States of Matter
The particle model, diffusion, purity and the separation techniques, the practical-paper section, where knowing why a method works is the difference between a description and an explanation.
Three states and the changes between them
The kinetic particle theory: everything is made of particles in constant motion, and temperature measures their average energy. Solid: particles touching, regular lattice, vibrating about fixed positions. Liquid: touching, irregular, sliding past each other. Gas: far apart, random, fast, colliding with each other and the walls.
Melting and boiling: particles gain energy, vibrate harder, and the forces between them are overcome. During a change of state the temperature stays constant because the energy is going into separating particles, not speeding them up. Condensing and freezing are the reverse. Sublimation is solid straight to gas, iodine and dry ice do it.
Gas pressure is particles hitting the walls. Heat a gas at fixed volume: faster particles, harder and more frequent collisions, higher pressure. Compress it at fixed temperature: same particles in less space, more frequent collisions, higher pressure. Both explanations are three-mark answers and the paper wants all three ideas.
Diffusion, and the two experiments
Diffusion is the net movement of particles from a region of higher concentration to lower, as a result of their random motion. It needs no stirring; it is what random motion does. Faster at higher temperature (more energy), and faster for lighter particles.
Bromine in a gas jar: the brown vapour spreads upward through air even though Br₂ (Mr 160) is far heavier than air. Random motion, not density, is the point.
Ammonia and hydrogen chloride from opposite ends of a tube: a white ring of ammonium chloride forms nearer the HCl end, because NH₃ (Mr 17) is lighter than HCl (36.5) and diffuses faster. If a question gives two gases and asks which reaches a detector first, compare Mr.
Purity, and how a melting point tells you
A pure substance is one substance, and it melts and boils at a sharp, fixed temperature. An impure one melts over a range and lower; it boils over a range and higher. That is the test: a solid that melts sharply at the data-book value is pure. In the exam, "pure" also has its everyday meaning stripped away, pure orange juice is a mixture, however natural.
Chromatography separates by how strongly each substance is attracted to the solvent versus the paper. Draw the base line in pencil (ink would run), keep the solvent below the line at the start, stop before the front reaches the top. Identical substances give identical Rf values in the same solvent: Rf = distance moved by substance ÷ distance moved by solvent front, always between 0 and 1. Colourless substances need a locating agent.
A pure substance gives one spot. A mixture gives several. Two samples giving the same Rf in two different solvents are almost certainly the same compound.
Choosing the separation method
The question names a mixture; you name the method and say why it works. Learn these pairings.
- Insoluble solid from liquid (sand from water), filtration. The residue stays on the paper; the filtrate passes through.
- Soluble solid from solution (salt from brine), evaporate to the point of crystallisation, then cool and crystallise. Do not boil to dryness for a hydrated salt; the water of crystallisation would be lost.
- Solvent from a solution (water from seawater), simple distillation. The solvent boils, the vapour condenses in the condenser, the solute stays behind.
- Two miscible liquids (ethanol from water), fractional distillation. The lower-boiling liquid vaporises first; the column with glass beads gives repeated condensing and re-evaporating so only the purest vapour reaches the top.
- Coloured substances in ink or dye: chromatography.
Rock salt to pure salt is the three-step classic: dissolve in water, filter off the sand, evaporate to crystallise. Order matters and each step needs its reason.
Where marks go
- "Particles expand when heated." They move faster and further apart. The particles themselves do not change size.
- Diffusion explained as "heavy gases sink". Random motion spreads them regardless of density.
- Melting range read as "melts at 75 °C". A range means impure.
- Chromatography base line in ink. Pencil.
- Rf greater than 1. Impossible; the substance cannot outrun the solvent.
- "Evaporate to dryness" for crystals. Evaporate partially, then cool and crystallise.
What to remember
- Solid/liquid/gas: arrangement, motion, spacing, three points each.
- Gas pressure = collisions with walls. More energy or less space = more pressure.
- Diffusion: random motion, high → low concentration, faster when hot and for lighter particles.
- Pure = sharp m.p./b.p. Impure = range, lower m.p. higher b.p.
- Rf = substance ÷ solvent, pencil line, solvent below the line.
- Filter, crystallise, distil, fractionally distil, chromatograph. Match each to its mixture.
Watch one worked through
Here is one done slowly, with the reason for every move. Then one for you.
A gas syringe at constant temperature is compressed from 60 cm³ to 20 cm³. The original pressure was 100 kPa. Find the new pressure and explain it with particles.
- Note that temperature is constant, so Boyle's law applies.P₁V₁ = P₂V₂Check the condition before using the law. If temperature changed, this equation would not hold.
- Substitute the known values.100 × 60 = P₂ × 20 6000 = 20 P₂Units cancel because the same unit appears on both sides, so cm³ and kPa can be used directly.
- Solve for the new pressure.P₂ = 6000 ÷ 20 = 300 kPaVolume fell to a third, so pressure tripled. Sanity-check that the two moved in opposite directions.
- Explain with particles.same particles, smaller space → more collisions per second with the walls → higher pressurePressure is the force of collisions on the walls. The particles do not get bigger or faster, there are just more hits per second.
Answer: 300 kPa, because the same particles in a smaller volume hit the walls more often
A gas at 200 kPa in 10 cm³ is expanded to 20 cm³ at constant temperature. New pressure in kPa?
Show the answer
100
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