An element contains one atom type. A compound contains two or more elements chemically joined in a fixed composition. A mixture contains multiple substances together. Pure O2 is an element because both atoms are oxygen. Pure water is a compound because hydrogen and oxygen are chemically joined.
Use atom types and joining patterns together. Identical particles containing two elements can represent a pure compound; several different substances together represent a mixture. Mixtures can contain elements, compounds or both. Chemical purity means one substance, not safe, healthy or natural. Physical methods can separate mixture components; separating a compound into its elements requires chemical change.
Example 1
A sample contains only identical molecules with one carbon atom joined to two oxygen atoms. Classify it.
A pure compound: every particle has the same composition and contains two elements chemically joined.
Example 2
A container holds oxygen gas and nitrogen gas without reacting. Classify it.
A mixture of elements: two substances coexist without forming one compound.
Watch out:A compound is one substance with chemically joined elements in a fixed composition. A mixture contains multiple substances.
Classifying elements, compounds and mixtures practice | Year 8 Science | Ascent Arcade