8.2 What Are Pure Substances?

You’ve seen “Pure” stamped on packs of milk, ghee and spices. But “pure” means something different in science than at the shop.

“Pure” in Science In everyday use, “pure” means good quality and unadulterated. In science, a pure substance is one that contains only a single kind of particle — no other substance mixed in. It cannot be separated into other kinds of matter by any physical process. By this strict test, even “pure” shop milk is actually a mixture, because it is made of more than one substance.

Try This — Sort by science Using the scientific meaning, decide which of these are mixtures and which are pure substances: milk, packed fruit juice, baking soda, sugar, soil. (Hint: a pure substance has just one kind of particle and can’t be physically separated into different substances.) Most “foods” turn out to be mixtures; baking soda and sugar are pure substances.

So matter divides cleanly:

The Big Map of Matter \text{Matter} \begin{cases} \textbf{Pure substances} \begin{cases} \textbf{Elements} \\ \textbf{Compounds} \end{cases} \\ \textbf{Mixtures} \;(\text{uniform or non-uniform}) \end{cases}

A pure substance is either an element or a compound. The whole rest of this chapter explores those two — and how to tell everything apart.

But how do we find out whether a pure-looking substance is an element or a compound? We try to break it down. Next, we’ll pass electricity through water — and watch it come apart.

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