Categories of Pure Substances:
The two main categories of pure substances are elements and compounds. These substances are composed of only one type of atom or molecule and have specific chemical and physical properties.

Elements:
- Elements are pure substances that consist of only one type of atom. They cannot be broken down into simpler substances by ordinary chemical reactions.
- Each element has its own unique set of properties, such as melting point, boiling point, and chemical reactivity.
- There are currently 118 known elements, with each one represented by a unique chemical symbol (e.g., H for hydrogen, O for oxygen, etc.).
- Elements are organized in the periodic table based on their atomic number, electron configuration, and chemical properties.

Compounds:
- Compounds are pure substances that are composed of two or more different elements chemically bonded together in fixed proportions.
- Unlike mixtures, compounds have a specific chemical formula that defines the ratio of elements in the compound.
- Compounds exhibit unique properties that are different from the properties of their constituent elements.
- Examples of compounds include water (H2O), salt (NaCl), and carbon dioxide (CO2).
- Compounds can be classified as organic compounds (containing carbon) or inorganic compounds (lacking carbon).
In conclusion, pure substances can be categorized as elements or compounds based on their composition and properties. Elements consist of only one type of atom, while compounds are made up of two or more different elements bonded together. Understanding the categories of pure substances is essential in the field of chemistry for identifying and studying the behavior of different substances.