Chemists can classify matter as solid, liquid, or gas. But there are other ways to classify matter, as well — such as pure substances and mixtures. Classification is one of the basic processes in science. All matter can be classified as either a pure substance or a mixture. A pure substance has a definite and constant composition — like salt or sugar. A pure substance can be either an element or a compound, but the composition of a pure substance doesn’t vary. Mixtures are physical combinations of pure substances that have no definite or constant composition — the composition of a mixture varies according to who prepares the mixture.
Although chemists have a difficult time separating compounds into their specific elements, the different parts of a mixture can be easily separated by physical means, such as filtration.
For example, suppose you have a mixture of salt and sand, and you want to purify the sand by removing the salt. You can do this by adding water, dissolving the salt, and then filtering the mixture. You then end up with pure sand.
Mixtures can be either homogeneous or heterogeneous:
A homogeneous mixture, sometimes called a solution, is relatively uniform in composition; every portion of the mixture is like every other portion.
For example, if you dissolve sugar in water and mix it really well, your mixture is basically the same no matter where you sample it.
A heterogeneous mixture is a mixture whose composition varies from position to position within the sample.
For example, if you put some sugar in a jar, add some sand, and then give the jar a couple of shakes, your mixture doesn’t have the same composition throughout the jar. Because the sand is heavier, there’s probably more sand at the bottom of the jar and more sugar at the top.
To determine if something is a substance or a mixture you can centrifuge it and/or look at its warming curve.
Explanation:
Centrifugation is a method of separating heterogeneous mixtures of solids with liquids or only liquids. One of the most important factors in centrifugation is density. This is because centrifugation separates what is denser from what is less dense. What is denser is at a lower level, while the less dense rises. Thus, centrifugation can be used as a test if something is a substance or a mixture. This is because if, after going through the centrifugation process, this "something" has two phases it means that it is a mixture; presenting a single phase is a substance.
Another very useful test in chemistry to identify if something is a substance or a mixture is the so-called warming curve. A solid-state sample is taken at a temperature and subjected to constant heating. The solid sample warms up until it begins to melt. Transformed into liquid continues to heat until boiling. Even after being turned into gas, it can still be heated.
If the boiling point and melting point have a constant temperature we can say that the sample is a pure substance. If one or both of them vary, it is a mixture.
