Elements, compounds and mixtures
The difference between an element, a compound and a mixture.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 2.1.1Describe the differences between elements, compounds and mixtures.
5 minute read
Elements, compounds and mixtures
Elements
An element is a substance made of only one type of atom. It cannot be broken down into anything simpler by chemical means. There are around 118 known elements, each represented by its own symbol on the Periodic Table.
Compounds
A compound forms when two or more different elements are chemically joined in a fixed proportion. Because the atoms are bonded together, a compound has different properties from the elements it is made of — for example, sodium (a reactive metal) and chlorine (a toxic gas) combine to form sodium chloride, an unreactive, edible salt. Compounds can only be separated back into their elements by a chemical reaction.
Mixtures
A mixture contains two or more substances that are not chemically joined. There is no fixed proportion — a mixture can contain any ratio of its components — and each substance keeps its own properties. Because nothing is chemically bonded, mixtures can be separated by physical methods such as filtration, evaporation or distillation.
Telling them apart
The two defining questions are: are the parts chemically joined? And is there a fixed proportion? A "yes" to both means a compound; a "no" to both means a mixture.
Think of it like this
Think of building blocks: a pure colour of blocks is an element, blocks permanently glued together in a fixed pattern to make a specific model is a compound, and blocks simply tipped into the same box unglued, in whatever amounts you like, is a mixture.
Common misconceptions
- Thinking air, which contains several gases, is a compound because it has a "typical" composition. It is a mixture — the gases are not chemically joined, and the proportions can vary slightly.
- Assuming a mixture must look uniform to be a mixture. Salt water looks completely uniform but is still a mixture, because the salt and water are not chemically combined.
- Believing a compound keeps the properties of the elements that formed it. A compound typically has entirely different properties from its constituent elements.
In the exam
- When asked to classify a substance, give a reason referring to bonding and proportion — "it is a mixture, because the components are not chemically joined and can be present in any ratio" scores more than just naming the category.
- A common exam trap is a compound name that "sounds like" a mixture, or vice versa — always check what the question tells you about how the substance was formed.