A mixture that contains only one substance is called a pure substance.

Here's why:

* Mixture: A combination of two or more substances that are not chemically bonded. Each substance retains its own properties.

* Pure Substance: A substance that is made up of only one type of atom or molecule.

So, if you have a container of only water, it's considered a pure substance because it's made up of only water molecules. The same goes for a jar of sugar, a bag of salt, or a block of pure gold.

It's important to note that the term "pure" is often used in a relative sense. Even a pure substance can contain impurities at a very low level. However, the main component of a pure substance is always the same single type of atom or molecule.