The main difference between a pure substance and a mixture is the uniformity of their composition. Here's a brief explanation of each:

1. Pure substance: A pure substance is a material that has a constant composition and properties throughout. A pure substance can be either an element or a compound. An element is a substance made up of only one type of atom, such as copper or oxygen. A compound is a substance made up of two or more different types of atoms chemically bonded together, such as water (H2O) or table salt (NaCl). The properties of a pure substance are uniform and can be used to identify the substance.
2. Mixture: A mixture is a material that consists of two or more substances physically combined together. Unlike a pure substance, the composition of a mixture can vary throughout the material. Mixtures can be classified into two types: homogeneous and heterogeneous. A homogeneous mixture has a uniform composition throughout the material, such as saltwater or air. A heterogeneous mixture has a non-uniform composition throughout the material, such as a salad or a rocky beach. The properties of a mixture can vary depending on the proportion and type of the substances that are combined.

In summary, the key difference between a pure substance and a mixture is that a pure substance has a constant composition and properties throughout, while a mixture can vary in composition and properties throughout the material.