1.1 Introduction to the particulate nature of matter

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.

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.

The kinetic molecular theory is a model that explains observable state properties using particle arrangement, movement, spacing, and attractions. Solid particles are closely packed and vibrate about fixed positions. Liquid particles remain close but change neighbours and flow past one another. Gas particles are far apart relative to their size and move freely, so a gas fills its container and is much more compressible.

Do not say that particles themselves melt, expand, or become a different substance. In a physical state change, the same particles adopt different motion and arrangement. For molecular substances, the energy change is associated with intermolecular attractions; it does not generally break the covalent bonds inside each molecule.

Temperature on the Kelvin scale is proportional to the average translational kinetic energy of particles. At the same Kelvin temperature, samples of different gases have the same average kinetic energy, although lighter particles have a higher typical speed than heavier particles. Use absolute temperature, not degrees Celsius, when comparing kinetic energies or temperature ratios.

Read a heating curve by first deciding whether the substance is within one state or changing state. On a sloping section, supplied energy increases average kinetic energy, so temperature rises. On a horizontal phase-change section at constant pressure, energy is still absorbed, but it is used to overcome intermolecular attractions and change particle arrangement; temperature and average kinetic energy stay constant until that change is complete. Cooling reverses the energy flow.

A 20 K temperature interval has the same size as a 20 °C interval, but 20 °C is not an absolute temperature of 20 K. Do not infer that heavier gas particles have greater average kinetic energy at the same temperature, and do not interpret a heating-curve plateau as a period when no energy is transferred.