Mass
Mass is the amount of matter in an object, measured in grams or kilograms with a balance, and it does not change with location.
Definition
Mass is how much stuff is in an object, measured with a balance in grams or kilograms. It is the amount of matter in an object and differs from weight, which is the force of gravity on the object and changes with location, whereas mass stays the same on Earth, on the Moon, or in orbit. Mass is also a measure of inertia, an object's resistance to acceleration, an intrinsic property that in ordinary chemistry and physics is conserved in every process. Laboratory mass is measured by comparison on a balance, which cancels local gravity, and is reported to the instrument's resolution, with a tared container, closed systems, and dry samples as the standard controls against systematic error.
Example
A rock on a balance reads $13.5\ \text{g}$ and a toy cart reads $350\ \text{g}$, so the cart has more mass. Dissolving $5.0\ \text{g}$ of salt in $100.0\ \text{g}$ of water gives salt water of $105.0\ \text{g}$, and on the Moon that salt water would still have a mass of $105.0\ \text{g}$ while weighing about one sixth as much. A sealed flask holding baking soda and a vial of vinegar reads $187.42\ \text{g}$, and after tipping the vial and letting the fizzing stop it reads $187.41\ \text{g}$, a change within the balance's resolution; repeating with an open flask gives $186.95\ \text{g}$ because $0.47\ \text{g}$ of carbon dioxide escaped.
Key Insight
Mass is not the same as size; a small steel ball can have more mass than a big foam ball. A balance measures mass and a spring scale measures weight, and the balance reads the same anywhere because gravity pulls equally on both sides of it. The open-flask result is not a violation of conservation but a leaky system, and when a lab appears to break a law the first suspect is always the boundary of the system, not the law.