Chemical reactions and conservation of mass
A chemical change produces a new substance, and in a sealed system no atoms are gained or lost, so the total mass before and after stays the same.
A worked example
A sealed flask holding 5.0 g of one reactant and 3.0 g of another, both fully used up, must contain 8.0 g of product afterwards.
The mistake people actually make
Mass only appears to change when a gas escapes or air is drawn in; in a sealed container it cannot. Colour change, heat released or absorbed, gas bubbles, and a solid forming from two liquids are all evidence of a chemical change, not proof by itself.