# Magnesium oxide is a compound of magnesium and oxygen. How do you know that it is a completely different substance from each of the two elements it is made up of?

Magnesium is a silvery metal with a metal point of $650$ ""^@C, and an atomic mass of $24.32 \cdot g \cdot m o {l}^{-} 1$. Oxygen is a colourless, room termperature gas with a boiling point of $- 182.3$ ""^@C, and a molecular mass of $32.00 \cdot g \cdot m o {l}^{-} 1$.
$M g O$ is a white, hygroscopic solid with a metal point of $2825$ ""^@C, and a formula mass of $40.31 \cdot g \cdot m o {l}^{-} 1$. The difference in physical AND chemical properties dictate a difference in chemical composition and formula.