Because the things in the numerator are identical to the things in the denominator, the fraction is identically 1. The magic of a unit factor is that it can be multipled by anything without changing the value of that thing, because anything multipled by 1 is unchanged. Of the two unit factors, you must use the correct one. Unit cancellation will tell you which is right and which is wrong. Whereas mass and volume are extensive properties of that particular sample, density is an intensive property, true for all samples of that material.
Different substances have different densities.
Fe These can then be used in calculations involving volume and mass of a material. Specific gravity is related to density. For a given substance it is the ratio of the density of that substance to the density of water. Heat and Temperature Heat and temperature are related, but different. Heat is a form of energy that can flow from a warmer object to a colder object.