Some Basic Concepts of Chemistry

Physical Properties

Physical properties are characteristics of a substance that can be measured or observed without altering the chemical composition of the substance.

• Definition: Properties such as mass, volume, and density that do not involve a change in the substance's chemical identity.

• Examples: Mass (amount of matter), volume (space occupied), density (mass per unit volume).

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform it into different substances.

• Definition: Properties that can only be observed or measured by changing the chemical identity of a substance.

• Examples: Reactivity with acids, flammability, oxidation states.

Classification of Matter

Matter can be classified based on its physical and chemical composition. The main categories are pure substances and mixtures.

• Pure Substances: Have a fixed composition and distinct properties. They are further divided into:

  • Elements: Substances that cannot be broken down into simpler substances by chemical means (e.g., O2, Fe).

  • Compounds: Substances composed of two or more elements chemically combined in fixed proportions (e.g., H2O, NaCl).

• Mixtures: Combinations of two or more substances where each retains its own properties. Mixtures can be:

  • Homogeneous Mixtures: Uniform composition throughout (also called solutions), e.g., saltwater.

  • Heterogeneous Mixtures: Non-uniform composition, e.g., sand in water.

Diagram Description: A flowchart showing: Matter → Pure Substance (Element, Compound) and Mixture (Homogeneous, Heterogeneous).

Law of Chemical Combination

Law of Conservation of Mass

The Law of Conservation of Mass is a fundamental principle in chemistry that states that mass is neither created nor destroyed in a chemical reaction.

• Statement: For any chemical change, the total mass of the reactants is always equal to the total mass of the products formed.

• Mathematical Expression:

• Example: When hydrogen reacts with oxygen to form water: , the combined mass of hydrogen and oxygen before the reaction equals the mass of water produced.

Application: This law is essential for balancing chemical equations and for quantitative chemical analysis.