Matter: States & Classification

Matter is the physical substance of the universe, defined by two simple tests — does it take up room, and does it weigh something.

• Matter is made up of tiny particles called atoms and molecules.
• The particles of matter have spaces between them and are in constant motion.
• The particles attract one another with a force called the force of attraction (cohesion).
• Matter can be classified physically (by state) and chemically (by composition).

Matter exists in different states depending on how strongly its particles are held together and how freely they move; the three common states are solid, liquid and gas, with plasma and Bose–Einstein condensate as two extra states.

• Plasma and Bose–Einstein condensate (BEC) are the fourth and fifth states of matter.
• The state of a substance depends on temperature and pressure.
• Inter-particle force is strongest in solids and weakest in gases.

In a solid the particles are packed tightly in a fixed pattern and can only vibrate about fixed positions, which gives a solid its rigidity.

• The particles are packed very closely in a regular arrangement.
• Solids are rigid and almost incompressible.
• The particles can only vibrate about their fixed mean positions.
• The force of attraction between particles is the strongest in solids.
• Examples: ice, iron, wood, stone, salt and sugar.

A liquid keeps its volume but takes the shape of its container, because its particles are close but free to slide past one another.

• A liquid takes the shape of the container it is poured into.
• The particles are close together but can move and slide past one another.
• Liquids can flow and are only slightly compressible.
• Liquids diffuse more slowly than gases.
• Examples: water, milk, oil, mercury and alcohol.

In a gas the particles are far apart and move freely at high speed, so a gas fills whatever space it is given and can be squeezed easily.

• A gas fills the entire container available to it.
• The particles are far apart and move randomly at high speed.
• Gases are highly compressible and have the lowest density.
• The force of attraction between particles is the weakest in gases.
• Gases diffuse rapidly and exert pressure on the walls of the container.
• Examples: oxygen, hydrogen, carbon dioxide, air and nitrogen.

Beyond the three common states lie two extreme states — plasma, formed at very high temperatures, and BEC, formed at temperatures near absolute zero.

• Plasma is formed at very high temperatures or in strong electric fields.
• Plasma is electrically conducting and is affected by magnetic fields.
• Plasma is the most abundant state of matter in the universe — stars and the Sun are made of plasma.
• Everyday examples: fluorescent tubes, neon signs and lightning.

• BEC forms at temperatures very near absolute zero (0 K).
• It was predicted by Satyendra Nath Bose and Albert Einstein.
• BEC was first produced in 1995 by Cornell, Wieman and Ketterle (who won the 2001 Nobel Prize).
• In a BEC the atoms lose their individual identity and behave as a single quantum entity.

Matter can be changed from one state to another by changing the temperature or pressure; each named change describes one specific transition between solid, liquid and gas.

• Heating usually moves matter from solid → liquid → gas; cooling reverses it.
• During a change of state the temperature stays constant while latent heat is absorbed or released.

The fixed temperatures at which a pure substance changes state are characteristic of that substance and are key exam values for water.

• Boiling point is the temperature at which a liquid turns into a gas throughout its bulk.
• For water: melting/freezing point = 0°C and boiling point = 100°C (at normal pressure).
• Freezing point and melting point of a pure substance are the same temperature.
• Latent heat is the heat absorbed or released during a change of state at constant temperature.

Both evaporation and boiling turn a liquid into vapour, but they differ in where it happens and at what temperature — a common exam comparison.

• Evaporation causes cooling because the fastest molecules escape and leave the liquid cooler.
• Evaporation increases with higher temperature, larger surface area, lower humidity and more wind.

Some substances bypass the liquid state entirely, while gases can return to liquid or solid form on cooling.

• Sublimation examples: camphor, naphthalene (mothballs), iodine, dry ice (solid CO₂) and ammonium chloride.
• Deposition is the direct change of a gas into a solid (the reverse of sublimation).
• Formation of frost on a cold surface is an example of deposition.
• Condensation is the change of a gas (vapour) into a liquid on cooling.
• Dew and the fogging of a cold mirror are examples of condensation.

Chemically, matter is divided into pure substances and mixtures; pure substances have a fixed composition, while mixtures are physical combinations in any proportion.

• A pure substance has a fixed composition throughout.
• Pure substances are of two types — elements and compounds.
• Mixtures are of two types — homogeneous and heterogeneous.
• Pure substances cannot be separated by physical methods; mixtures can be.

• Matter → Pure Substances (Elements, Compounds) and Mixtures (Homogeneous, Heterogeneous).

A pure substance is made of only one kind of particle; it is either an element, made of one kind of atom, or a compound, made of atoms chemically combined.

• An element cannot be broken down into simpler substances by chemical means.
• There are about 118 known elements in the periodic table.
• Examples: hydrogen, oxygen, gold, iron, carbon and copper.

• A compound is formed when two or more elements combine chemically in a fixed ratio.
• A compound has a fixed ratio of elements by mass.
• A compound has properties different from its constituent elements.
• A compound can be split into elements only by chemical methods, not physical ones.
• Examples: water (H₂O), carbon dioxide (CO₂), common salt (NaCl) and sugar.

A mixture contains two or more substances physically mixed in any proportion, keeping their own properties; it is uniform (homogeneous) or non-uniform (heterogeneous).

• The components of a mixture keep their own properties.
• Mixtures can be separated by physical methods.

• A homogeneous mixture has a uniform composition throughout with no visible boundaries.
• Solutions are homogeneous mixtures.
• Examples: salt water, sugar solution, air, and brass (alloy).

• A heterogeneous mixture has a non-uniform composition with visible separate parts.
• Examples: sand and water, oil and water, a mixture of sand and iron filings.

Telling a mixture apart from a compound is a frequent exam question; the difference lies in how the components combine and whether the composition is fixed.

Elements are grouped by their properties into metals, non-metals and metalloids, which sit between the two with mixed character.

• Most elements are metals.

• Metals are lustrous, malleable, ductile and good conductors of heat and electricity.
• Metals are usually solid at room temperature — except mercury, which is a liquid metal.
• Examples: iron, copper, gold, aluminium, silver.

• Non-metals are dull, brittle and poor conductors of heat and electricity (except graphite).
• Non-metals exist as solids, liquids or gases.
• Examples: oxygen, hydrogen, carbon, sulphur, nitrogen (bromine is a liquid non-metal).

• Metalloids show properties of both metals and non-metals.
• Examples: silicon, boron, germanium, arsenic, antimony.
• Silicon and germanium are used as semiconductors.

The atom is the smallest unit of an element, while a molecule is the smallest unit of a substance that can exist freely — a basic distinction tested in exams.

• A molecule is the smallest particle of a substance that can exist independently.
• A molecule is formed when two or more atoms combine.
• The term atom was coined by John Dalton; the word means "indivisible".

• A molecule of an element has the same kind of atoms (e.g. O₂).
• A molecule of a compound has different kinds of atoms (e.g. H₂O).

The components of a mixture are separated by physical methods chosen according to differences in their properties — size, boiling point, magnetism, density or solubility.

• Filtration separates by particle size; distillation by boiling point; magnetic separation by magnetism.

• Filtration separates an insoluble solid from a liquid using filter paper.
• Evaporation recovers a dissolved solid by driving off the liquid.
• Distillation separates miscible liquids by their difference in boiling points — the liquid is boiled to vapour and then condensed back.
• Fractional distillation separates miscible liquids with close boiling points, as in refining petroleum and separating air's gases.
• Chromatography separates the components of a mixture by their different speeds through an absorbing medium.
• Centrifugation separates denser particles by spinning the mixture at high speed.