Nature of Matter: Elements, Compounds, and Mixtures — Science STD 8 — Question

(i) Elements
- Definition: A pure substance consisting of only one type of atom is called an atom.
- Composition: It is made up of identical atoms, each with the same number of protons.
- Properties: Elements possess unique and distinct physical and chemical properties.
- Separation: It cannot be broken down into simpler substances by ordinary chemical or physical means.
- Examples: Gold (Au), Oxygen (O), Iron (Fe), Sodium (Na).
(ii) Compounds
- Definition: A pure substance formed when two or more different elements chemically combine in a fixed proportion by mass.
- Composition: It consists of molecules where different types of atoms are chemically bonded together in a specific ratio.
- Properties: Compounds exhibit entirely new properties different from their constituent elements. For example, water (H₂O) is a liquid at room temperature, while its constituent elements, hydrogen and oxygen, are gases.
- Separation: It can only be broken down into its constituent elements through chemical or electrochemical reactions.
- Examples: Water (H₂O)
(iii) Mixtures
Definition: A combination of two or more substances (elements or compounds) that are physically blended but not chemically bonded.
- Composition: The components can be present in any proportion, and the composition can be variable.
- Properties: Each component retains its individual physical and chemical properties within the mixture. For example, in a mixture of iron filings and sulfur powder, both retain their respective magnetic and yellow color properties.
- Separation: It can be separated into its constituent substances using physical methods based on differences in their physical properties (e.g., filtration, evaporation, magnetism).
- Types: It can be either homogeneous (uniform composition throughout, like saltwater or air) or heterogeneous (non-uniform composition with visibly distinct parts, like sand and water or a salad).
- Examples: Air, saltwater, soil, sugar solution.