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1.1 Introduction to the particulate nature of matter

Syllabus
First assessment 2025
Topic
1.1
Level
SL

Learning objectives

1.1.1Elements, compounds, and mixtures• Elements: primary constituents of matter, cannot be chemically broken down• Compounds: atoms of different elements bonded in fixed ratio• Mixtures: more than one element/compound in no fixed ratio, separated by physical methods• Distinguish homogeneous and heterogeneous mixtures and choose separation methods1.1.2Kinetic molecular theory• Explains physical properties of matter (solids, liquids, gases) and changes of state• State symbols: s, l, g, aq• Name changes of state: melting, freezing, vaporization, condensation, sublimation, deposition1.1.3Temperature and kinetic energy• Temperature T (Kelvin) = measure of average kinetic energy Ek• Convert between Celsius and Kelvin scales• Interpret temperature and physical property changes during changes of state

Classifying Matter by Composition

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.

Separating Mixtures

4 marks

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

Explaining States with Particles

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.

The same particles are closely ordered in a solid, close but mobile in a liquid, and far apart in a gas, with arrows naming each state change.
State Particle model Fixed volume? Fixed shape? Symbol
Solid Close, ordered or fixed positions; vibrate Yes Yes (s)
Liquid Close, disordered; move past neighbours Yes No (l)
Gas Widely spaced; rapid random motion No No (g)

(aq) does not name a fourth state of pure matter. It means the stated species is dissolved in water.

Change Direction Energy transfer for the substance
Melting solid → liquid absorbed
Freezing liquid → solid released
Vaporization liquid → gas absorbed
Condensation gas → liquid released
Sublimation solid → gas absorbed
Deposition gas → solid released

Heating increases particle motion until a transition begins. During the transition, supplied energy changes the extent of intermolecular attraction and particle arrangement; the chemical identity remains the same. Evaporation can occur at a liquid surface below the boiling point, whereas boiling occurs throughout a liquid when its vapour pressure matches the external pressure.

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.

State Change Reasoning

Practice only · progress is not saved

Temperature and Average Kinetic Energy

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.

A heating curve rises within each state and stays flat during melting and boiling while added energy changes particle arrangement.

T/K=T/°C+273.15T / K = T / °C + 273.15

Worked example — convert before interpreting

For 25.0 ∘C25.0\,^\circ\mathrm{C}, substitute into the conversion: T=25.0+273.15=298.15 KT=25.0+273.15=298.15\,\mathrm{K}, reported as 298.2 K298.2\,\mathrm{K} to one decimal place. The Kelvin value is the absolute temperature used for kinetic-energy comparisons; a rise of 10 K10\,\mathrm{K} is the same temperature interval as a rise of 10 ∘C10\,^\circ\mathrm{C}.

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.

Temperature Calculations

1 mark

What happens to the average kinetic energy, KE , of the particles in a gas when the absolute temperature is doubled?
KE=12mv2\mathrm{KE}=\frac{1}{2} \mathrm{mv}^{2}

Particulate Matter Summary

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?