1.1.1—Elements, compounds, and mixtures

Classify matter by asking what particles are present, whether different elements are chemically bonded, and whether the composition is fixed. An element contains one type of atom and cannot be chemically broken down. A compound contains atoms of different elements bonded in a fixed ratio, so it has its own properties. A mixture contains two or more substances in no fixed ratio; the components are not chemically bonded to one another and retain properties that can be used for separation.

Class	Particle-level test	Composition	How components can be obtained
Element	One type of atom	Fixed identity	Cannot be chemically broken into simpler substances
Compound	Different elements chemically bonded	Fixed ratio	Requires a chemical change
Mixture	More than one substance without bonding between components	Variable ratio	Uses a physical-property difference

Formation of a compound creates new properties: sodium and chlorine do not keep their separate properties after forming sodium chloride. Mixing substances does not have that effect. A homogeneous mixture is uniform at the scale observed; a heterogeneous mixture has distinguishable regions. Both remain mixtures.

Choose a separation method in three linked moves: identify a physical-property difference, select the operation that exploits it, then state which component is recovered where. Use magnetism for a magnetic solid; filtration for an insoluble solid suspended in a fluid; crystallization or evaporation to recover a dissolved solid; and simple distillation to recover a solvent or separate liquids with a sufficiently wide boiling-point gap. Use fractional distillation when miscible liquids have closer boiling points. Chromatography separates through different relative attractions to the mobile and stationary phases. A multi-component mixture may require a sequence of methods.

Uniform appearance is not evidence of a compound: air and salt solution are homogeneous mixtures. Also, do not justify filtration by saying only that substances have different solubilities. The separated solid must be insoluble, and the filter works because its particles do not pass through the pores.

Diagnostic check: a particle box containing two unbonded particle types represents a mixture even if it looks uniform. For an unknown mixture, complete three columns—chosen method, physical-property reason, and observable recovered fraction. Reject any proposal that changes the substances chemically or cannot state where each component goes.

How it is assessed

Questions test classification and definition of elements, compounds, and mixtures, then apply the classification to choosing or sequencing physical separation methods; both MCQ and structured formats occur.

Command terms

describe / suggest

What earns marks

State the defining contrast explicitly: one atom type versus different elements chemically bonded in fixed ratios, and no-fixed-ratio unbonded mixture components. For separation procedures, name each operation and connect it to the component recovered.

Watch for

Confusing fixed-ratio compounds with no-fixed-ratio mixtures, or naming a separation method without linking it to the relevant component or physical property.

Representative question

[Maximum number: 4]

Suggest a set of experimental steps required to obtain pure samples of each component of the mixture.

Retrieve the progression: classify matter as an element, compound, or mixture; use particle movement and spacing to explain states and state changes; then connect absolute temperature in kelvin with average kinetic energy.

When checking an answer, ask three questions: Is the composition fixed or separable physically? Which direction do the state symbols show? Am I using kelvin when the claim concerns average kinetic energy?

1.1.1—Elements, compounds, and mixtures

Classifying Matter by Composition

Separating Mixtures

Assessment in practice

Particulate Matter Summary

Question 1