The difference between a substance and a mixture is that a substance is one of a kind, a material of the same composition throughout, on the contrary, a mixture is one or more different substances brought together and mixed together without changing the nature of each single substance.
One way to test it is to take two substances like sand and table salt. They should each be in granular form and in adequate amount to mix. Neither substance has changed after mixing the two. Even though it may not be easy or convenient to accomplish, each substance could be separated out from the mixture.
When it comes to two substances in lump form, it would not be a mixture when one lump is positioned next to the other lump because there are not enough pieces to combine.
However, there could be a mixture of three substances, like sand, table salt and graphite powder and there could be a mixture with four substances, etc., ad infinitum.
Mixtures are of solid substances in general. On the other hand, one starts referring to solutions when liquids are involved. Gases can be a mixture like for example, air is a mixture with nitrogen, oxygen, argon, etc.
