What are pure substances classified as?

The correct answer is option 3. Elements and Compounds.

Pure substances are classified into two main categories: elements and compounds.

1. Elements:

• Elements are substances that consist of only one type of atom.
• Each element is represented by a unique chemical symbol (e.g., O for oxygen, H for hydrogen).
• Elements cannot be broken down into simpler substances by chemical means.

2. Compounds:

• Compounds are substances composed of two or more different elements chemically combined in fixed ratios.
• The constituent elements in a compound are held together by chemical bonds.
• Compounds have unique chemical formulas (e.g., H₂O for water, CO₂ for carbon dioxide).
• Compounds can be broken down into their constituent elements by chemical reactions.

In summary, pure substances are either elements or compounds. Elements are made up of only one type of atom, while compounds consist of two or more different elements chemically bonded together. This classification is fundamental to understanding the composition and behavior of matter in chemistry.