Conservation of mass

What is the law of conservation of mass?

In a chemical reaction, mass is ALWAYS conserved

What does it mean if the mass seems to change?

There is gas involved

What happens to the mass of one of the products is a gas and why?

The mass decreases and all the reactants are solid, liquid or aqueous.
This is because before the reaction, all the reactants aee contained in the reaction vessels. If the vessel isn’t enclosed, then the gas can escape from the reaction vessel as it’s formed. It is no longer in the reaction vessel, so you can’t account for its mass.

What happens to the mass of one of the reactants is a gas and why?

The mass increases and all the products are solid, liquids or aqueous.
This is because before the reatcion, the gas is floating aroung the air. It’s there, but it’s not contained in the reaction vessel, so you can’t account for its mass.

When the gas reacts to form part of the product it becomes contained inside the reaction vessel - so the stuff inside the reaction vessel increases.