if matter is uniform throughout and can be separated into simpler substance by physical process, it is?
A substance is a kind of matter that can not be separated into simpler kinds of matter by physical processes. For example, pure water (distilled water) is a substance. There is nothing you can physically do with water to separate it into 2 or more other substances. Certainly, we know that water is composed of hydrogen and oxygen. Almost everyone, even those with no formal exposure to chemistry, have seen the formula H2O. We can separate water into hydrogen and oxygen, but this involves a chemical change, not just a physical one. A mixture is a kind of matter than can be separated into two or more simpler kinds of matter by physical processes. Here, we can recognize two distinct substances that are mixed together to make this kind of matter we call "salt water". It is possible to separate the salt water using only physical processes -- that is, without changing the chemical identity of anything.