Anything that has mass and takes up space is matter — air, water, a stone, our body. Matter is made of tiny particles that keep moving. How close the particles are decides whether it is a solid, a liquid or a gas. Exams ask about changes of state, which things are mixtures, and the right way to separate them.
On this page you will learn:
- The three states of matter, and why ice floats
- Melting, boiling, condensation and sublimation
- Elements, compounds and mixtures: air, sea water, iodised salt
- Solutions, colloids and suspensions
- Separation methods; physical and chemical changes
States of matter
Rule in one line: solids keep shape and volume, liquids keep only volume, gases keep neither.
Most liquids shrink when they become solid. Water is different. When it freezes, its particles lock into an open pattern with gaps. So ice takes more space than the same water. Ice is lighter (less dense) than water and floats. Water pipes can burst on very cold nights for this reason.
Besides solid, liquid and gas, matter can be plasma — the glowing gas inside stars and neon signs — or a Bose–Einstein condensate, a very cold state named after the Indian scientist Satyendra Nath Bose and Albert Einstein.
Changing state
Rule in one line: heating or cooling (and pressure) moves matter from one state to another.
| Change | From → to | Example |
|---|---|---|
| Melting | Solid → liquid | Ice melts at 0 °C |
| Freezing | Liquid → solid | Water turns to ice |
| Boiling | Liquid → gas, all through the liquid | Water boils at 100 °C |
| Evaporation | Liquid → gas, only at the surface, at any temperature | Wet clothes dry |
| Condensation | Gas → liquid | Dew; drops outside a cold glass |
| Sublimation | Solid → gas directly | Camphor, naphthalene balls, dry ice |
| Deposition | Gas → solid directly | Frost on very cold nights |
Evaporating particles take heat from their surroundings. So the place they leave becomes cooler. Sweat cools the body, and water in an earthen pot (ghara) stays cool. Evaporation is faster with a larger surface, higher temperature, dry air and wind.
Elements, compounds and mixtures
Rule in one line: a compound has a fixed make-up and new properties; a mixture does not.
- MatterAnything with mass that takes up space
- Pure substancesElements (iron, gold, sulphur, diamond) and compounds (water, salt)
- MixturesUniform: air, sea water, brass. Not uniform: soil, sand in water
Common salt (sodium chloride) with a very small amount of potassium iodide (or potassium iodate) mixed in. The two do not react; they are only mixed. So iodised salt is a mixture. The iodine prevents goitre.
Air: a mixture of gases
Rule in one line: the share of each gas in air is counted by volume.
Solutions, colloids and suspensions
Rule in one line: the smaller the particles, the clearer the mixture and the less it settles.
| Type | Particles | Behaviour | Examples |
|---|---|---|---|
| Solution | Smallest | Clear; never settles; passes through filter paper | Salt water, soda water, air |
| Colloid | Medium | Looks uniform; scatters a light beam (Tyndall effect) | Milk, fog, smoke, jelly |
| Suspension | Largest | Cloudy; settles on standing; can be filtered | Muddy water, chalk in water |
Soda water (an aerated drink) is carbon dioxide gas dissolved in water under pressure. So it is a gas-in-liquid solution: carbon dioxide is the solute and water is the solvent. When the bottle is opened, the pressure falls and the gas bubbles out.
Separating mixtures
Rule in one line: pick the method that uses a difference between the parts.
| Method | Uses the difference in | Example |
|---|---|---|
| Evaporation | One part turns to vapour | Salt from sea water |
| Distillation | Boiling points far apart | Pure water from salt water |
| Fractional distillation | Boiling points close together | Gases of air; parts of petroleum |
| Separating funnel | Liquids that do not mix | Oil from water |
| Centrifugation | Density, by fast spinning | Cream from milk; blood tests |
| Chromatography | How far each part travels | Colours in ink |
| Sublimation | One part sublimes | Ammonium chloride from salt |
| Crystallisation | Solubility on cooling | Pure copper sulphate, alum, sugar |
| Magnet | Magnetic property | Iron filings from sand |
| Winnowing | Weight, using wind | Husk from grain |
Physical and chemical changes
Rule in one line: a chemical change makes a new substance; a physical change does not.
Traps and practice
Last revision — where the options try to fool you.
- Water expands on freezing; that is why ice floats.
- Gas → liquid is condensation; solid → gas is sublimation.
- Diamond is an element (carbon), not a mixture. Air, soil and sea water are mixtures.
- Iodised salt is a mixture with potassium iodide, not a compound, and not 'molecular iodine'.
- Nitrogen is 78% of air by volume, not by mass.
- In soda water, CO2 is the solute and water the solvent.
- Milk and fog are colloids, not true solutions.
Convert 27 °C to kelvin, and 373 K to °C.
Show answer
27 + 273 = 300 K; 373 − 273 = 100 °C.
- Gas to liquid
- Condensation
- Solid straight to gas
- Sublimation
- Solid carbon dioxide
- Dry ice
- Main gas of air
- Nitrogen, 78% by volume
- Main salt in sea water
- Common salt (sodium chloride)
- Cream from milk
- Centrifugation
- Colours in ink
- Chromatography
- Light beam seen in fog
- Tyndall effect
- Solid: fixed shape and volume; gas: neither
- Ice is lighter than water and floats
- Sublimes: camphor, naphthalene, dry ice, ammonium chloride
- Air: N2 78%, O2 21%, Ar 0.93%, CO2 0.04% by volume
- Mixtures: air, soil, sea water, iodised salt, brass
- Soda water: CO2 dissolved in water
- K = °C + 273