Mass

Mass is the amount of matter in a sample, expressed as a number with units. A value for mass must always include a number and measurement units; without both, the value is meaningless. A mass value can be any positive real number together with a measurement unit. For example, a mass value may be 10 kilograms, or 0.35 grams. To say that something has mass without saying how much mass is wrong.

Mass is the example Merrill returns to most often when teaching the discipline of property and value. A definition often given for matter is that it has mass. As soon as that statement is made, one must immediately ask "how much mass?" When speaking of matter in general, it is impossible to say how much, so matter does not have the property of mass. Only a particular sample, in front of us, in some container, has a particular mass. This is why the concept "water" does not have mass either, but the water in this glass does, and a scale will read it.

Mass is extensive, and decisively so. Pour half the water out, and the mass of the water in the glass is half what it was. Add more, and the mass goes up. The value tracks the amount, the way a tally tracks the items being counted. This is exactly the property that conservation talks about: when no material disappears in a change, the total mass before equals the total mass after. The arithmetic of conservation is mass arithmetic.

Mass takes its place among the core features of a phase, alongside volume, temperature, pressure, energy, entropy, and the others. It is one of the extensive ones, paired naturally with volume, and the ratio of the two has its own name (density) and its own work to do. Mass is where chemistry's quantitative discipline begins, because mass is what the scale gives you, and the scale is one of the first instruments anyone learns to trust.