What is defined as a unit that measures the amount of substance in chemistry?

The term that measures the amount of substance in chemistry is the mole. This unit is fundamental to the field of chemistry because it establishes a link between the macroscopic scale of substances that we can measure and the microscopic scale of atoms and molecules. One mole is defined as exactly 6.022 x 10²³ particles, which could be atoms, molecules, ions, or other entities, depending on the context. This relationship allows chemists to quantify and compare amounts of different substances in reactions, providing a foundational basis for stoichiometry, the calculation of reactants and products in chemical reactions.

In contrast, other options represent different types of measurements. A liter is a unit of volume, often used to measure liquids rather than the quantity of particles. A gram measures mass, which does not directly translate to the number of particles without considering the molar mass of the substance. A joule is a unit of energy, unrelated to the measurement of substance quantity. Therefore, the mole stands out as the correct choice when referring to the specific measure of the amount of substance in chemistry.