化学式是用元素符号来表示纯净物组成的式子。各种纯净物都有固定的组成，任何一种纯净物的化学式，其元素的种类和原子的个数都是固定不变的，所以，一种物质都只有一个化学式。化学式所表示的意义包括质（由哪些元素组成）和量（如化学式中各元素的原子个数）两方面。因此， The chemical formula is an equation composed of pure elements. All kinds of pure substances have a fixed composition. The chemical formula of any kind of pure substance, the type of its elements and the number of atoms are all fixed and unchanged. Therefore, a substance has only one chemical formula. The meaning represented by the chemical formula includes two aspects: quality (consisting of which elements) and quantity (such as the number of atoms of each element in the chemical formula). therefore,