Grade 8 · Matter & Materials · Lesson 3

Compounds & Mixtures

Understand how elements combine to form compounds, and how mixtures differ from pure substances.

Elements, Compounds, and Mixtures

All matter can be classified as either a pure substance or a mixture. Pure substances are made of only one type of particle and have fixed, definite properties. Mixtures contain two or more substances that are physically combined but not chemically joined.

Three key categories:
• Element — a pure substance made of only one type of atom (e.g. oxygen gas, O₂; iron, Fe)
• Compound — a pure substance made of two or more different elements chemically bonded together in a fixed ratio (e.g. water, H₂O; salt, NaCl)
• Mixture — two or more substances physically combined but not chemically bonded (e.g. air, seawater, soil)

Elements

An element is a pure substance that cannot be broken down into simpler substances by ordinary chemical means. There are 118 known elements, all listed on the periodic table. Elements exist as individual atoms (e.g. helium, He) or as molecules of the same type of atom (e.g. oxygen gas, O₂; hydrogen gas, H₂).

If a substance contains only ONE type of atom, it is an element. If it contains two or more DIFFERENT types of atom chemically bonded, it is a compound.

Compounds

A compound is formed when two or more different elements are chemically combined. The elements are joined by chemical bonds (ionic or covalent) in a fixed, definite ratio. Compounds have entirely different properties from the elements they are made of.

Key point: In a compound, the elements are present in a fixed ratio by mass. Water is always H₂O — never H₃O or HO. This is the Law of Definite Proportions. Compounds can only be separated by chemical reactions, not physical means.

Mixtures

A mixture contains two or more substances that are physically combined. The substances keep their own properties and can usually be separated by physical methods (e.g. filtration, evaporation).

Homogeneous vs Heterogeneous Mixtures

FeatureHomogeneous mixture (solution)Heterogeneous mixture
AppearanceUniform throughout — looks the same in every partNon-uniform — different parts look different
Components visible?No — components are not visible to the naked eyeOften yes — distinct parts can be seen
ExamplesSalt water, vinegar, air, bronze (alloy), soft drinkSand + water, soil, oil + water, salad, granite
SeparationUsually harder — requires evaporation or distillationEasier — filtration, sieving, or decanting

Air is a homogeneous mixture of nitrogen (~78%), oxygen (~21%), argon (~1%), and small amounts of other gases. It looks uniform, but can be separated into its component gases by fractional distillation of liquid air.

Physical vs Chemical Changes

Understanding how compounds and mixtures form requires knowing the difference between physical and chemical changes:

Signs of a chemical change: colour change, gas produced (bubbling), temperature change, light produced, precipitate forms, new smell. Forming a compound = chemical change. Making a mixture = physical change.

Molecule Viewer & Mixture Quiz

Select a molecule to see its ball-and-stick model rotating on the canvas. Then test yourself with the mixture quiz on the right.

Drag the canvas to rotate · Ball-and-stick model
Select Molecule
H₂O
Water
Compound
Water is one of the most important compounds on Earth. It is made of 2 hydrogen atoms covalently bonded to 1 oxygen atom. Its bent shape gives it unusual properties, including a high boiling point.
Is It a Mixture? — Mini Quiz
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Review the explanations above for any you missed.
Answer these questions in your exercise book. Show all reasoning where required.
Question 1 · (3 marks)
Define each of the following and give one example of each: (a) element, (b) compound, (c) mixture.
Question 2 · (3 marks)
Sodium (Na) is a reactive metal and chlorine (Cl) is a toxic gas. Yet when they combine to form sodium chloride (NaCl), the product is harmless table salt. (a) Is this a physical or chemical change? (b) What type of substance is NaCl? (c) Explain why the properties of NaCl are so different from its constituent elements.
Question 3 · (2 marks)
Explain the difference between a homogeneous mixture and a heterogeneous mixture. Classify each of the following: (i) salt water, (ii) soil, (iii) air, (iv) oil and water.
Question 4 · (4 marks)
Classify each of the following as an element, compound, or mixture. Give a reason for each answer: (a) pure iron (Fe), (b) carbon dioxide (CO₂), (c) seawater, (d) pure gold (Au).
Question 5 · (2 marks)
List FOUR signs that a chemical change has taken place. Then decide: is dissolving salt in water a physical change or a chemical change? Explain your answer.
Question 6 · (4 marks)
Air is described as a homogeneous mixture. (a) Name THREE gases found in air and give approximate percentages for each. (b) Explain why air is classified as a mixture rather than a compound. (c) Name the industrial process used to separate the gases in air.
Question 7 · (4 marks)
Liquid air is warmed slowly so its gases can be separated. The table below shows the boiling point of each gas.
GasBoiling point (°C)
Nitrogen (N₂)−196
Argon (Ar)−186
Oxygen (O₂)−183
(a) Using the data, which gas boils off FIRST as the liquid air warms up? Explain your answer. (b) Calculate the temperature difference, in °C, between the boiling points of nitrogen and oxygen. (c) Name this separation process, and explain why it is able to separate the gases even though air is a homogeneous mixture.