Is this true or false about the characteristics of a substance? a certain substance will always be made up of the same element or compound.

A chemical substance is defined as the substance which is made up of single molecule or atom.

Thus, the given statement is true.

Now, law of multiple proportions: The combination of two elements with each other to give two or more compounds, the mass ratio of one element which combines with the fixed mass of another element are present in form of simpler whole numbers. For example: and .

Hence, a substance is made up of the single element or compound.