Why does change in mass happen

The mass in a chemical reaction is constant in a closed system.

Explanation:

It is important to remember that mass changes only occur because a gas is being released into the atmosphere, or because a gas from the atmosphere is being used as a reactant - no atoms are being created or destroyed, just rearranged.

The mass in a chemical reaction is constant in a closed system ... It is important to remember that mass changes only occur because a gas is being released into the atmosphere, or because a gas from the atmosphere is being used as a reactant - no atoms are being created or destroyed, just rearranged.