Modelling matter as particles

Matter is any material that has mass and occupies volume. Chemists describe matter with a particulate model: a visible sample is treated as a huge collection of tiny particles. The model explains why substances have fixed compositions, why mixtures can be separated, and why chemical reactions rearrange particles instead of making matter appear or disappear.

An atom is the smallest particle of an element that retains that element’s chemical identity. We open up the atom properly in Structure 1.2. For now, the useful point is simple: atoms of different elements can be rearranged and bonded in different ways.

Elements, compounds and mixtures

A pure substance is a material made of only one chemical substance and therefore has a fixed composition throughout. Elements and compounds are pure substances.

An element is a pure substance composed of only one type of atom and cannot be chemically decomposed into simpler substances. Copper, oxygen and sulfur are elements. You cannot split an element into simpler chemical substances by heating, filtering or dissolving it. In chemical symbols, an element is shown by one or two letters, such as C, Na or Fe.

A compound is a pure substance composed of atoms of two or more different elements chemically bonded in a fixed ratio. Water contains hydrogen and oxygen in a fixed particle ratio; sodium chloride contains sodium and chloride ions in a fixed ratio. A compound has its own properties, not just an average of the properties of the elements in it. For example, a compound containing a reactive metal and a poisonous gas may be a stable crystalline solid.

A chemical bond is an electrostatic attraction that holds atoms or ions together in a substance. Since compounds involve bonding, you need a chemical change to separate a compound into its elements, such as thermal decomposition or electrolysis. A simple physical technique will not do it.

A mixture is a material containing two or more elements or compounds that are not chemically bonded together and are present in no fixed ratio. The key phrase is “no fixed ratio”. One sample of salty water can be more concentrated than another and still be salty water. The components of a mixture usually keep their own chemical identities, so physical methods can separate mixtures.

A quick comparison is often the easiest way to keep the categories straight:

Homogeneous and heterogeneous mixtures

A homogeneous mixture is a mixture with uniform composition and properties throughout a sample. Air is a good model example: any small sample is still mainly the same gases mixed together. A solution is homogeneous too, because the dissolved particles are spread through the solvent at particle level.

A heterogeneous mixture is a mixture with non-uniform composition, so different regions of the sample have different properties or composition. Muddy water, oil-and-water dressing and granite are heterogeneous. A sample taken from one part may not have the same composition as a sample taken from another.

Do not decide “compound or mixture” just by appearance. A clear colourless liquid might be pure water, a solution, or a mixture of miscible liquids. Chemists look for evidence: fixed composition, sharp melting or boiling behaviour, whether components retain properties, and whether physical separation works.

Purity and melting point

A melting point is the temperature at which a solid and its liquid are in equilibrium at a stated pressure. Pure substances usually melt over a very narrow temperature range. Impurities usually lower the melting point and broaden the melting range, so melting point determination is a useful Tool 1 technique when checking whether a product has been purified successfully.

In a practical, you pack the sample into a thin capillary tube and place it in a melting point apparatus. Record the temperature when melting first begins and when the last solid disappears. That range tells you more than a single neat number when the sample is impure.

Choosing a separation method

A physical separation works when the components differ in a physical property. The property you use determines the technique:

• particle size → filtration;
• solubility in a chosen solvent → solvation followed by filtration or crystallization;
• volatility or boiling point → evaporation or distillation;
• attraction to a mobile phase and a stationary phase → chromatography;
• magnetism or density → useful in some mixtures, though not the main syllabus list here.

Practical details matter as well: safety, toxicity, flammability, cost, scale of the sample, environmental disposal, whether heating might decompose a substance, and the purity required. In class I always ask: “What property is different enough for us to use?” If you cannot answer that, you have not chosen a separation method yet.

Solvation, filtration, recrystallization and evaporation

Solvation is a process in which solute particles become surrounded by solvent particles and disperse through the solvent. If one solid dissolves and another does not, solvation gives a neat route to separation. Add a suitable solvent, stir, and let the soluble component enter solution while the insoluble component stays as a solid.

Filtration is a separation technique in which a mixture passes through a porous barrier that retains insoluble solid particles while liquid or solution passes through. The solid left on the filter paper is the residue, a solid material retained after a separation process. The liquid that passes through is the filtrate, a liquid or solution collected after filtration.

Evaporation is a separation technique in which solvent is vaporized to leave behind a dissolved solid. Use it when the dissolved solid is stable on heating and you do not need to collect the solvent. Evaporation to dryness is simple, but if it is done too strongly it may give small, impure crystals.

Recrystallization is a purification technique in which an impure solid is dissolved in hot solvent and then forms purer crystals as the solution cools. The desired substance is more soluble in hot solvent than in cold solvent, while many impurities either stay dissolved or are removed by filtration. The usual sequence is: dissolve in minimum hot solvent, filter if insoluble impurities remain, cool slowly, filter the crystals, wash with a little cold solvent, and dry.

For products of a reaction, this pattern is common: remove excess insoluble reactant by filtration, concentrate the filtrate by gentle evaporation, allow crystals to form on cooling, then filter and dry the crystals. The details change with the reaction, but the logic does not.

Distillation and chromatography

Distillation is a separation technique in which a liquid is vaporized and then condensed, allowing components with different volatilities or boiling points to be separated. Simple distillation is often used to separate a solvent from a dissolved solid or to separate liquids with sufficiently different boiling points. The thermometer, condenser direction, and collection flask all matter. Vapour should be cooled efficiently so the distillate is collected as a liquid.

Chromatography is a separation technique in which components of a mixture move at different rates because they have different relative attractions to a mobile phase and a stationary phase. In paper chromatography, the solvent is the mobile phase and the paper is the stationary phase. A component more strongly attracted to the solvent moves further; a component more strongly attracted to the paper moves less far.

This is where Structure 2.2 begins to peep through the door: intermolecular forces influence whether substances mix, dissolve or separate. If particles of two substances attract each other strongly enough, a homogeneous mixture may form. If not, the mixture may remain heterogeneous or separate into layers.

Alloys are a useful link to Structure 2.3. An alloy is a mixture containing a metal and one or more other elements, usually with metallic bonding between particles. Alloys are generally treated as mixtures because their composition can vary over a range and their properties change with composition; they do not usually have the single fixed ratio expected for a compound.

The kinetic molecular theory

The kinetic molecular theory is a model that describes matter as particles in constant motion with spaces and attractions between them. It doesn’t try to show every particle exactly as it is. Instead, it helps explain what we can observe: shape, volume, compressibility, diffusion and changes of state.

In this model, particles in a solid are held close together and mostly vibrate about fixed positions. In a liquid, the particles are still close together, but they can move past one another. In a gas, the particles are far apart compared with their own size, and they move rapidly in random directions.

Solids, liquids and gases

A solid is a state of matter with a fixed shape and fixed volume because its particles are held in fixed positions. Solids are not easily compressed, since the particles are already close together.

A liquid is a state of matter with fixed volume but no fixed shape because its particles remain close together while moving past one another. Liquids take the shape of the bottom of their container. Like solids, they are not easily compressed.

A gas is a state of matter with no fixed shape and no fixed volume because its particles are widely spaced and move independently. Gases fill their container. They are compressible because there is much more empty space between particles.

Whether a substance is solid or fluid under standard conditions depends on the balance between particle attractions and particle motion. Stronger attractions tend to favour solids. Weaker attractions and greater particle motion favour liquids or gases. Structure 2 develops this using bonding and intermolecular forces.

State symbols in equations

State symbols show the physical state of each substance in a chemical equation:

• (s) means solid;
• (l) means liquid;
• (g) means gas;
• (aq) means aqueous, meaning dissolved in water.

For example, H₂O(s), H₂O(l) and H₂O(g) all represent the same compound in different physical states. NaCl(s) is solid sodium chloride, while NaCl(aq) means sodium chloride dissolved in water. The symbol (aq) is not a fourth state of matter; it is a solution in water.

State symbols carry chemical information. If a reaction is done in solution, a gas is released, or an insoluble solid forms, the state symbols help show what you would actually observe.

Changes of state

Melting is a change of state in which a solid becomes a liquid. Freezing is a change of state in which a liquid becomes a solid. These two processes are opposites.

Vaporization is a change of state in which a liquid becomes a gas. It includes evaporation, a surface process that can occur below the boiling point, and boiling, a process throughout the liquid when bubbles of vapour form within the liquid. Condensation is a change of state in which a gas becomes a liquid.

Sublimation is a change of state in which a solid becomes a gas without first becoming a liquid. Deposition is a change of state in which a gas becomes a solid without first becoming a liquid. Solid carbon dioxide subliming and frost forming from water vapour are useful mental pictures.

These changes are physical changes, not chemical changes. The particles themselves do not become different substances; their arrangement, spacing and motion change.

Energy changes during changes of state

Moving from a more ordered or condensed state to a less condensed state requires energy input. Melting, vaporization and sublimation are endothermic processes, meaning processes that absorb energy from the surroundings.

Moving from a less condensed state to a more condensed state releases energy. Freezing, condensation and deposition are exothermic processes, meaning processes that transfer energy to the surroundings.

This links to Reactivity 1.2: energy is not destroyed during a change of state; it is transferred. During melting or boiling, energy is used mainly to overcome attractions between particles rather than to increase the temperature of the sample. During freezing or condensation, attractions form more strongly and energy is released.