What term refers to a small particle or a tiny piece of a substance?

The term that accurately refers to a small particle or a tiny piece of a substance is "mote." A mote typically describes a very small particle, often used to denote dust or specks that are barely visible to the naked eye. This term emphasizes the small size and is often associated with particles found in the air or very minute fragments of a material.

While "atom" and "molecule" are fundamental concepts in chemistry referring to the basic building blocks of matter, they denote specific structural elements rather than a general term for tiny pieces of any substance. An atom is the smallest unit of an element, and a molecule is formed when two or more atoms bond together.

The term "granule" refers to a small grain or particle but is often used in specific contexts related to the size and texture of materials, such as grains of sand or sugar. Thus, while it implies smallness, "mote" best captures the general essence of a small, indistinct particle, making it the most appropriate choice in this context.