What you'll learn
States of matter and the particle theory is the foundation topic of CXC CSEC Chemistry, and every later unit depends on it. The kinetic theory holds that all matter is made of particles in constant motion, and that the arrangement, spacing and energy of those particles determine whether a substance is a solid, a liquid or a gas. Once that model is secure, changes of state, diffusion, gas behaviour and the separation of mixtures all follow from it. By the end of this guide you should be able to describe the arrangement, motion and energy of particles in each state, explain every change of state including sublimation, interpret heating and cooling curves and explain the plateaus, describe and explain diffusion, distinguish elements, compounds and mixtures, and select an appropriate separation technique for any mixture.
Key terms and definitions
Kinetic theory — the model stating that all matter consists of particles in constant motion
Solid — a state in which particles are closely packed in a regular arrangement and vibrate about fixed positions
Liquid — a state in which particles are close together but randomly arranged and able to move past one another
Gas — a state in which particles are far apart and move rapidly in random directions
Melting — the change from solid to liquid at the melting point
Boiling — the change from liquid to gas throughout the liquid at the boiling point
Evaporation — the change from liquid to gas at the surface, occurring below the boiling point
Sublimation — the direct change from solid to gas without passing through the liquid state
Diffusion — the net movement of particles from a region of higher concentration to one of lower concentration
Element — a substance containing only one type of atom
Compound — two or more elements chemically combined in fixed proportions
Mixture — two or more substances not chemically combined, each keeping its own properties
Core concepts
The three states of matter
In a solid, the particles are packed closely together in a regular, orderly arrangement. Strong forces of attraction hold them in fixed positions, and they can only vibrate. A solid therefore has a fixed shape and a fixed volume, and it cannot be compressed because there is almost no space between the particles.
In a liquid, the particles are still close together, so the volume is fixed, but they are randomly arranged and the forces between them are weaker. The particles can slide past one another, so a liquid flows and takes the shape of its container. Liquids are almost incompressible.
In a gas, the particles are very far apart with negligible forces of attraction between them, and they move rapidly in random directions. A gas has neither a fixed shape nor a fixed volume; it fills whatever container it occupies and is easily compressed because of the large spaces between particles.
Density follows directly from spacing. Solids are usually densest, liquids slightly less dense, and gases far less dense than either.
Changes of state
Adding energy to a solid makes the particles vibrate more strongly. At the melting point the vibrations are sufficient to overcome the forces holding the particles in fixed positions, and the solid melts.
Further energy makes the liquid particles move faster. At the boiling point the particles have enough energy to overcome the attractive forces completely and escape as a gas, forming bubbles throughout the liquid.
Removing energy reverses both processes: condensation from gas to liquid, and freezing from liquid to solid. The freezing point and the melting point of a substance are the same temperature.
Sublimation is the direct change from solid to gas without any liquid stage. Iodine, solid carbon dioxide and ammonium chloride all sublime, and each appears regularly in examination questions.
Evaporation differs from boiling in three examinable ways: it occurs only at the surface of a liquid, it happens at any temperature below the boiling point, and it involves only those particles with above-average energy escaping. Because the fastest particles leave, the average energy of those remaining falls, which is why evaporation causes cooling. This explains why sweating cools the body and why a wet cloth feels cold.
The rate of evaporation increases with higher temperature, larger surface area, moving air and lower humidity.
Heating and cooling curves
A graph of temperature against time for a substance being heated steadily has a distinctive shape: sloping sections separated by horizontal plateaus.
The sloping sections show the temperature rising while the substance remains in one state. The energy supplied increases the kinetic energy of the particles, so they move faster and the temperature rises.
The horizontal plateaus show changes of state. During a plateau the temperature stays constant even though energy is still being supplied, because the energy is being used to overcome the forces of attraction between the particles rather than to increase their kinetic energy. The first plateau corresponds to melting and the second to boiling, and the boiling plateau is the longer of the two because more energy is needed to separate the particles completely than merely to let them move past one another.
Explaining why the plateau is flat, in terms of overcoming forces rather than increasing kinetic energy, is the single most valuable explanation in this topic.
A pure substance melts and boils at sharp, fixed temperatures, whereas a mixture melts and boils over a range. This gives a practical test for purity.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, resulting from the random motion of particles. It explains why a perfume released in one corner of a room is eventually smelled throughout it.
Diffusion is fastest in gases, because the particles move quickly and are widely spaced; slower in liquids, because particles are closer together and collide more often; and negligible in solids, because the particles cannot change position.
Three factors increase the rate of diffusion: a higher temperature, because particles have more kinetic energy and move faster; a steeper concentration gradient; and a lower relative molecular mass, because lighter particles move faster at a given temperature.
The classic demonstration uses cotton wool soaked in concentrated ammonia solution at one end of a long glass tube and cotton wool soaked in concentrated hydrochloric acid at the other. A white ring of ammonium chloride forms where the two gases meet, and it forms nearer the hydrochloric acid end. The reason is that ammonia has a lower relative molecular mass than hydrogen chloride, so ammonia particles move faster and travel further before the two meet.
Elements, compounds and mixtures
An element contains only one type of atom and cannot be broken down into simpler substances by chemical means.
A compound contains two or more elements chemically combined in fixed proportions. Its properties differ entirely from those of its constituent elements, and separating it requires a chemical reaction. Energy is usually released or absorbed when a compound forms.
A mixture contains two or more substances that are not chemically combined. Each substance keeps its own properties, the proportions can vary, and separation is achieved by physical means with no chemical reaction and no significant energy change.
The comparison between iron and sulfur illustrates all of this. A mixture of iron filings and sulfur powder can be separated with a magnet, and the iron remains magnetic. Heated together they react to form iron sulfide, a compound that is not magnetic, cannot be separated by a magnet, and has properties belonging to neither element.
Separating mixtures
Because the components of a mixture keep their own properties, separation exploits a difference between them, and choosing the right method means identifying which property differs.
Filtration separates an insoluble solid from a liquid, using the difference in particle size. The solid collected in the filter paper is the residue and the liquid passing through is the filtrate.
Evaporation and crystallisation recover a dissolved solid from its solution. Gentle heating evaporates some of the solvent, and slow cooling then allows pure crystals to form. Slow cooling produces larger, purer crystals.
Simple distillation separates a solvent from a solution, using the difference in boiling points. The solution is heated, the vapour passes into a condenser where it cools and condenses, and the pure liquid is collected as the distillate.
Fractional distillation separates two or more miscible liquids with different boiling points. A fractionating column provides a temperature gradient so that the liquid with the lowest boiling point reaches the top first. It is used to separate ethanol from water and to separate crude oil into fractions.
Chromatography separates dissolved substances that travel at different rates through a stationary phase.
Using a separating funnel separates two immiscible liquids, such as oil and water, which form distinct layers.
Using a magnet separates a magnetic solid from a non-magnetic one.
Sublimation separates a subliming solid, such as ammonium chloride or iodine, from one that does not sublime.
Worked examples
Example 1: Explaining a flat section on a heating curve (4 marks)
A solid is heated steadily. Explain why its temperature remains constant for a period even though heating continues.
During this period the substance is changing state. The energy supplied is being used to overcome the forces of attraction between the particles, allowing them to move apart, rather than to increase their kinetic energy.
Temperature is a measure of the average kinetic energy of the particles. Since the kinetic energy is not increasing during the change of state, the temperature remains constant. Once the change of state is complete, further energy again increases the kinetic energy and the temperature begins to rise.
Example 2: Explaining the ammonia and hydrogen chloride experiment (4 marks)
Cotton wool soaked in concentrated ammonia is placed at one end of a glass tube and cotton wool soaked in concentrated hydrochloric acid at the other. A white ring forms nearer the hydrochloric acid end. Explain.
Both liquids produce gases that diffuse along the tube, and where they meet they react to form solid ammonium chloride, seen as a white ring.
Ammonia has a lower relative molecular mass than hydrogen chloride. At the same temperature, particles of lower mass move faster, so the ammonia particles diffuse more rapidly along the tube. The ammonia therefore travels further before the two gases meet, which is why the white ring forms closer to the hydrochloric acid end.
Example 3: Choosing separation techniques (4 marks)
Describe how you would obtain pure water and pure salt from a mixture of sand, salt and water.
First filter the mixture. The sand is insoluble and is retained as the residue in the filter paper, while the salt solution passes through as the filtrate.
Then separate the filtrate by simple distillation. The solution is heated, the water evaporates and the vapour passes into a condenser, where it cools and condenses to give pure water collected as the distillate. The salt remains in the flask, since it does not evaporate.
Alternatively, if only the salt is required, the filtrate can be heated to evaporate most of the water and then left to cool slowly so that pure salt crystals form.
Common mistakes and how to avoid them
The most frequent error in this topic is stating that particles expand when a substance is heated. Particles do not change size; they move faster and further apart, so the substance expands.
Students often confuse evaporation with boiling. Evaporation occurs only at the surface and at any temperature; boiling occurs throughout the liquid at a fixed temperature.
Another common slip is saying that energy during a change of state is used to make particles move faster. It is used to overcome the forces of attraction between them, which is exactly why the temperature does not rise.
In the diffusion experiment, many candidates attribute the position of the ring to concentration rather than to relative molecular mass. The masses are the reason.
Finally, candidates frequently describe a compound as a mixture of elements. A compound involves chemical combination in fixed proportions and can only be separated chemically.
Exam technique for "States of Matter and the Particle Theory"
When describing a state, give all three features — arrangement, spacing and movement of particles — since marks are usually distributed across them.
For heating curve questions, identify what is happening in each section before writing, and use the phrase overcoming forces of attraction for every plateau.
For separation questions, name the property that differs between the components before naming the method. Solubility, boiling point, particle size, magnetism and density each point to a different technique.
Where a question asks you to distinguish a compound from a mixture, use three criteria: whether the components are chemically combined, whether the proportions are fixed, and whether separation is physical or chemical.
Quick revision summary
Kinetic theory holds that all matter is made of particles in constant motion. Solids have particles closely packed in a regular arrangement vibrating in fixed positions, giving fixed shape and volume; liquids have particles close but randomly arranged and able to slide, giving fixed volume but no fixed shape; gases have widely spaced fast-moving particles with neither. Melting, boiling, condensing and freezing involve overcoming or re-forming forces of attraction, and sublimation goes directly from solid to gas. Evaporation occurs at the surface at any temperature and causes cooling because the most energetic particles escape. On a heating curve, sloping sections show rising kinetic energy and flat plateaus show changes of state where energy overcomes forces instead. Diffusion is fastest in gases and faster for particles of lower relative molecular mass, which is why the ammonium chloride ring forms nearer the hydrochloric acid end. Elements contain one type of atom, compounds are chemically combined in fixed proportions, and mixtures keep their components' properties and separate physically by filtration, crystallisation, distillation, chromatography, a separating funnel, a magnet or sublimation.