A pure substance is a substance with a constant composition and distinct properties. It is composed of only one type of atom, molecule, or compound. Examples of pure substances include elements (e.g., iron, gold, oxygen) and compounds (e.g., water, salt, sugar).

A jar of pebbles, on the other hand, is a mixture of different types of stones and minerals. Each pebble may have a different composition, color, texture, and shape. Therefore, a jar of pebbles is a heterogeneous mixture, not a pure substance.