SL & HL Structure · Models of the particulate nature of matter BETA S1.1

Particulate nature of matter

IB syllabus topic Structure 1.1 — Introduction to the particulate nature of matter

The particulate nature of matter explains that all substances are made of tiny moving particles; this revision note covers elements, compounds and mixtures, separation techniques, the kinetic molecular theory of solids, liquids and gases, and converting between Celsius and the absolute Kelvin temperature scale.

Part of IB Chemistry (2025-2030 syllabus) — Standard and Higher Level.

Key points

Matter is classified as elements (one type of atom), compounds (two or more elements chemically bonded in fixed ratios) or mixtures (two or more substances physically combined, in variable ratios and keeping their own properties).
Compounds can only be split by chemical means and have properties different from their elements; mixtures can be separated by physical means and keep the properties of their components.
Separation techniques exploit physical-property differences: filtration (particle size), evaporation/crystallisation (volatility), distillation (boiling point), and chromatography (relative attraction to a mobile vs stationary phase).
The kinetic molecular theory states that all matter is made of constantly moving particles; the amount of movement (kinetic energy) increases with temperature.
Solids have fixed shape and volume (close, ordered, vibrating particles), liquids have fixed volume but no fixed shape (close, disordered, sliding particles), and gases fill any container (far apart, fast, random particles).
Changes of state (melting, freezing, boiling, condensing, sublimation, deposition) are physical changes that involve gaining or losing energy without changing the chemical identity of the particles.
The Kelvin (absolute) temperature scale starts at absolute zero (0 K), the temperature at which particle motion is at a minimum; convert using temperature in K = temperature in degrees C + 273.

Subtopic by subtopic

Elements, compounds and mixtures

All matter can be sorted into three classes based on what its particles are.

An element contains only one type of atom (e.g. copper Cu, oxygen $\text{O}_2$ ). It cannot be broken down chemically.
A compound contains two or more elements chemically bonded in a fixed ratio (e.g. water $\text{H}_2\text{O}$ , carbon dioxide $\text{CO}_2$ ). Its properties differ from those of the elements it contains, and it can only be split by chemical means (reaction, electrolysis).
A mixture contains two or more substances that are physically combined, in variable proportions. Each component keeps its own properties, and the mixture can be separated by physical methods.

Mixtures are further classed as:

Homogeneous - uniform throughout, no visible boundaries (e.g. salt water, air, brass).
Heterogeneous - non-uniform, with distinguishable components or phases (e.g. sand and iron filings, oil and water).

A quick test: if a fixed formula can be written, it is an element or compound (a pure substance); if the composition can vary, it is a mixture. Pure substances also have sharp, fixed melting and boiling points, whereas mixtures melt/boil over a range of temperatures - a useful way to judge purity.

Separation techniques

Because the components of a mixture keep their own physical properties, we separate them by exploiting a difference in a physical property - never by reacting them.

Filtration - separates an insoluble solid from a liquid using a difference in particle size; the solid (residue) stays on the filter paper, the liquid (filtrate) passes through.
Evaporation / crystallisation - recovers a dissolved solute from a solution by removing the solvent; relies on a difference in volatility (the solvent evaporates, the solute does not).
Distillation - separates liquids using a difference in boiling point. Simple distillation separates a liquid from a non-volatile solute; fractional distillation separates two miscible liquids with close boiling points.
Chromatography - separates dissolved substances by their relative attraction to a moving mobile phase and a fixed stationary phase; components that cling to the stationary phase move slowly.

Choosing a method: identify the property that differs between the components (size, volatility, boiling point, solubility/attraction), then pick the technique that uses that difference.

States of matter and the kinetic molecular theory

The kinetic molecular theory says all matter is made of particles in constant motion, and that their average kinetic energy rises with temperature. This model explains the three states.

Solid - particles are close together in a regular, fixed arrangement; they only vibrate about fixed positions. Fixed shape and fixed volume; cannot be compressed.
Liquid - particles are close together but disordered; they can slide past one another. Fixed volume but takes the shape of its container; barely compressible.
Gas - particles are far apart and move fast and randomly in all directions. No fixed shape or volume - a gas fills its container and is easily compressed.

Changes of state are physical changes (the particles themselves are unchanged):

$\text{solid} \underset{\text{freezing}}{\overset{\text{melting}}{\rightleftharpoons}} \text{liquid} \underset{\text{condensing}}{\overset{\text{boiling}}{\rightleftharpoons}} \text{gas}$

Sublimation is solid to gas directly; deposition is gas to solid. Heating gives particles energy to overcome forces between them (melting, boiling); cooling removes energy so forces pull particles closer (condensing, freezing).

Temperature and the Kelvin scale

Temperature is a measure of the average kinetic energy of the particles in a sample. The hotter the substance, the faster its particles move on average.

Chemists use the Kelvin (absolute) scale because it has no negative values and is directly proportional to average kinetic energy. Its zero point, absolute zero (0 K), is the temperature at which particle motion is at a minimum - it is about $-273\ ^{\circ}\text{C}$ .

Convert between the scales with:

$T\,(\text{K}) = \theta\,(^{\circ}\text{C}) + 273$

(and subtract $273$ to convert back). For example, $25\ ^{\circ}\text{C} = 298\ \text{K}$ and $0\ \text{K} = -273\ ^{\circ}\text{C}$ .

Key ideas:

A 1 K change equals a 1 $^{\circ}\text{C}$ change - the scales have the same size of degree, just a different zero.
Always work in Kelvin for gas laws and energetics; using Celsius there gives wrong (sometimes negative) answers.
STP in the data booklet is defined at $273\ \text{K}$ (0 $^{\circ}\text{C}$ ) and 100 kPa, where one mole of gas occupies $22.7\ \text{dm}^3$ .

Formulae

T\,(\text{K}) = \theta\,(^{\circ}\text{C}) + 273

Converting a Celsius temperature $\theta$ to the absolute Kelvin scale (and subtract $273$ to go back). You CONSTRUCT this relation yourself; it is not printed in the data booklet. Always use Kelvin in gas-law and energetics calculations.

V_m = 22.7\ \text{dm}^3\,\text{mol}^{-1}

The molar volume of an ideal gas at STP, given in the data booklet ( $V_m = 2.27\times10^{-2}\ \text{m}^3\,\text{mol}^{-1}$ ). It quantifies how far apart gas particles are: one mole of any gas occupies the same large volume because gas particles are widely spaced. (Used in detail in later quantitative topics.)

Definitions

Element: A pure substance made of only one type of atom that cannot be broken down into simpler substances by chemical reactions.
Compound: A pure substance formed when two or more different elements are chemically bonded together in a fixed ratio; it can only be separated into its elements by chemical means.
Mixture: Two or more substances physically combined (not chemically bonded), present in variable proportions and separable by physical methods; each component keeps its own properties.
Homogeneous mixture: A mixture with uniform composition and properties throughout, with no visible boundaries between components (e.g. a salt solution, air).
Heterogeneous mixture: A mixture with a non-uniform composition in which the separate components or phases can be distinguished (e.g. sand in water, oil and water).
Kinetic molecular theory: The model that all matter consists of tiny particles in constant motion, whose average kinetic energy increases as temperature rises.
Absolute zero: The lowest possible temperature, 0 K (about -273 degrees C), at which particle motion and kinetic energy are at their minimum.

Worked examples

(SL) Classify each of the following as an element, a compound or a mixture: (a) nitrogen gas, $\text{N}_2$ ; (b) sodium chloride, $\text{NaCl}$ ; (c) clean air; (d) iron filings stirred into sulfur powder.

1
Ask: how many types of atom, and are they chemically bonded in a fixed ratio?
2
(a) $\text{N}_2$ is only nitrogen atoms - one type of atom - so it is an element (even though it is diatomic).
3
(b) $\text{NaCl}$ has two different elements chemically bonded in a fixed 1:1 ratio - a compound.
4
(c) Air contains several gases physically combined in variable proportions, uniform throughout - a homogeneous mixture.
5
(d) Iron and sulfur are just physically mixed (not heated/reacted), in any ratio, and can be separated by a magnet - a heterogeneous mixture.

Answer: (a) element; (b) compound; (c) mixture (homogeneous); (d) mixture (heterogeneous).

(SL) A student has a mixture of sand, table salt and water. Describe, in order, the separation techniques needed to obtain the dry sand and dry salt separately, and state the property each technique relies on.

1
Sand is insoluble in water; salt is soluble - so first remove the sand from the liquid.
2
Filter the mixture: sand stays as the residue on the filter paper; salt solution passes through as the filtrate. This relies on a difference in particle size (insoluble solid vs liquid).
3
Rinse and dry the sand residue.
4
Take the salt solution and heat it to evaporate the water (evaporation/crystallisation), leaving solid salt behind. This relies on a difference in volatility (water evaporates, salt does not).

Answer: 1) Filtration gives the sand (residue) - difference in particle size. 2) Evaporation/crystallisation of the filtrate gives the salt - difference in volatility.

(SL) (a) Convert $37\ ^{\circ}\text{C}$ and $-196\ ^{\circ}\text{C}$ to Kelvin. (b) Explain, in terms of particle motion, why $-196\ ^{\circ}\text{C}$ is a positive temperature on the Kelvin scale.

1
Use $T(\text{K}) = \theta(^{\circ}\text{C}) + 273$ .
2
(a) $37 + 273 = 310\ \text{K}$ .
3
$-196 + 273 = 77\ \text{K}$ .
4
(b) The Kelvin scale starts at absolute zero (0 K), the point of minimum particle motion. At $-196\ ^{\circ}\text{C}$ the particles still have kinetic energy (they are still moving), so the temperature must be above 0 K and therefore positive.

Answer: (a) $310\ \text{K}$ and $77\ \text{K}$ . (b) Because particles are still moving (have kinetic energy), the temperature is above absolute zero (0 K), so it is positive on the Kelvin scale.

Common mistakes

×Calling a diatomic molecule like $\text{O}_2$ or $\text{N}_2$ a compound. It is still an element because it contains only one type of atom; a compound needs two or more different elements.
×Confusing 'pure' in everyday language with chemistry: a pure substance is a single element or compound (one fixed formula), so even bottled mineral water is a mixture, not chemically pure.
×Saying particles 'melt', 'expand' or 'disappear' during a change of state. The particles themselves do not change - only their arrangement, spacing and energy change.
×Forgetting the +273 and doing gas-law or energetics maths in degrees Celsius. Always convert to Kelvin first; a Celsius value can even be negative, which breaks the calculation.

Exam tips

✓To classify a substance, decide first whether it is pure (fixed formula = element or compound) or a mixture (variable composition); then check if a pure substance has one type of atom (element) or more (compound).
✓When choosing a separation technique, always name the physical property the method exploits (particle size, volatility, boiling point, or attraction to a phase) - examiners reward the reasoning, not just the technique.
✓Link every state-of-matter answer back to the three particle features: spacing/arrangement, the type of movement, and the forces between particles - that is what gains the marks.
✓Remember that 0 K is absolute zero (minimum motion) and a 1 K change equals a 1 degree C change; convert with $T(\text{K}) = \theta(^{\circ}\text{C}) + 273$ before any gas or energy calculation.

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