What did Antione Lavoiser discover?

Answer 1

Antione Lavoiser discovered that matter's form or shape can be changed, but its mass will stay the same.

So the most common example would be: when boiling water, you will see steam rising from the pot, and you will notice the water level in the pot lowering. That water is changing its form to steam, but that steam has the same mass as when it was water in the pot. The same goes for when water is frozen into ice. The ice cube has the same mass as when it was in its liquid state before being frozen.