In chemistry, a pure substance is defined as a material that consists of only one type of particle, which can be either an element or a compound.

To elaborate, a pure substance is a form of matter that maintains a constant composition and exhibits uniform properties throughout the entire sample. It is composed solely of one type of particle, which may be atoms, molecules, or ions. Pure substances are further classified into two categories: elements and compounds.

An element is a pure substance that cannot be decomposed into simpler substances by any chemical means. Examples of elements include hydrogen, oxygen, gold, and iron. Each element is uniquely identified by the number of protons in the nucleus of its atoms, a value known as its atomic number.

In contrast, a compound is a pure substance formed from two or more different elements that are chemically combined in a fixed ratio. For example, water is a compound consisting of two hydrogen atoms and one oxygen atom, represented chemically as $H_2O$. Compounds can be broken down into simpler substances, but this can only be achieved through chemical reactions, not by physical methods.

Pure substances possess specific physical and chemical properties, such as boiling point, melting point, density, and reactivity, which remain consistent throughout the sample. For instance, pure water has a boiling point of $100^\circ C$ at sea level, a characteristic that holds true for all samples of pure water.

In contrast, mixtures are composed of two or more different substances that are physically combined rather than chemically. The components of a mixture can be separated using physical methods, such as filtration or distillation. Unlike pure substances, mixtures do not have fixed compositions or uniform properties throughout the sample.