How is matter classified as substances elements compounds and mixtures?

Matter is classified as substances and mixtures.
Substances include elements and compounds. Element is something that cannot be broken down into anything more simple even by chemical methods. There are over 100 elements on earth and the most common ones include oxygen or aluminum or silicon.
Compound is something where 2 or more elements are chemically combined together, such as by electrolysis or heating. It cannot be broken down by simple physical methods, such as evaporation or filtration. Common Examples include carbon dioxide, sodium chloride etc.
Mixture is where elements or compounds are mixed together, and can be separated by simple physical methods. Examples include air or sea water. The proportion of the elements and compounds are not fixed.