1The idea in one paragraph

Everything you can pick up, pour or breathe is made of particles too small to see: atoms, molecules or ions. They are always moving and they attract one another, and this subtopic follows from that picture. Whether a sample is an element, a compound or a mixture depends on what the particles are and whether they are bonded. Whether it is a solid, a liquid or a gas depends on their energy of motion compared with the attractions between them. Temperature measures that energy of motion, on average. This picture is the kinetic molecular theory, the first model of many in the course: not a photograph of matter, but a simplified description that explains and predicts what we observe.

2Elements, compounds and mixtures

An element is a substance that cannot be broken down into simpler substances by a chemical reaction. All of its atoms have the same number of protons (S1.2 makes that exact). Copper, carbon and oxygen are elements. An element can exist as single atoms, as argon does, or as molecules of the same atom bonded together, as oxygen does in O₂. Both are still elements.

A compound is made of atoms of two or more different elements chemically bonded together in a fixed ratio. Water is always two hydrogen atoms to one oxygen atom; sodium chloride is always one sodium to one chlorine. Because the atoms are bonded, a compound has properties of its own that are nothing like those of its elements. Sodium is a soft metal that fizzes in water and chlorine is a toxic green gas, yet sodium chloride is table salt. A compound can only be split into its elements by a chemical reaction.

A mixture contains two or more elements or compounds that are not chemically bonded to each other and are present in no fixed ratio. You can stir a little salt or a lot of salt into water and it is still salt water. Each substance in a mixture keeps its own properties, and that is why a mixture can be separated by physical methods that exploit the differences.

Figure 1 sorts all matter this way, and Figure 2 shows what each case looks like at the particle level.

The test for a particle diagram is simple: count the different kinds of particle, where a particle is an atom on its own or a group of atoms bonded together. One kind of particle containing one kind of atom is an element, as in panels (a) and (b). One kind of particle containing two or more kinds of atom is a compound, as in panel (c). More than one kind of particle is a mixture, as in panel (d), even if one of those particles is a compound.

Homogeneous and heterogeneous mixtures. A homogeneous mixture has the same composition throughout and you cannot see its separate parts. It is one phase: salt water, air, and an alloy such as brass are all homogeneous. A heterogeneous mixture is not uniform. It has more than one phase and you can often see the boundaries between them: sand in water, oil on water, granite with its visible crystals.

Alloys look like compounds but are not. Brass is copper and zinc held together by metallic bonding, yet it is a mixture: the proportion of zinc can vary widely, so there is no fixed ratio. Bonding does not make a compound; a fixed ratio of different elements does.

3Separating mixtures

Every separation method works the same way. Find a physical property that differs between the components, then use it. Figure 3 sets out the methods the guide names and the property behind each one.

Solvation (dissolving). When a solid dissolves, particles of the solvent surround particles of the solute and carry them into solution. Only some solids dissolve in a given solvent, so dissolving is a way of pulling one substance away from others. Salt dissolves in water; sand does not.

Filtration separates an insoluble solid from a liquid. The liquid and anything dissolved in it pass through the tiny holes in the filter paper as the filtrate; the solid particles are too large and are left on the paper as the residue.

Evaporation removes a volatile solvent by heating, leaving the dissolved solid behind. If you want well-formed crystals, heat only until the solution is concentrated, then let it cool slowly: that is crystallization. Heating to dryness gives a powder, and can decompose some solids.

Recrystallization purifies an impure solid. Dissolve it in the minimum volume of hot solvent, so the solution is saturated when hot. As it cools, the solubility of the main substance falls and it crystallizes out. The impurities are present in much smaller amounts, so they stay dissolved in the cold solvent. Filter off the crystals, rinse with a little cold solvent, and dry. It works because solubility changes with temperature, and because the impurity never reaches its own saturation point.

Distillation separates a liquid from a solution when you want the liquid. The mixture is boiled; the vapour of the component with the lower boiling point passes into a condenser, is cooled back to a liquid, and is collected as the distillate. Figure 4 shows the apparatus. Two liquids with close boiling points need fractional distillation, which adds a column packed with glass beads so the vapour condenses and re-boils many times on its way up.

Chromatography separates small amounts of dissolved substances. In paper chromatography a spot of the mixture is placed on a pencil line near the bottom of a strip of paper, and the paper stands in a solvent. The solvent (the mobile phase) rises up the paper (the stationary phase). Each substance spends part of its time dissolved in the moving solvent and part of its time held on the paper. A substance that is more attracted to the solvent travels further. Figure 5 shows a mixture splitting into two spots.

Each spot is described by its retardation factor:

Rf = distance moved by the spot ÷ distance moved by the solvent front, both measured from the baseline.

Rf has no units and always lies between 0 and 1. Under the same conditions (same paper, same solvent, same temperature) a substance always gives the same Rf, so matching an unknown spot to a known one identifies it. In Figure 5 the pure dye run beside the mixture rises to the same height as the teal spot, so the mixture contains that dye.

Putting methods together. To get dry salt from rock salt (salt with sand and grit): dissolve in water, filter off the sand, evaporate the filtrate until crystals start to form, let it crystallize, then filter and dry the crystals. Purifying the product of a reaction is the same thinking: filter and wash an insoluble product, recrystallize a soluble solid, distil a liquid.

4The kinetic molecular theory and the three states

The kinetic molecular theory describes matter as particles in constant, random motion, with attractive forces between them. The kinetic energy of the particles is their energy of motion, and it increases with temperature. The state of a substance depends on a contest between two things: the kinetic energy of the particles, which tends to push them apart, and the attractions between them, which hold them together. Figure 6 shows the three results.

Each property has its reason in the particle picture: a gas can be squashed because most of its volume is empty space, a liquid flows because its particles slide past one another, a solid keeps its shape because its particles cannot leave their positions. "Use the kinetic molecular theory" means: property, then particles.

5State symbols

A balanced equation says which substances react and in what amounts. State symbols add the physical state of each one, in brackets after its formula: (s) solid, (l) liquid, (g) gas, (aq) aqueous, meaning dissolved in water.

• Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
• NaCl(s) → Na⁺(aq) + Cl⁻(aq), which is salt dissolving in water
• H₂O(l) → H₂O(g), which is water boiling

Two traps. Water itself is (l), never (aq): aqueous means dissolved in water. And the state is the one under the reaction's conditions, so water made in a flame is H₂O(g), a choice that changes answers in Reactivity 1.

6The six changes of state

Figure 7 names every change of state and shows which way energy flows in each.

Understand the pattern instead of memorising it. Moving towards the gas separates particles against their attractions, which needs energy, so melting, vaporization and sublimation are endothermic. The reverse changes re-form attractions and release energy, so they are exothermic.

Vaporization has two forms, and the exam separates them. Evaporation happens only at the surface of a liquid, at any temperature below the boiling point: the fastest-moving particles at the surface escape. Boiling happens throughout the liquid at one temperature, the boiling point, and you see it as bubbles of vapour forming inside the liquid.

Solid carbon dioxide ("dry ice") sublimes at atmospheric pressure, which is why it leaves no puddle. Frost forming on a cold window overnight is deposition: water vapour goes straight to ice.

7Temperature, kinetic energy and the Kelvin scale

In a sample, the particles do not all move at the same speed. Some are fast, some slow, and collisions constantly swap energy between them. The temperature of the sample measures the average kinetic energy of its particles. Heat the sample and the average rises.

The Kelvin scale makes that relationship direct: the temperature T in kelvin is proportional to the average kinetic energy of the particles. Its zero, absolute zero (0 K), is the temperature at which the particles would have the least possible energy of motion; nothing can be colder. The kelvin is the SI unit of temperature, and a change of 1 K is exactly the same size as a change of 1 °C. Only the starting point differs, as Figure 8 shows.

T (K) = θ (°C) + 273.15, and a temperature change is the same number in K and in °C.

Write "K", not "°K". Mark schemes usually accept 273, but 273.15 costs nothing.

One consequence for S1.5: at the same temperature every gas has the same average kinetic energy per particle. Kinetic energy is ½mv², so light hydrogen molecules move faster on average than heavy carbon dioxide molecules. The spread of energies around the average is a graph you meet in Reactivity 2.2.

8Heating and cooling curves

Heat a block of ice at a steady rate, read a thermometer every minute, and plot temperature against time. Figure 9 is the result.

Read it section by section.

Sloping sections: one state, temperature rising. The energy supplied increases the kinetic energy of the particles, so the temperature goes up. On the first slope all of it is ice, on the middle slope all of it is water, on the last slope all of it is steam.

Flat sections: a change of state, temperature constant. At 0 °C the ice melts. Energy is still going in, but it is used to overcome the attractions between the particles, not to speed them up. The average kinetic energy does not change, so the temperature does not change. Solid and liquid are both present until the last of the ice has gone. At 100 °C the same thing happens as the water boils, with liquid and gas both present.

Why the boiling plateau is longer. Melting only loosens the particles: they are still touching in the liquid. Boiling separates them completely, which means overcoming almost all of the remaining attraction. For water, boiling takes about 40.7 kJ per mole against about 6.0 kJ per mole for melting, nearly seven times as much, so at a steady rate of heating the boiling plateau lasts nearly seven times as long. Figure 9 is not drawn to that scale, but the boiling plateau is always the longer one.

A cooling curve is the same graph run backwards. The temperature falls on each slope. It holds steady while the liquid freezes, because the attractions re-forming release energy that balances the energy being lost to the surroundings.

What you would see is part of the skill: solid and liquid together on the melting plateau, bubbles forming throughout the liquid on the boiling plateau, and an invisible gas above 100 °C (the white cloud over a kettle is droplets that have already condensed).

Purity from a curve. A pure substance melts and boils at one sharp temperature, so its plateau is flat. An impure substance changes state over a range of temperatures, and its melting point is lowered, so its "plateau" slopes. A sharp melting point matching the known value is how chemists check that a recrystallized product is pure.