2SO4 and their atomic counts.

Pure substance that cannot be broken down. Na, K, Fe, Ca.
(a) H2O (b) CO2 (c) NaCl.
Compound: chemical combination, fixed ratio, different properties. Mixture: physical mixing, variable ratio, retains constituent properties.
(a) Filtration (b) Winnowing (c) Magnetic separation.
(a) Homogeneous (b) Heterogeneous (c) Homogeneous.
Metals are shiny and conduct electricity; non-metals are dull and do not conduct electricity.
H2SO4: Hydrogen (2 atoms), Sulphur (1 atom), Oxygen (4 atoms).