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Look around you and almost everything you can see — the air, a glass of water, the metal of a spoon, a spoonful of salt — is built from the same set of around 100 different kinds of atom. What makes one substance behave so differently from another is not new kinds of atom but the way those atoms are joined together, and whether they are joined at all. This is the heart of Topic C2 of OCR Gateway Science A: learning to sort every substance into one of three categories — element, compound or mixture — and to recognise from a formula, a diagram or a melting point which one you are dealing with. Getting this classification right is the foundation for the bonding, separation and periodic-table ideas that fill the rest of the topic.
By the end of this lesson you should be able to define an element, a compound and a mixture, classify substances from their formulae or particle diagrams, distinguish a molecule from a compound, explain the chemistry meaning of a "pure" substance, and use a melting or boiling point to judge purity.
An element is a substance made of only one type of atom — atoms that all have the same number of protons. There are about 100 elements, and each has its own one- or two-letter symbol: O for oxygen, Na for sodium, Fe for iron. The first letter is always a capital and the second (if there is one) is always lower-case, so "Co" is the element cobalt but "CO" is the compound carbon monoxide. That small distinction matters a great deal in chemistry.
The defining feature of an element is that it cannot be broken down into anything simpler by chemical means. You can melt it, dissolve it or react it, but you cannot turn an element into two different substances by an ordinary chemical reaction, because there is only one kind of atom to begin with. All the elements are listed in the periodic table, which you will study in detail later in this topic.
Exam Tip: "An element contains only one type of atom and cannot be broken down into simpler substances chemically." Memorise that sentence — it scores the definition mark every time, and the phrase "one type of atom" is the part examiners look for.
A compound is formed when two or more different elements are chemically combined in fixed proportions. The atoms are held together by chemical bonds (you will meet ionic and covalent bonds later in C2), and forming or breaking those bonds is a chemical change.
Three ideas define a compound:
That last point is the most striking. Sodium is a soft, silvery metal so reactive it bursts into flame on water; chlorine is a poisonous green gas. Yet when they combine the compound sodium chloride (NaCl) is the harmless white salt you sprinkle on chips. The properties of a compound are not an average of its elements — they are genuinely new, because chemical bonding has changed how the atoms behave.
A compound can only be separated back into its elements by a chemical reaction (for example by electrolysis or by heating with a more reactive element), never by simple physical methods such as filtering. That is the key contrast with a mixture.
| Substance | Formula | Elements combined |
|---|---|---|
| Water | H2O | hydrogen, oxygen |
| Carbon dioxide | CO2 | carbon, oxygen |
| Sodium chloride | NaCl | sodium, chlorine |
| Magnesium oxide | MgO | magnesium, oxygen |
| Aluminium oxide | Al2O3 | aluminium, oxygen |
Exam Tip: A formula with more than one capital letter is a compound (e.g. CO2, NaCl). A formula with only one element's symbol — even if there are several atoms, like O2 — is still an element.
A mixture contains two or more substances (elements and/or compounds) that are not chemically combined. Because there are no chemical bonds between the different substances, two things follow:
A familiar example is air, which is a mixture of nitrogen, oxygen, argon, carbon dioxide and water vapour, all just mingled together. Sea water is a mixture of water and dissolved salts; brass is a mixture of copper and zinc; crude oil is a mixture of many hydrocarbons. In none of these is there a fixed formula, and in every one the parts can be pulled apart physically.
The contrast between a compound and a mixture is one of the most important ideas in C2, so it is worth laying out side by side.
| Compound | Mixture | |
|---|---|---|
| Joined by chemical bonds? | Yes | No |
| Fixed proportions? | Yes (a formula) | No — any proportions |
| Properties | New properties, unlike the elements | Each component keeps its own properties |
| How to separate | Only by a chemical reaction | By physical methods |
| Example | Water, H2O | Air; sea water; brass |
A molecule is two or more atoms joined by covalent bonds. It is easy to muddle "molecule" with "compound", but they answer different questions: molecule is about how many atoms are joined, while element and compound are about how many types of atom.
So a molecule can be an element (like O2, N2, Cl2) or a compound (like H2O, CO2, NH3). The two ideas overlap but are not the same.
In a particle diagram, identical circles = an element; different circles bonded together in the same repeating unit = a compound; separate particles, not all bonded together = a mixture. Learn to read these at a glance, because exam questions often show them.
In everyday language "pure" can mean "natural" or "with nothing added" — pure orange juice, pure mountain spring water. In chemistry the word has a stricter meaning: a pure substance is a single element or a single compound, with nothing else mixed in. By that definition "pure" spring water is not pure at all, because it contains dissolved minerals — it is a mixture.
This chemistry meaning gives us a powerful way to test purity, because pure substances and mixtures behave differently when heated or cooled:
So if a substance melts cleanly at one temperature, it is pure; if it softens and melts over a spread of several degrees, it is a mixture. Chemists use this constantly — comparing a measured melting point with the known value for the pure compound is a quick check on how pure a sample is.
A student has two white solids. Solid A melts sharply at 801°C. Solid B begins to soften at 670°C and is not fully molten until 710°C. The melting point of pure sodium chloride is 801°C. Which solid is the pure sodium chloride, and what can you say about the other?
Step 1 — recall the rule: a pure substance melts at a single, sharp temperature; a mixture melts over a range.
Step 2 — apply it to A: solid A melts sharply at exactly 801°C, matching pure sodium chloride. So A is the pure sodium chloride.
Step 3 — apply it to B: solid B melts over a range of 670–710°C (40°C wide), and the melting starts below 801°C. A melting range shows it is impure — a mixture — not pure sodium chloride.
Answer: A is pure sodium chloride (sharp melting point at the known value); B is a mixture (it melts over a range and below the pure value).
Exam Tip: If a question gives a melting point (one temperature) call the substance pure; if it gives a melting range (two temperatures) call it a mixture / impure. Saying "melts over a range, so it is impure" earns the mark.
A formulation is a special kind of mixture — one that has been made to a precise formula (a fixed recipe of measured amounts), where each component is included for a particular purpose. It is still a mixture, because the components are not chemically bonded together, but it is a designed mixture rather than an accidental one.
Formulations are everywhere in industry and daily life:
| Formulation | Example components, each with a job |
|---|---|
| Paint | pigment (colour), solvent (lets it spread), binder (sticks it to the surface) |
| Alloy | a metal plus other elements added to improve strength or hardness (e.g. steel) |
| Fertiliser | compounds supplying nitrogen, phosphorus and potassium for plant growth |
| Fuel | a blend of hydrocarbons mixed for the right ignition and energy properties |
| Medicine (a tablet) | the active drug plus binders, fillers and a coating |
| Cleaning product | detergents, fragrance, water and other additives |
The key idea for the exam is that the proportions are carefully controlled so the product does its job — change the recipe and the paint will not stick or the tablet will not release its drug correctly. That deliberate design is what separates a formulation from an ordinary mixture such as muddy water.
Exam Tip: A formulation is "a mixture made in fixed proportions where each component has a particular purpose". Naming an example (paint, an alloy, a fertiliser) and the job of one component is a reliable way to score.
| Misconception | The correct idea |
|---|---|
| "A compound is just a mixture of its elements" | No — in a compound the elements are chemically bonded in fixed proportions with new properties; a mixture has no bonds |
| "Air is a compound" | Air is a mixture of gases (mainly nitrogen and oxygen) — they are not chemically combined and can be separated physically |
| "O2 is a compound because it has two atoms" | O2 is an element (one type of atom); it is a molecule, but a molecule of an element |
| "A pure substance is anything natural with nothing added" | In chemistry, pure means a single element or compound; spring water is a mixture, so it is not pure |
| "A mixture has a sharp melting point like a pure substance" | A mixture melts/boils over a range; only a pure substance has a sharp, fixed melting point |
| "You can separate a compound by filtering" | A compound can only be split by a chemical reaction; only mixtures separate by physical methods |
Question (6 marks): Iron and sulfur can be mixed together, or heated together to react and form iron sulfide. Explain the difference between the mixture of iron and sulfur and the compound iron sulfide, referring to bonding, properties and how each could be separated.
Mid-band response: "The mixture is just iron and sulfur together and they are not joined. The compound iron sulfide is joined together. You can separate the mixture with a magnet but not the compound."
Examiner-style commentary: The basic distinction (joined versus not joined) and one valid separation point are present, but the answer is thin. To climb a band, use the word chemically bonded, explain that the compound has new properties unlike the elements, and state that the compound can only be separated by a chemical reaction.
Stronger response: "In the mixture, the iron and sulfur are not chemically combined, so each keeps its own properties — the iron is still magnetic and can be removed with a magnet. In iron sulfide the iron and sulfur are chemically bonded, so it has new properties and is not magnetic. The compound cannot be separated by a magnet because the atoms are bonded together."
Examiner-style commentary: A clear answer that contrasts bonding, properties and one separation method. To reach the top band, state explicitly that the compound has a fixed composition / formula (FeS), that separating it needs a chemical reaction (not a physical method), and give the contrast in properties more fully.
Top-band response: "In the mixture, the iron and sulfur are simply mingled and are not chemically combined, so there are no bonds between them. Each element therefore keeps its own properties: the iron stays magnetic, so it can be pulled out with a magnet, and the sulfur could be dissolved away — both are physical separation methods. When the two are heated they react chemically to form the compound iron sulfide (FeS), in which the atoms are held together by chemical bonds in a fixed ratio. The compound has new properties quite different from the elements — it is not magnetic and does not behave like either iron or sulfur — so a magnet no longer works. The only way to get the elements back from iron sulfide is by a chemical reaction, because physical methods cannot break chemical bonds. This is the essential difference: a mixture is held together by nothing and separates physically, whereas a compound is held by bonds, has a fixed formula and new properties, and separates only chemically."
Examiner-style commentary: Full marks. It contrasts bonding (none versus chemical bonds), composition (any ratio versus fixed formula), properties (retained versus new), and separation (physical versus chemical) — covering every dimension the question asks for, with a correct named example throughout.
This content is aligned with OCR Gateway Science A GCSE Chemistry (J248), Topic C2 Elements, compounds and mixtures. Refer to the official OCR specification document for the exact wording.