Matter: particles, elements, compounds and mixtures: WASSCE Chemistry

Everything around you, from the air you breathe to the rice in your bowl, is matter. Chemists explain the behaviour of matter by imagining it made of tiny particles: atoms, molecules and ions. This lesson introduces those particles and the three ways they come together: as elements, compounds and mixtures.

Learning objectives

By the end of this lesson you should be able to:

define matter and describe the evidence that it is made of particles
distinguish atoms, molecules and ions
distinguish elements, compounds and mixtures, and metals from non-metals
state the differences between compounds and mixtures
distinguish physical changes from chemical changes

Matter and its particles

Definition

Matter is anything that has mass and occupies space (has volume). It exists as solid, liquid or gas.

The particulate theory of matter says matter is made of very small particles that are always moving.

Evidence for particles:

Diffusion: the smell of fried fish or perfume spreads through a room, because particles move and mix.
Dissolving: sugar "disappears" in tea because its particles spread out between the water particles.
Brownian motion: pollen grains or smoke specks jiggle randomly, pushed by invisible moving particles.

Particle	Definition	Examples
Atom	The smallest particle of an element that can take part in a chemical reaction	Na, Fe, C
Molecule	The smallest particle of an element or compound that can exist on its own, made of two or more atoms joined together	O₂, H₂, H₂O, CO₂
Ion	An atom or group of atoms carrying an electric charge	Na⁺, Cl⁻, SO₄²⁻, NH₄⁺

Elements, compounds and mixtures

Particle diagrams: an element has one kind of atom; a compound has atoms of different elements joined in a fixed ratio; a mixture has different substances not joined together

Figure 1. Elements, compounds and mixtures at the particle level. WASSCE Tutor diagram.

Definition

An element is a substance that cannot be split into simpler substances by chemical means. It contains only one kind of atom.
A compound is a substance formed when two or more elements are chemically combined in a fixed ratio by mass.
A mixture contains two or more substances not chemically combined, in any proportion.

Metals and non-metals

Metals	Non-metals
Shiny (lustrous)	Usually dull
Good conductors of heat and electricity	Poor conductors (except graphite)
Malleable and ductile	Brittle when solid
Usually high melting points	Usually low melting points
Sonorous (ring when struck)	Not sonorous
Form basic oxides	Form acidic or neutral oxides
Form positive ions	Form negative ions (or share electrons)
Examples: Fe, Al, Cu, Na, Au	Examples: C, O, N, S, Cl

Some elements, such as silicon, have properties of both and are called metalloids.

Compounds and mixtures compared

Compound	Mixture
Elements chemically combined	Substances not chemically combined
Fixed composition (water is always 2 H : 1 O)	Variable composition (any amount of salt in water)
Properties differ from those of its elements	Each part keeps its own properties
Separated only by chemical means	Separated by physical means (filtration, distillation)
Formation usually gives out or takes in heat	Usually little or no heat change on mixing
Sharp, fixed melting and boiling points	Melts and boils over a range

Worked example

Iron and sulphur. Iron filings and yellow sulphur powder are mixed.

Mixture: a magnet pulls out the iron, and both keep their properties.
Heated: the mixture glows and forms black iron(II) sulphide, FeS, a compound.
After heating: a magnet no longer picks out the iron, the product is not yellow, and it reacts with dilute acid to give hydrogen sulphide, which smells of rotten eggs. Iron alone gives hydrogen, which has no smell.

The new properties show a chemical change has made a compound.

Physical and chemical changes

Physical change	Chemical change
No new substance forms	New substance(s) form
Usually easily reversed	Usually not easily reversed
Mass of substance unchanged	Total mass is conserved, but substances change
Little energy change	Often large energy change (heat, light)
Examples: melting ice, dissolving salt, boiling water, magnetising iron, crushing stones	Examples: burning charcoal, rusting, cooking an egg, fermenting palm wine, souring of milk

In Sierra Leone

Making gari involves both kinds of change. Grating and pressing the cassava are physical changes. Fermentation, which sours the pulp and breaks down poisonous cyanide compounds, is a chemical change. Roasting dries it (physical) and partly cooks the starch (chemical).

Common mistake

Dissolving is a physical change, even though the solid "disappears". Evaporating the water gets the salt back unchanged. Students often wrongly call it a chemical change.

Key points

Matter has mass and occupies space; it is made of moving particles: atoms, molecules and ions.
Elements have one kind of atom; compounds have elements chemically combined in a fixed ratio; mixtures are not chemically combined.
Metals are shiny conductors that form positive ions and basic oxides; non-metals form negative ions and acidic oxides.
Physical changes make no new substance; chemical changes make new substances.

WASSCE examination focus

WAEC tests this

A favourite question is "State four differences between a compound and a mixture"; another is classifying changes as physical or chemical with reasons. Expect to define atom, molecule and ion with examples, and to compare metals with non-metals.

Quick check

Define a molecule and give one example.
Is air an element, a compound or a mixture? Give a reason.
Give two differences between a compound and a mixture.
Is the rusting of iron a physical or a chemical change? Why?

Answers:

The smallest particle of a substance that can exist on its own, made of two or more atoms joined together, e.g. O₂ or H₂O.
A mixture: its gases are not chemically combined and its composition varies.
Any two: chemically combined or not; fixed or variable composition; properties differ from or match the components; separated by chemical or physical means; sharp or range of melting point.
Chemical: a new substance (hydrated iron(III) oxide) forms and the change is not easily reversed.

Matter: particles, elements, compounds and mixtures