Pure substances are substances that are made up of only one kind of particle and have a fixed or constant structure. They are mostly homogeneous in nature containing only one type of atom or molecule. Pure substances are further classified as elements and compounds. An element is a substance that consists of only one type or kind of atom, and it cannot be broken down or transformed into a new substance even by using some physical or chemical means. A compound is a substance that consists of two or more elements that are chemically combined in a fixed ratio.

The properties of pure substances are constant and predictable, and they can be used to identify and classify them. These properties include:

• Physical properties: These are properties that can be observed or measured without changing the chemical composition of the substance. Examples of physical properties include mass, compressibility, density, color, boiling point, melting point, heat capacity, vapor pressure (for liquids), heat of vaporization (for liquids), and surface tension (for liquids) .

• Chemical properties: These are properties that describe how a substance interacts with other substances to form new substances. Examples of chemical properties include flammability, toxicity, acidity or basicity, reactivity, solubility, electronegativity, and ionization.

• Homogeneity: Pure substances are homogeneous, meaning they have a uniform composition throughout.

• Definite composition: Pure substances always have a definite and constant composition.

• Fixed boiling and melting points: Pure substances have fixed boiling and melting points, which can be used to identify them.

• Phases: Pure substances may exist in different phases, including solid, liquid, and gas.

In summary, pure substances are homogeneous substances that have a fixed or constant structure and predictable properties. They can be classified as elements or compounds, and their properties include physical and chemical properties, homogeneity, definite composition, fixed boiling and melting points, and phases.