Explain what it means to have a change in mass.
In simplest terms, a change in mass occurs when the amount of a substance changes (in an open system).
In the lab, the amount of a substance is measured as mass. The Law of Conservation of Mass states that matter is neither created or destroyed. This refers to a closed system. If your system is open, such as an uncovered beaker, an uncovered flask, an uncovered pot, the substance in the container can be spilled, contaminated, or, in the case of a liquid, perhaps vaporized.
The mass of a flask changes when water is added to the flask. The mass of the flask with water changes when sugar is added.
If you are carrying a weighing paper holding 3.00 g of a salt and you bump into someone and spill some of the salt, the mass of salt on the weighing paper changes.
If you have an open beaker containing 75.00 g of liquid water sitting on a lab bench and leave it there for 2 days, the mass of the liquid water will change as some liquid water evaporates (becomes vapor no longer contained in the beaker.
If you filter a solid, such as charcoal, from a liquid, such as water, the mass of the dry solid will be less than the mass of the wet solid. The wet solid will change in mass until all of the water is evaporated.
When doing an experiment to carefully measure the residue remaining after a decomposition reaction, a ceramic crucible is used. Before doing the experiment, the crucible must be dried by heating it over a flame until the mass of the crucible stops changing. By flaming the crucible, water withing the pores of the ceramic material is vaporized. As the water is driven from the ceramic, the mass of the crucible changes. You know all of the water has been removed from the ceramic when the mass of the crucible remains constant after continued heating.