Chemistry - Substances and Their Properties, Separation, States of Matter
This domain tests how physical properties identify substances, how mixtures are separated, and how the particle model explains changes of state. Questions often ask you to choose a separation method and justify it using a property difference.
Properties and particle model
Density = mass ÷ volume. Pure substances have fixed melting and boiling points; impurities lower melting point and raise boiling point. Solids have fixed particle positions that vibrate; liquids have close, randomly arranged particles that slide; gases have far-apart, fast particles. Diffusion is faster at higher temperature because particles move faster. State changes-melting, boiling, condensation, freezing, sublimation-involve energy but not a change in substance identity.
Separation methods
Filtration separates an insoluble solid from a liquid. Evaporation or crystallisation recovers a dissolved solid. Distillation separates liquids with different boiling points. Chromatography separates dyes by different solubilities and adsorption. Decantation and magnetic separation use density or magnetism. Choose the method from the property difference, not from habit.
Gas collection
Collect insoluble gases by water displacement. Collect gases denser than air by upward displacement of air; collect less dense gases by downward displacement. Warming a liquid usually decreases gas solubility.
Worked example
A metal sample has mass 54 g and displaces 20 cm³ of water. Density = 54 ÷ 20 = 2.7 g/cm³, consistent with aluminium. If the sample were a mixture, the density would be an average and would not prove a single pure metal.
Traps
Do not use filtration to separate dissolved salt. Distillation separates by boiling point, not particle size. Evaporation can occur below the boiling point. Density units must match: 1 g/cm³ = 1000 kg/m³. Read the meniscus at eye level, and remember that boiling point changes with pressure.