How do elements, compounds and mixtures differ at the level of particles?
Distinguish between elements, compounds and mixtures, describe the difference between a compound and a mixture in terms of bonding and properties, and classify substances accordingly
A focused answer to the O-Level Chemistry outcome on classifying matter. The difference between elements, compounds and mixtures, why a compound differs from a mixture of the same elements, and how to classify substances at the particle level.
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What this dot point is asking
SEAB wants you to tell apart elements, compounds and mixtures, to explain clearly how a compound differs from a mixture of the same elements (in terms of bonding, fixed ratio and properties), and to classify given substances into the three categories. This is foundational vocabulary that underpins formulae, equations and the whole of chemistry, and the iron and sulfur comparison is a recurring exam favourite.
The answer
Elements
An element is a substance made of only one kind of atom. It cannot be broken down into anything simpler by chemical means. The roughly 100 elements are listed in the Periodic Table; examples are oxygen, iron, carbon and copper. An element may exist as single atoms (helium) or as molecules of identical atoms (oxygen, ).
Compounds
A compound is a substance in which two or more different elements are chemically combined in a fixed ratio. Forming a compound is a chemical change, and the compound has its own new properties, different from the elements that made it. Water (), carbon dioxide () and sodium chloride () are compounds. Because the elements are bonded together, a compound can only be separated into its elements by a chemical reaction, not by physical methods.
Mixtures
A mixture contains two or more substances (elements or compounds) that are not chemically combined. The substances keep their own properties and can be present in any ratio. Because nothing is bonded, a mixture can be separated by physical means such as filtration, distillation or a magnet. Air, sea water and a solution of salt in water are mixtures.
Compound versus mixture: the key comparison
The difference between a compound and a mixture of the same elements is examined constantly. Compare iron and sulfur:
- A mixture of iron filings and sulfur powder keeps the properties of both (the iron is still magnetic), can be made in any ratio, and is separated by physical means.
- The compound iron(II) sulfide, made by heating them, is a single new substance with its own properties (not magnetic), has the elements combined in a fixed ratio, and can only be split by a chemical reaction.
So forming the compound is a chemical change with new properties; mixing is a physical change that preserves the original properties.
Examples in context
Example 1. Air as a mixture. Air is a mixture of nitrogen, oxygen, argon, carbon dioxide and water vapour, not chemically combined, which is why fractional distillation of liquid air (a physical method) can separate it into nitrogen and oxygen. If air were a compound, it would have a fixed formula and could not be split this way.
Example 2. Water as a compound. Pure water is always two hydrogen atoms to one oxygen atom, a fixed ratio, with properties (a liquid that puts out fire) quite unlike its elements (two flammable or fire-supporting gases). Only electrolysis, a chemical change, splits water back into hydrogen and oxygen.
Try this
Q1. State what is meant by an element. [1 mark]
- Cue. A substance made of only one kind of atom that cannot be broken down by chemical means.
Q2. Give two ways a compound differs from a mixture of the same elements. [2 marks]
- Cue. A compound has its elements chemically combined in a fixed ratio with new properties; a mixture has them merely mixed in any ratio, keeping their own properties and separable physically.
Q3. Classify sea water and explain your answer. [2 marks]
- Cue. Sea water is a mixture: it contains water and dissolved salts that are not chemically combined and can be separated by physical means such as distillation.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksIron filings and powdered sulfur can be mixed, or heated together to form iron(II) sulfide. (a) State two differences between the mixture and the compound. (b) Explain, in terms of the substances present, why a magnet separates the iron from the mixture but not from the compound.Show worked answer →
(a) Any two of: the mixture can be separated by physical means (such as a magnet) but the compound cannot; the mixture keeps the properties of iron and sulfur but the compound has new properties of its own; the elements in the mixture can be present in any ratio but in the compound they are chemically combined in a fixed ratio; making the compound is a chemical change that gives out heat.
(b) In the mixture the iron is still present as iron, which is magnetic, so a magnet pulls it out. In the compound the iron has chemically combined with sulfur to form iron(II) sulfide, a new substance that is not magnetic, so the magnet has no effect.
Markers reward two valid differences and the explanation that iron keeps its properties in a mixture but loses them when chemically combined in the compound.
Original3 marksClassify each of the following as an element, a compound or a mixture: (a) oxygen gas, (b) carbon dioxide, (c) air. Give a brief reason for each.Show worked answer →
(a) Oxygen gas is an element: it contains only one kind of atom.
(b) Carbon dioxide is a compound: it contains two different elements (carbon and oxygen) chemically combined in a fixed ratio.
(c) Air is a mixture: it contains several substances (nitrogen, oxygen, and others) not chemically combined, which can be separated by physical means.
Markers reward the correct classification of each with a reason tied to one kind of atom (element), chemically combined elements (compound), or physically mixed substances (mixture).
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