Sea water, air, milk, ink and brass are all mixtures. Learn how to describe them, measure them and take them apart.
Almost nothing you touch is a single pure substance. Air, sea water, milk, ink, brass and the soil in a garden are all mixtures. Chemists need to describe mixtures, measure how much of a substance is dissolved in them, and separate them into pure parts. This article gives you the vocabulary, the calculations and the methods.
A pure substance contains only one element or only one compound. An element has one kind of atom (such as oxygen, O₂, or iron, Fe). A compound has two or more elements chemically joined in a fixed ratio (such as water, H₂O). A mixture contains two or more substances that are not chemically joined, so they can be present in any proportion.
| Property | Compound | Mixture |
|---|---|---|
| Composition | Fixed ratio of elements | Any proportions |
| How it is made | Chemical reaction | Simply mixing; no new substance forms |
| How it is separated | Only by chemical reactions | Physical methods (filtering, distilling, etc.) |
| Properties | Different from its elements | Keep the properties of the parts |
| Melting or boiling point | Sharp, fixed temperature | Melts or boils over a range of temperatures |
For example, iron filings mixed with sulfur powder are a mixture: a magnet can pull out the iron. Heat them strongly and they react to form the compound iron(II) sulfide, which is not magnetic and cannot be separated by a magnet.
Mixtures differ in how finely the parts are spread through each other.
| Type | Particle size (rough) | Behaviour | Examples |
|---|---|---|---|
| Solution | Smaller than about 1 nm (single molecules or ions) | Clear, does not settle, passes through filter paper | Salt water, air, brass |
| Colloid | About 1 nm to 1000 nm | Looks uniform but scatters light (Tyndall effect); does not settle quickly | Milk, fog, mayonnaise, blood |
| Suspension | Larger than about 1000 nm | Cloudy; particles settle on standing; filtered out | Muddy water, chalk in water |
The Tyndall effect is the scattering of a light beam by colloid particles, so the beam becomes visible, like sunlight through mist or a torch beam in smoky air. A true solution does not scatter light in this way.
A solution is a mixture in which one substance (the solute) has dissolved completely in another (the solvent) so that it is spread evenly as separate particles. Salt in water is a solution of solute salt and solvent water. When the solvent is water, we call it an aqueous solution.
Several factors change how fast a solid dissolves: stirring, heating and using smaller pieces (a larger surface area) all speed it up. Only temperature, however, changes how much can dissolve at equilibrium.
Solubility is the maximum mass of a solute that dissolves in 100 g of solvent at a given temperature, giving a saturated solution. For most solids, solubility increases as the temperature rises, but not equally for all substances.
| Temperature | KNO₃ (g per 100 g water) | NaCl (g per 100 g water) |
|---|---|---|
| 0 °C | 13 | 36 |
| 20 °C | 32 | 36 |
| 40 °C | 64 | 36 |
| 60 °C | 110 | 37 |
| 80 °C | 169 | 38 |
| 100 °C | 245 | 39 |
These values are approximate and rounded; textbooks give slightly different figures. For gases, the trend is the opposite: solubility decreases as the temperature rises. This is why a warm fizzy drink loses its gas faster than a cold one, and why very warm lakes can hold less oxygen for fish.
Question: A saturated solution of potassium nitrate made with 100 g of water at 60 °C is cooled to 20 °C. What mass of crystals appears? Use the values in the table.
Answer: about 78 g of potassium nitrate crystals.
Cooling a hot saturated solution is the basis of crystallisation. If a solution holds more solute than normal for its temperature, because it was cooled very carefully without disturbance, it is called supersaturated; adding a tiny crystal or scratching the glass then makes crystals form quickly.
Concentration tells you how much solute is in a given volume of solution. Two units are common in school chemistry:
Remember that 1 dm³ = 1000 cm³, so convert cm³ to dm³ by dividing by 1000. The amount in moles is mass ÷ molar mass (Mr in g/mol).
Question: 5.85 g of sodium chloride is dissolved in water and made up to 500 cm³. Give the concentration in g/dm³ and mol/dm³. (Mr of NaCl = 58.5)
Dilution means adding solvent to lower the concentration. The amount of solute does not change, so concentration × volume stays the same before and after:
Question: 25 cm³ of 2.0 mol/dm³ hydrochloric acid is diluted with water to a total volume of 250 cm³. What is the new concentration?
The right method depends on the physical property that differs between the parts of the mixture, such as particle size, solubility, boiling point or magnetism.
| Method | Separates | Works because | Example |
|---|---|---|---|
| Decanting | An insoluble solid from a liquid after it settles | Solid sinks; liquid is poured off | Sand from water |
| Filtration | Insoluble solid from a liquid | Solid is too big to pass through filter paper pores | Sand from salt solution |
| Evaporation | Dissolved solid from a solution | Solvent evaporates; solute stays | Salt from sea water |
| Crystallisation | A dissolved solid, as pure crystals | Solubility falls as a hot saturated solution cools | Copper(II) sulfate crystals |
| Simple distillation | A solvent from a solution | Solvent boils, vapour is cooled and condensed | Pure water from salt water |
| Fractional distillation | Liquids with different boiling points | The fractionating column lets the lower boiling liquid reach the top first | Ethanol (78 °C) from water (100 °C) |
| Separating funnel | Immiscible liquids | Denser liquid is run off from the bottom | Oil and water |
| Chromatography | Dissolved substances such as dyes | Substances move at different speeds through the paper | Colours in ink |
| Magnet | Magnetic from non-magnetic solids | Iron, cobalt and nickel are attracted to magnets | Iron filings from sulfur |
Wear eye protection when heating liquids and use heat-resistant gloves or tongs. Never taste anything in a laboratory. Keep flammable liquids such as ethanol away from naked flames and heat them with a water bath or electric heater. Copper(II) sulfate is harmful if swallowed; wash your hands after the experiment.
A spot of the mixture is placed on a pencil start line on chromatography paper, and the bottom edge of the paper dips into a solvent (the starting line must be above the solvent level). The solvent rises by capillary action and carries the dissolved substances up the paper at different speeds, depending on how soluble each is in the solvent and how strongly it sticks to the paper. The start line is drawn in pencil because pencil lead does not dissolve, but pen ink would run.
The Rf value (retention factor) compares how far a substance travelled with how far the solvent travelled. It has no units and is always between 0 and 1:
Question: In Figure 3, the yellow spot moved 4.0 cm and the solvent front moved 8.0 cm. Find its Rf, and say what the match with Dye P tells us.
Note: The blue spot matches neither reference dye, so the ink contains at least one other dye.
| Mixture | Best method | Reason |
|---|---|---|
| Sand and salt | Add water, filter, then evaporate the filtrate | Sand is insoluble and stays on the filter paper; salt dissolves and is recovered by evaporation |
| Pure water from sea water | Simple distillation | Water boils off; salt remains behind |
| Ethanol and water | Fractional distillation | Both are liquids that mix, with different boiling points |
| Black ink into its dyes | Chromatography | Dyes travel at different speeds |
| Oil and water | Separating funnel | They do not mix and have different densities |
When a question asks “how would you separate…”, name the property that differs, then name the method, and finally give the order of steps. This pattern earns most of the marks.
In daily life, “pure” can mean “nothing added”: pure orange juice is still a mixture. In chemistry, pure means a single substance. Two tests are common:
Purity matters in medicines, food and electronics, where even tiny impurities can cause harm or failure.
Air is a mixture of gases, by volume about 78% nitrogen, 21% oxygen, just under 1% argon and about 0.04% carbon dioxide, plus variable water vapour. These are separated industrially by cooling air until it becomes liquid and then using fractional distillation, because the gases have different boiling points.
Sea water is a solution of salts, mainly sodium chloride, in water, and is a source of drinking water through distillation or reverse osmosis in places with little fresh water. Alloys such as brass and steel are solid solutions of metals. Blood and milk are complex mixtures that include colloids and suspended cells or fat droplets.
Try each question first, then tap it to reveal the answer.
Answer: A mixture of gases, mainly nitrogen and oxygen.
Answer: Sugar is the solute; water is the solvent.
Answer: About 37 g.
Answer: 20 ÷ 0.250 = 80 g/dm³.
Answer: Simple distillation.
Answer: 3.0 ÷ 7.5 = 0.40.
Answer: It forms a suspension: the particles are large enough to settle under gravity.
Suggested use: give students a mixed sample (sand, salt and iron filings) and ask them to plan a flow chart of separation steps before doing it. Compare black felt-tip pens by chromatography, using pencil start lines. Heating and distillation need teacher supervision. Parents can encourage discussion of kitchen examples such as tea, coffee, salt water and sieving flour. Please follow your own syllabus for required methods and notation.
These reputable sources were consulted to verify the facts in this article:
Copyright note: This article is original EDUSAMBAM educational writing. The diagrams are original inline SVG schematics created for this page and are not copied from any source. Links open external websites; the sources were used to check facts and are suggested for further study.
Last updated: 21 October 2026. Written for students up to age 18 and checked against standard school-level chemistry references. This is general education, not a substitute for your teacher, textbook or syllabus.
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