This topic classifies all matter as an element, a compound or a mixture. Elements and compounds are pure substances. Mixtures are not pure, and the two types met most often at this level are solutions and suspensions.
Later work on the particulate model of matter, separation techniques and chemical changes all builds on this classification, so it should be learnt first. Telling a compound from a mixture, and giving the evidence for the choice, is one of the most heavily tested skills in the topic.
What these notes cover
- •Classifying matter: elements, compounds and mixtures
- •Decision test: element, compound or mixture?
- •Elements and chemical symbols
- •Metals and non-metals, and the exceptions
- •Compounds: fixed ratio and new properties
- •Mixtures and alloys
- •Iron and sulfur: mixture versus compound
- •Solutions and suspensions
- •Rate of dissolving
- •Solubility and solubility curves (optional extension)
- •Worked examples
- •Common mistakes and exam technique
Key definitions
- Element
- A pure substance that cannot be broken down into simpler substances by chemical methods. It is made up of only one kind of atom.
- Compound
- A pure substance made of two or more different elements chemically combined in a fixed ratio by mass. Its properties are different from those of its constituent elements.
- Mixture
- Two or more substances (elements, compounds or both) that are physically combined, not chemically combined, in no fixed ratio. Each component keeps its own properties.
- Alloy
- A mixture of a metal with one or more other elements, which may be metals or non-metals, e.g. steel (iron + carbon), brass (copper + zinc) and bronze (copper + tin).
- Suspension
- A mixture in which small pieces of an insoluble substance are scattered through a liquid without dissolving. It is cloudy, settles on standing and leaves a residue when filtered.
Key formulas
Forming a solution
solute + solvent → solution
The solute is the substance that dissolves, the solvent does the dissolving. A solution is a homogeneous mixture, not a compound.
Solubility
solubility = maximum mass of solute (g) that dissolves in 100 g of solvent
Must always be given at a stated temperature, e.g. 32 g per 100 g of water at 20 °C.
Scaling solubility to a different mass of solvent
maximum mass of solute = solubility × (mass of solvent ÷ 100 g)
The mass of solute that can dissolve is proportional to the mass of solvent, at the same temperature.
Mass of crystals formed on cooling
mass of crystals = mass dissolved at higher temperature − maximum mass that can stay dissolved at lower temperature
Applies when a saturated solution is cooled and the solubility falls. The remaining solution is still saturated.
Fixed mass ratio in iron sulfide
mass of iron : mass of sulfur = 7 : 4, so mass of sulfur = (4 ÷ 7) × mass of iron
Applies to the compound only. For example, 14.0 g of iron needs 8.0 g of sulfur. A mixture of iron and sulfur has no fixed ratio.
Fixed mass ratio in water
mass of hydrogen : mass of oxygen = 1 : 8
Every pure sample of water has this composition: 9 g of water always contains 1 g of hydrogen and 8 g of oxygen.
Worked example
The solubility of solid X in water is 32 g per 100 g of water at 20 °C and 106 g per 100 g of water at 60 °C. (a) Find the maximum mass of X that can dissolve in 250 g of water at 60 °C. (b) The saturated solution from (a) is cooled to 20 °C. Find the mass of crystals of X that form. (c) State whether crushing the solid into a fine powder, or stirring, would change the answer to (a). Explain your answer.
- 1Solubility is quoted for 100 g of water, so the given mass of water is 250 g ÷ 100 g = 2.5 times larger.
- 2(a) Maximum mass of X at 60 °C = 106 g × 2.5 = 265 g.
- 3(b) The mass of water does not change on cooling, so it is still 250 g, and the maximum mass of X at 20 °C = 32 g × 2.5 = 80 g.
- 4The extra solute cannot stay dissolved, so mass of crystals = 265 g − 80 g = 185 g, and the remaining solution is still saturated with 80 g of X in 250 g of water.
- 5(c) Crushing gives a larger surface area in contact with the water and stirring brings fresh water into contact with the solid, so both increase only the rate of dissolving.
- 6Solubility of X depends on temperature, not particle size or stirring, so at 60 °C in 250 g of water the maximum is still 265 g; it simply dissolves in a shorter time. The answer to (a) does not change.
Watch out: Solubility is quoted per 100 g of solvent, so scale it to the mass of water given before subtracting.
Common mistakes
Calling an alloy a compound
Wrong: An alloy such as brass or steel is a compound because the metals are joined together.
Right: An alloy is a mixture of a metal with one or more other elements. The elements are not chemically combined and their proportions can be varied: brass can be made with more or less zinc and is still brass.
Counting atoms instead of types of atom
Wrong: Oxygen gas is O₂, which has two atoms, so it is a compound.
Right: Count the types of atom, not the number of atoms. O₂ contains only one type of atom, so it is an element. CO₂ contains two types of atom chemically combined, so it is a compound.
Confusing clear with colourless
Wrong: The solution is clear, so it is colourless.
Right: Clear means transparent: light passes through and no particles can be seen. Colourless means it has no colour. Copper(II) sulfate solution is clear and blue; water is clear and colourless.
Questions students ask
What is the difference between a compound and a mixture?
In a compound the elements are chemically combined in a fixed ratio, the compound has new properties, and it can only be broken down by chemical means such as thermal decomposition or electrolysis. In a mixture the components are physically combined in any ratio, each keeps its own properties, and they can be separated by physical methods such as filtration, evaporation, distillation or a magnet.
Is brass an element, compound or mixture?
Brass is a mixture. It is an alloy of copper and zinc that are not chemically combined, and the proportions can be varied. It is not found in the Periodic Table and has no fixed ratio of copper to zinc.
How do you tell a solution from a suspension?
A solution is clear, does not settle on standing, leaves no residue when filtered and lets light pass straight through. A suspension is cloudy, its insoluble particles settle to the bottom, it leaves a residue on the filter paper and it scatters or blocks light.
Does stirring increase solubility?
No. Stirring and using smaller particles increase only the rate of dissolving, so the solute dissolves in a shorter time. Solubility, the maximum mass that can dissolve, depends on the temperature and on the solute and solvent, not on stirring or particle size.
In the full PDF
- •Classification tree of matter and a simplified Periodic Table outline showing where metals and non-metals sit
- •Full table of chemical symbols with their Latin origins, the metals versus non-metals property table and the exceptions to know
- •Iron and sulfur particle diagram with the full compound-versus-mixture comparison table
- •Solubility curves for solids X and Y with a worked example on reading them
- •Two further worked examples: classifying six substances with reasons, and using experimental evidence to tell a mixture from a compound
- •Remaining common mistakes, model exam answers, the fair-test variables table, a final checklist and the topic summary