A substance cannot have less mass after a chemical change than it had before. This is because of the Law of Conservation of Mass, which states that mass cannot be created or destroyed in a closed system.

Here's why you might *think* a substance has less mass:

* Gas production: A chemical reaction might produce a gas that escapes into the atmosphere. This loss of gas makes it *appear* as though the substance has lost mass, but the gas is still present, just not in the container.

* Evaporation or sublimation: If a liquid or solid evaporates or sublimates (turns directly into a gas), it might seem like mass is lost. However, the evaporated or sublimated material is still present in the air as a gas.

* Incomplete measurements: Sometimes, measuring techniques might not be precise enough to account for all the mass involved in a reaction. This can create the illusion of mass loss.

In summary, even though it may appear that mass is lost during a chemical reaction, the total mass of the system remains constant. It's just that some of the mass might be in a different form or location than it was initially.