When you put a stelzer tablet into a small amount of water in a small container the tablet fizzes and dissolves and it explodes, that is a chemical change because the tablet is no longer a solid
When you mix two scientific liquids and some dish soap then you create elephant toothpaste. This is a chemical reaction because two liquids become a solid
The law of conservation and mass is in my life everyday when I eat one kg of food then I weigh one kg more than before I ate it. Or if I put one L of water in a bottle then the bottle will weigh one kg more than before.