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Nearly everything around you — the air you breathe, the water in a glass, a steel spoon, a pinch of salt — is built from about one hundred different kinds of atom. What makes salt behave so differently from the metals it is made of is not some special new atom, but the way the atoms are joined together, and whether they are joined at all. This lesson opens Topic C2 of your OCR Gateway Combined Science course by teaching you to sort any substance into one of three groups: an element, a compound or a mixture. Learning to read a formula, a particle diagram or a melting point and place a substance correctly is the foundation on which the bonding, separation and periodic-table ideas in the rest of C2 are built.
By the end of this lesson you should be able to define an element, a compound and a mixture, sort substances from their formulae or particle diagrams, tell a molecule apart from a compound, explain what "pure" means in chemistry, and use a melting or boiling point to judge whether a sample is pure.
This lesson develops AO1 (defining element, compound and mixture) and AO2 (classifying substances from their formulae or particle diagrams), extending to AO3 when you interpret a melting or boiling point to judge whether a sample is pure.
An element is a substance made from just one type of atom — atoms that all contain the same number of protons. There are roughly 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 any second letter is always lower-case, so "Co" means the element cobalt while "CO" means the compound carbon monoxide. That tiny difference in capitalisation carries real chemical meaning.
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 no ordinary chemical reaction can split an element into two different substances, because there is only one kind of atom present to begin with. Every element appears in the periodic table, which you will study more closely later in this topic.
Exam Tip: Learn the sentence "an element contains only one type of atom and cannot be broken down into simpler substances chemically." The phrase "one type of atom" is the part examiners look for, and it scores the definition mark reliably.
A compound forms when two or more different elements are chemically combined in fixed proportions. The atoms are joined by chemical bonds (you will meet ionic and covalent bonds later in C2), and making or breaking those bonds is a chemical change.
Three ideas together define a compound:
That last point is the most striking of the three. Sodium is a soft, silvery metal so reactive it catches fire on water; chlorine is a choking green gas. Yet when they combine, the compound sodium chloride (NaCl) is the harmless white salt you sprinkle on chips. A compound's properties are not an average of its elements — they are genuinely new, because chemical bonding has changed the way the atoms behave.
A compound can only be separated back into its elements by a chemical reaction (for example by electrolysis, or by heating it with a more reactive element), never by a simple physical method such as filtering. That is the crucial 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 containing more than one capital letter shows a compound (for example CO2 or NaCl). A formula built from only one element's symbol — even when it has 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 joining the different substances, two consequences follow:
A familiar example is air, a mixture of nitrogen, oxygen, argon, carbon dioxide and water vapour all simply 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. None of these has a fixed formula, and in every case the parts can be pulled apart physically.
The difference between a compound and a mixture is one of the most important ideas in C2, so it is worth setting 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 part 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 confuse "molecule" with "compound", but the two words answer different questions: molecule is about how many atoms are joined, whereas element and compound are about how many types of atom.
So a molecule can be an element (like O2, N2 or Cl2) or a compound (like H2O, CO2 or NH3). The two ideas overlap, but they are not the same.
Reading a particle diagram: identical circles show an element; different circles bonded together in the same repeating unit show a compound; separate particles that are not all bonded show a mixture. Learn to spot these at a glance, because exam questions often show them.
In everyday language, "pure" often means "natural" or "with nothing added" — pure orange juice, pure spring water. In chemistry the word is stricter: 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 chemical meaning gives us a powerful way to test for 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 across several degrees it is a mixture. Chemists use this constantly — comparing a measured melting point with the known value for a pure compound is a quick check on how pure a sample is.
Two white solids are tested. Solid A melts sharply at 801°C. Solid B begins to soften at 670°C and is not fully molten until 710°C. Pure sodium chloride melts at 801°C. Which solid is 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, while 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 (a spread of 40°C) and starts melting below 801°C. A melting range shows it is impure — a mixture — not pure sodium chloride.
Answer: A is pure sodium chloride (a 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 single melting point, call the substance pure; if it gives a melting range (two temperatures), call it a mixture / impure. Writing "it melts over a range, so it is impure" earns the mark. A common misconception is that a mixture has a sharp melting point like a pure substance — it does not.
A formulation is a special kind of mixture — one made to a precise formula (a fixed recipe of measured amounts) in which 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 everyday 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 properly. That deliberate design is what marks a formulation out from an ordinary mixture such as muddy water.
Exam Tip: Define a formulation as "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 of its components 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 and 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 so that they 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 there, but the answer is thin. To move up a band, use the phrase 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 with a magnet because the atoms are bonded together."
Examiner-style commentary: A clear answer contrasting 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 behaves like neither iron nor sulfur — so a magnet no longer works. The only way to recover the elements from iron sulfide is by a chemical reaction, because physical methods cannot break chemical bonds. That 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 a fixed formula), properties (retained versus new) and separation (physical versus chemical) — covering every dimension the question asks for, with a correct named example used throughout.
This content is aligned with OCR Gateway Combined Science A (J250), Topic C2 Elements, compounds and mixtures. Refer to the official OCR specification for exact wording.