A glass of clear liquid can contain pure water, salt water, or a mixture of several dissolved substances. Its appearance alone cannot tell you which. To distinguish a pure substance from a mixture, ask what it contains and whether its composition can vary.
Start with chemical identity
In introductory chemistry, a pure substance has a definite composition. It may be an element, such as copper, or a compound, such as water. An element cannot be separated into simpler substances by ordinary chemical reactions. A compound contains chemically combined elements: water contains hydrogen and oxygen in the proportions described by H₂O.
A mixture contains more than one substance. Its proportions can change without requiring a new name for every possible recipe. A little salt dissolved in water and a much saltier solution are both mixtures of salt and water.
Uniform does not necessarily mean pure
A homogeneous mixture has a uniform composition at the scale being considered. A fully dissolved salt solution is an example. A heterogeneous mixture has regions with different compositions, such as sand mixed with water. Seeing distinct pieces is helpful evidence for a mixture, but not seeing them is not proof of purity.
This also explains why the everyday meaning of “pure” can mislead. A product described as pure juice can still contain water, sugars, acids, and many other chemical substances. A marketing description does not replace a chemical classification.
Use separation as a second test
The components of mixtures can be separated using physical differences. Filtering can separate insoluble sand from water. It will not remove dissolved salt with an ordinary paper filter: the dissolved material passes through with the liquid. Evaporation can leave salt behind, while distillation can collect water vapor after it condenses.
Breaking a compound into different substances requires a chemical change. Melting ice is not such a change: solid water and liquid water have the same chemical identity. A sample containing only water can therefore have ice and liquid present without becoming a mixture of different chemical substances.
Try the decision yourself
- Copper metal: an element, when the sample is chemically pure.
- Pure sodium chloride: a compound, even though it contains two elements.
- Salt dissolved evenly in water: a homogeneous mixture.
- Sand and water: a heterogeneous mixture.
For an unfamiliar sample, explain your reasoning in this order: identify its components, ask whether their proportions can vary, then consider an appropriate physical separation. That is more reliable than judging by color or transparency.