Pure substances and mixtures
A pure substance has only one kind of particle. Water, salt and gold are pure. A mixture has two or more substances mixed in any amount. Each part keeps its own properties. Sand in water is still sand.
Pure substances are of two types: elements (one kind of atom, like iron or oxygen) and compounds (atoms of different elements joined in a fixed ratio, like water H₂O).
| Mixture | Compound |
|---|---|
| Parts in any ratio | Fixed ratio by mass |
| Parts keep their properties | New properties |
| Separated by simple physical methods | Only by chemical reactions |
| Iron filings + sulphur powder (a magnet pulls out iron) | Iron sulphide (a magnet cannot pull out iron) |
Homogeneous and heterogeneous mixtures
Homogeneous means "the same all through". Sugar in water, salt in water, air and brass are homogeneous. You cannot see separate parts, even with a magnifier.
Heterogeneous means "not the same all through". Sand in water, oil in water, a fruit salad and soil are heterogeneous. You can see the parts, or they settle.
Tip: a homogeneous mixture is called a solution. Solutions can be solid (alloys like brass), liquid (lemonade) or gas (air).
Solution, colloid and suspension
A solution has a solute (the part that dissolves, like salt) and a solvent (the part that dissolves it, like water). Its particles are smaller than 1 nm. They pass through filter paper, never settle and do not scatter light.
A suspension has solid particles bigger than about 1000 nm floating in a liquid, like chalk in water or muddy water. You can see the particles. They settle when left alone and a filter paper can hold them back. It is heterogeneous.
A colloid is in between (1–1000 nm). Milk, fog, smoke, jelly and shaving cream are colloids. The particles do not settle and pass through filter paper, but they are big enough to scatter light. A colloid looks uniform but is really heterogeneous.
Tyndall effect
When a beam of light passes through a colloid or a suspension, its path becomes visible because the particles scatter the light. In a true solution the path is not seen. This is the Tyndall effect. Example: sunlight through gaps in a forest canopy on a misty day.
Parts of a colloid
The particles are the dispersed phase; the medium they sit in is the dispersion medium. Aerosol (fog: liquid in gas), emulsion (milk: liquid in liquid), foam (shaving cream: gas in liquid), gel (jelly: liquid in solid), sol (paint: solid in liquid).
Concentration of a solution
Concentration tells how much solute is in a given amount of solution. More solute = more concentrated; less = dilute.
- Mass by mass % = (mass of solute ÷ mass of solution) × 100
- Mass by volume % = (mass of solute in g ÷ volume of solution in mL) × 100
- Volume by volume % = (volume of solute ÷ volume of solution) × 100
Remember: solution = solute + solvent. Always divide by the whole solution, not just the solvent.
A saturated solution cannot dissolve any more solute at that temperature. The amount that dissolves in 100 g of water to make it saturated is the solubility. Hot water usually dissolves more solid than cold water.
Separation methods
Every method uses a difference between the parts of a mixture.
- Evaporation: heat a solution until the liquid leaves (dye from ink, salt from sea water).
- Crystallisation: heat a solution to make it strong, then cool it. Pure crystals form and impurities stay in the liquid. It gives purer solids than evaporation (alum, copper sulphate, sugar).
- Distillation: for two liquids that mix well and whose boiling points differ by more than about 25 °C (acetone and water). The one with the lower boiling point boils first, its vapour cools in a condenser and drips out. If the gap is smaller, fractional distillation is used (petroleum, gases of air).
- Chromatography: for colours that dissolve in the same solvent (dyes in ink, pigments in leaves, drugs in blood tests). The colour that dissolves better rises faster on the paper.
- Sublimation: for a solid that turns straight into gas (ammonium chloride, camphor, naphthalene, iodine) mixed with one that does not (salt).
- Centrifugation: spin fast; heavier particles go to the bottom (cream from milk, blood tests, washing machine drier).
- Coagulation: add alum to muddy water; tiny particles stick into bigger lumps and settle. Then decant and filter.
- Also: separating funnel for liquids that do not mix (oil and water), magnet for iron, filtration for insoluble solids.
Try it at home
- Tyndall torch test: fill three clear glasses with salt water, water with a few drops of milk, and chalk water. Shine a phone torch from the side in a dark room. In which glass can you see the beam?
- Kitchen chromatography: draw a dot with a black sketch pen 2 cm above the end of a strip of filter paper or white tissue. Dip the end (not the dot) in water. Wait 10 minutes and count the colours.
- Predict then check in the 3D: at step 4, set the slider to 10 g. Predict the mass % before you look. (Answer: 10 ÷ 100 × 100 = 10%.)
Key formulas and definitions
- Mass % = (mass of solute ÷ mass of solution) × 100
- Mass by volume % = (mass of solute in g ÷ volume of solution in mL) × 100
- Volume % = (volume of solute ÷ volume of solution) × 100
- Mass of solution = mass of solute + mass of solvent
- Solution < 1 nm · Colloid 1–1000 nm · Suspension > 1000 nm
Worked examples
1. Classify: air, soil, brass, milk, sand in water, sugar in water.
Homogeneous: air, brass, sugar in water. Heterogeneous: soil, sand in water. Milk is a colloid, which looks uniform but is really heterogeneous.
2. 20 g of salt is dissolved in 180 g of water. Find the mass by mass %.
Mass of solution = 20 + 180 = 200 g. Mass % = 20 ÷ 200 × 100 = 10%.
3. A solution has 5 g of sugar in 250 mL of solution. Find the mass by volume %.
Mass by volume % = 5 ÷ 250 × 100 = 2%.
4. 30 mL of alcohol is mixed with water to make 150 mL of solution. Find the volume %.
Volume % = 30 ÷ 150 × 100 = 20%.
5. How much sugar is needed to make 250 g of a 12% (mass) sugar solution? How much water?
Sugar = 12% of 250 g = 0.12 × 250 = 30 g. Water = 250 − 30 = 220 g.
6. How would you separate a mixture of salt, sand and ammonium chloride?
Step 1: heat the dry mixture under an inverted funnel; ammonium chloride sublimes and collects on the funnel. Step 2: add water to the rest; salt dissolves, sand does not. Step 3: filter to get sand. Step 4: evaporate (or crystallise) the filtrate to get salt.
7. Why is distillation not enough to separate the gases of air, and what is used instead?
The boiling points of nitrogen (−196 °C) and argon (−186 °C) are only 10 °C apart. Air is first cooled and squeezed into a liquid, then warmed slowly in a tall fractionating column. Each gas comes off at its own height. This is fractional distillation.
Common mistakes
- Dividing by the mass of the solvent instead of the whole solution. For 20 g salt + 180 g water, use 200 g, not 180 g.
- Calling milk a solution. Milk is a colloid: it shows the Tyndall effect.
- Thinking a mixture has a fixed ratio. Only compounds have a fixed ratio; mixtures can be made in any amount.
- Using evaporation when pure crystals are needed. Evaporation leaves impurities in the solid and may burn it; crystallisation gives purer crystals.