Pure Substances & Mixtures
Elements, Compounds, Solutions & Separation Techniques
Classification of Matter
All matter can be systematically classified based on its composition. The first division is between pure substances — which have a fixed, uniform composition — and mixtures — which are combinations of two or more substances that are physically, but not chemically, combined.
This classification is the framework that underpins much of chemistry. Understanding where a substance falls in this scheme tells you how it can be separated, what properties it has, and how it will behave in reactions.
Pure Substances
A pure substance has a fixed composition throughout — every sample of it is identical, with the same properties regardless of its source. Pure substances have sharp, definite melting and boiling points. They can only be separated into simpler substances through chemical means, not physical ones.
Pure substances are divided into two categories: elements and compounds.
A pure substance composed of only one type of atom. Cannot be broken down into simpler substances by chemical means. There are 118 known elements, each with a unique symbol on the periodic table.
A pure substance formed when two or more different elements are chemically bonded together in a fixed ratio. Has properties completely different from the elements that make it up.
Hydrogen (H₂) and oxygen (O₂) are both elements — colourless, flammable or reactive gases. When they chemically combine in a 2:1 ratio, they form water (H₂O) — a liquid at room temperature with completely different properties. The compound is not a simple mix of the two gases; it is an entirely new substance with its own unique identity.
Some elements exist as diatomic molecules — pairs of atoms bonded together — in their standard state. The common diatomic elements are: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. Memorising these is important — when you write “oxygen” in a chemical equation, it appears as O₂, not O.
A compound always contains the same elements in the same ratio by mass, regardless of how it was made or where it was found. Water from the ocean, a river, or a laboratory is always 88.9% oxygen and 11.1% hydrogen by mass. This law, established by Joseph Proust in 1799, is a defining feature of compounds.
Mixtures
A mixture is a combination of two or more substances that are physically combined — not chemically bonded. The components retain their individual properties. Mixtures have variable composition: you can have saltwater with a little salt or a lot of salt — both are still saltwater.
Crucially, the components of a mixture can always be separated by physical means — without any chemical reaction occurring.
Uniform composition throughout — every sample from any point is identical. Also called a solution. The components are not visibly distinguishable. E.g. saltwater, vinegar, air, brass (copper + zinc).
Non-uniform composition — different parts of the mixture have different compositions. Components are visibly distinguishable. E.g. sand and water, oil and water, pizza, granite, blood.
In a solution (homogeneous mixture), the substance present in the larger amount is the solvent and the substance dissolved in it is the solute. In saltwater, water is the solvent and salt is the solute. The solvent does not have to be water — in many industrial and pharmaceutical applications, organic solvents like ethanol or acetone are used.
Between homogeneous and heterogeneous mixtures sit two intermediate types. A colloid contains particles larger than dissolved molecules but too small to settle — milk, fog, and gelatin are colloids. A suspension contains larger particles that will eventually settle — muddy water and orange juice with pulp are suspensions.
Comparison: Elements, Compounds & Mixtures
| Property | Element | Compound | Mixture |
|---|---|---|---|
| Composition | One type of atom only | Two or more elements in a fixed ratio | Two or more substances in any ratio |
| Chemical formula | Single symbol: Fe, O₂, Au | Fixed formula: H₂O, NaCl, CO₂ | No fixed formula |
| Properties | Unique to that element | Entirely different from component elements | Each component retains its own properties |
| Melting/boiling point | Sharp, definite point | Sharp, definite point | Range, varies with composition |
| Separation method | Cannot be separated chemically | Only by chemical reaction | Physical methods (filtration, distillation, etc.) |
| Examples | Gold, nitrogen, sulfur, silicon | Water, salt, sugar, aspirin | Air, seawater, soil, bronze |
Separation Techniques
Because the components of a mixture retain their individual physical properties, those properties can be used to separate them. The key is choosing the right technique based on the difference in properties between the components.
A mixture is passed through filter paper or a porous material. Solid particles (residue) too large to pass through are retained; liquid (filtrate) passes through.
A liquid mixture is heated until one component vaporises, the vapour is cooled and condensed back into a liquid in a separate container. Used for liquids with sufficiently different boiling points.
The solvent is evaporated by heating, leaving the dissolved solute behind as a solid residue. Works when the solute is non-volatile (does not evaporate at the boiling point of the solvent).
Components in a mixture travel through a medium (paper, silica) at different rates depending on their affinity for the stationary phase vs the moving solvent. Separates mixtures into their components as distinct bands or spots.
A hot, concentrated solution is cooled slowly. The desired solute crystallises out (precipitates) as a pure solid while impurities remain dissolved. Produces very pure crystals.
A magnet is used to attract and remove magnetic components from a non-magnetic mixture. Simple and effective when one component is magnetic.
When a mixture contains several liquids with similar but different boiling points, fractional distillation is used — a tall column allows repeated vaporisation and condensation, separating components into “fractions.” This is how crude oil is refined into petrol, diesel, kerosene, and other products.
Worked Examples
Practice Questions
Q1. Which of the following is a pure substance?
Q2. A compound differs from a mixture in that a compound:
Q3. Which separation technique would best separate a mixture of sand (insoluble solid) and water?
Q4. Air is classified as a homogeneous mixture because:
Q5. The Law of Definite Proportions states that:
Key Takeaways
- All matter is classified as either a pure substance (fixed composition) or a mixture (variable composition).
- Pure substances are either elements (one type of atom) or compounds (two or more elements chemically bonded in a fixed ratio).
- Compounds have completely different properties from the elements that form them — water and hydrogen/oxygen are nothing alike.
- The Law of Definite Proportions: a compound always has the same composition by mass, regardless of its origin.
- Mixtures are homogeneous (uniform, e.g. solutions) or heterogeneous (non-uniform, e.g. sand in water).
- The components of a mixture retain their individual properties and can be separated by physical methods.
- Separation techniques exploit differences in physical properties: filtration (size), distillation (boiling point), evaporation (volatility), chromatography (affinity), crystallisation (solubility), and magnetic separation (magnetic properties).