A Level Chemistry OCR Practice Exam 2026 – All-in-One Guide to Master Your Chemistry Exam!

The term "amount of substance" refers specifically to the quantity that is measured in moles, which is a fundamental concept in chemistry. The mole is a unit that allows chemists to count particles, such as atoms, molecules, or ions, in a specific sample of material. This relationship is crucial because it provides a bridge between the macroscopic scale of substances (the quantities we can measure like grams or liters) and the microscopic scale (the number of entities like atoms or molecules).

Using the mole as a unit helps chemists perform calculations related to the stoichiometry of reactions, allowing them to determine how much of each reactant is needed and how much product can be expected. This is essential for understanding chemical reactions quantitively and is basic to both theoretical and practical chemistry.

The other options, while they relate to various aspects of chemical processes, do not accurately define "amount of substance." For example, mass and concentration are related concepts, but they do not encompass the idea of counting discrete particles, which is the primary significance of the concept of moles.