Chapter 3: Exploring Diversity of Matter by its Chemical Composition (Summary Sheet)

Part A · Elements

1 What an Element Is

An element is a pure substance that cannot be broken down into simpler substances by chemical means.
Elements are the basic building blocks of all living and non-living matter — every substance, everywhere, is built from them.
An element is made up of only one type of atom. That is what makes it pure: every particle is the same kind.
Each element has its own chemical symbol — one or two letters, always with the first capital (Mg for magnesium, O for oxygen).
Every element known on Earth is listed in the Periodic Table of Elements.

2 Metals and Non-metals in the Periodic Table

The elements in the Periodic Table fall into metals and non-metals, separated by a zig-zag line.

Property	Metals	Non-metals
Position	Left of the zig-zag line	Right of the zig-zag line
Electrical conductivity	Conductors — current passes through	Insulators — current does not pass through
Thermal conductivity	Good conductors of heat	Poor conductors of heat
Melting and boiling points	Usually high	Usually low
Examples	Copper, silver, magnesium	Neon, oxygen, chlorine

Knowing these properties lets you identify an element and pick it for a job: copper conducts and bends, so it is used for wires.
Mercury is the only metal that is a liquid at room temperature. Its volume grows evenly as it gets hotter, which is why it was used in thermometers.

3 Properties an Element Always Keeps

An element has fixed physical properties — its melting point, boiling point and density never change from sample to sample.
It also has fixed chemical properties: how it reacts with other substances is always the same.
Elements exist in two forms: as single atoms (neon, argon) or as molecules (two or more of the same atom joined together, like O₂).
Seven elements are found as pairs — the diatomic elements: hydrogen H₂, nitrogen N₂, fluorine F₂, oxygen O₂, iodine I₂, chlorine Cl₂, bromine Br₂. In an exam, write these with the subscript 2, not as a single atom.

Part B · Compounds

4 What a Compound Is

A compound is a pure substance made of two or more different elements that are chemically combined — joined by chemical bonds, not just mixed.
There is only one type of particle in a compound, and every particle is identical. That is why a compound is pure.
Its particles are either molecules (non-metals joined together, like carbon dioxide) or ions.
Its chemical formula shows the fixed ratio of the different atoms in it: CO₂ always means one carbon atom to two oxygen atoms.

5 A Compound Is Not Like Its Elements

A compound has different properties from the elements it is made of — from both its physical and its chemical behaviour.
A compound has a fixed proportion by mass of its elements, so it has its own formula and its own properties.
A pure compound has fixed melting and boiling points, exactly like an element.

Compound	Made of these elements	How the compound differs
Water, H₂O	Hydrogen and oxygen	A liquid that puts out a flame — yet hydrogen burns easily and oxygen helps things burn
Sodium chloride, NaCl	Sodium and chlorine	An everyday white seasoning — yet sodium is a metal that reacts violently with water and chlorine is a poisonous gas

6 How a Compound Can Be Broken Down

Only chemical methods can break a compound into its elements — because chemical bonds have to be broken. Physical methods cannot do it.

Chemical method	What has to be done
Electrolysis	Pass electricity through a compound (or its molten/solution form) until it splits
Thermal decomposition	Heat the compound until the bonds break and it breaks down

Part C · Mixtures, Solutions and Suspensions

7 What a Mixture Is

A mixture is two or more elements and/or compounds that are not chemically combined — they are only placed together, so each part keeps its own identity.
A mixture may be a solid, a liquid or a gas.

State	Everyday example	What it is a mixture of
Solid	Brass	Metals mixed together — copper and zinc (steel and bronze are other alloys)
Liquid	Seawater	Water with sodium chloride dissolved in it (sugar solution, salt solution)
Gas	Air	Elements (nitrogen, oxygen, argon) and compounds (carbon dioxide, water vapour); natural gas is another

8 Properties a Mixture Always Shows

A mixture displays the characteristics of its constituents — every part behaves as it would on its own, so each one can still be recognised.
The amounts are variable: the proportion of each substance is not fixed, so a mixture has no single formula.
Its melting and boiling points are variable — it melts and boils over a range of temperatures instead of at one.
A mixture can be separated by physical methods such as filtration or distillation, because making the mixture involved no chemical change.

Mixture	What it is made of	How each constituent still shows itself
Salt solution	Salt and water	Tastes salty, and still evaporates like water — both constituents can be recovered separately
Air	Nitrogen, oxygen, argon, carbon dioxide	The oxygen still supports burning although the nitrogen does not; nothing new has been formed
Brass	Copper and zinc	Looks and conducts like both metals, but melts over a range of temperatures instead of at one point
Iron filings with sulfur	Iron and sulfur	The iron is still attracted by a magnet and the sulfur is still yellow — a magnet separates them again

9 Solute, Solvent and Solution

The solute is the substance that dissolves; the solvent is the substance it dissolves in; together they form a solution.
A solution is a mixture, and a special one: it looks the same throughout, because the solute particles are spread evenly between the solvent particles and are too small to be seen.
Filtering a solution leaves no residue, and leaving it to stand gives no sediment — the solute does not settle out. Light passes straight through it.

Solution	Solute	Solvent
Salt solution	Salt	Water
Vinegar	Ethanoic acid	Water
Soda water	Carbon dioxide	Water
Air	Oxygen, carbon dioxide	Nitrogen

Alloys such as brass and stainless steel are solid solutions — one metal spread through another while both stay solid.

10 Suspensions

A suspension is a mixture of an insoluble solid held in a liquid or gas. The solid does not dissolve — it is just floating through.
Its particles are large enough to be seen, so it looks cloudy and scatters light instead of letting it pass.
Filtering it leaves a residue on the filter paper, with the liquid coming through as the filtrate.
Left to stand, it separates into distinct layers, the solid settling at the bottom. This settling is called sedimentation.
Muddy water after rain is a suspension: soil particles are held in the water, the mixture looks cloudy, and left to stand the soil settles to the bottom as sediment.
Sand stirred into water is the same kind of mixture — the sand does not dissolve, so it can be filtered out and left behind on the paper.
Both settle because the particles are heavy enough to sink. A dissolved solute never does this, which is the quickest way to tell the two apart.

11 Solutions Against Suspensions

Compare	Solution	Suspension
Composition	Homogeneous — the same throughout	Heterogeneous — not the same throughout
Physical and chemical properties	Uniform throughout the mixture	Different throughout the mixture
Filtration	No residue on the filter paper	Residue is left behind
On standing	Solute stays dissolved — no sedimentation	Separates into layers as the solid settles
Light	Passes through; the mixture is transparent	Scattered by the particles; the mixture looks opaque

12 Solubility and What Changes It

Solubility is the maximum mass of a solute that will dissolve in a fixed volume of solvent at a given temperature. It is measured in g per 100 cm³ of solvent.
Because a temperature is part of the definition, a solubility figure without one is incomplete.

Factor	How it changes the solubility
Type of solute	Different solutes dissolve to different amounts in the same solvent — sugar is more soluble in water than salt is
Type of solvent	The same solute behaves differently in different solvents — salt dissolves in water but not in oil
Temperature	Raising the temperature makes most solid solutes more soluble in water (calcium sulfate and calcium hydroxide are the exceptions)

13 The Rate of Dissolving

The rate of dissolving is how much solute dissolves in a given volume per unit time — how fast it happens. This is not the same question as how much can dissolve.

Factor	How it changes the rate
Temperature	The hotter the solvent, the faster a solid solute dissolves
Size of the solute particles	Powder dissolves faster than a lump: breaking it up increases the exposed surface area for the solvent to work on
Stirring	Stirring spreads the solute particles through the solvent, so they meet more solvent and dissolve faster

Trap: a bigger beaker, or more water, does not make the solute dissolve faster. Only temperature, particle size and stirring change the rate.

Part D · Classifying Matter, and Why Composition Matters

14 Elements, Compounds and Mixtures Side by Side

Compare	Element	Compound	Mixture
Particles it is made of	Atoms or molecules	Molecules or ions	Atoms, molecules and/or ions
Types of atom present	Only one type	Two or more types, chemically combined	Two or more types, not chemically combined
Composition by mass	Fixed	Fixed	Variable
Melting and boiling points	Fixed	Fixed	Variable — it boils over a range
Chemical properties	Fixed	Fixed, and different from its elements	Variable — the same as its constituents
How it can be separated	Cannot be broken down further	Only by chemical methods	By physical methods

15 Classifying Matter from a Particle Diagram

In these diagrams each circle is one atom; a different colour or size means a different type of atom; circles that touch are chemically combined into one molecule.
Ask two questions in this order: how many types of atom are there, and are the atoms joined or separate?
“Pure” means only one kind of particle; a pure compound is one kind of particle with more than one type of atom in it.

Element — single atoms. One type of atom, not joined to anything. Argon (Ar) is like this.

Element — molecules. One type of atom, joined in pairs. Oxygen (O₂) is like this.

Compound. One kind of particle, each with two different atoms joined. Carbon dioxide (CO₂) is like this.

Mixture. Different kinds of particle not joined to one another — oxygen, carbon dioxide and argon in air.

So: one type of atom = element (separate circles: single atoms; joined pairs: molecules). Two or more types joined in every particle = compound. Two or more types present but not joined = mixture.

16 Why Chemical Composition Matters

Situation	What the composition tells you
Sorting waste before recycling	Classifying waste by its chemical composition — metals with metals, plastics with plastics — is what makes recycling and reuse of precious materials possible. Mixed waste cannot be recycled; sorted waste can
Melamine in plastic containers	Knowing it is there is beneficial: melamine makes a hard, heat-resistant plastic that is right for plates and cups
Melamine in milk powder	Knowing it is there is harmful: it is not food, and adding it to make the protein reading look higher has caused serious illness
Any everyday item	Read the composition before deciding whether it is safe to use, safe to heat, or safe to eat

So the same substance can be helpful in one product and dangerous in another — it is the use, and the amount, that decides.

Part E · Exam Tips

17 Exam Tips: What Each Syllabus Pointer Asks For

The pointer	What earns the mark
State that elements are the basic building blocks of living and non-living matter	Say an element is a pure substance of one type of atom that cannot be broken down by chemical means, and that everything living and non-living is built from elements
Recognise different types of elements in the Periodic Table, e.g. metals and non-metals	Name the two groups and the line that separates them — metals to the left of the zig-zag line, non-metals to the right — and give a property that tells them apart, such as electrical conductivity
Show an understanding that compounds are two or more chemically combined elements	Use the words chemically combined, and add that the elements are joined in a fixed ratio shown by the formula
Show an understanding that compounds differ from their constituent elements	Give one example and state the difference in properties — water puts out a flame although hydrogen burns (or salt is edible although sodium and chlorine are not)
Show an understanding that mixtures are not chemically combined	Say the substances are not chemically combined, so each keeps its own properties
Show an understanding that mixtures display the characteristics of their constituents	Point to a part of the mixture and say which constituent it comes from, and note that the proportions can be varied
Distinguish between solute, solvent and solution	Name the substance that dissolves (solute), the one it dissolves in (solvent), and the mixture formed (solution) — usually the larger amount is the solvent
Show an understanding that solutions and suspensions are mixtures	Say both are mixtures, then separate them: a solution is homogeneous with nothing left on the filter paper, a suspension is heterogeneous and settles on standing
Distinguish between elements, compounds and mixtures	Use the two tests: how many types of atom, and whether the atoms are chemically combined
Classify matter as elements, compounds and mixtures based on chemical composition	State the class first, then give the evidence from the particle diagram or from the composition
Investigate the factors that affect the rate of dissolving and solubility	Keep the two ideas apart: solubility = how much can dissolve (changes with solute, solvent and temperature); rate = how fast (changes with temperature, particle size and stirring). Name what is changed and what is kept the same
Show an appreciation of classifying waste by composition to recycle and reuse precious materials	Explain that waste sorted by its chemical composition can be recycled or reused, while mixed waste cannot
Show an awareness of the importance of knowing the composition of everyday items	Give both sides with an example: the same substance can be beneficial in one product (melamine in plastic containers) and harmful in another (melamine in milk powder)