Some Basic Concepts of Chemistry

Physical Properties

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

• Definition: Properties such as mass, volume, and density that do not alter the identity of the substance.

• Examples: Mass, volume, density, melting point, boiling point, color, and state of matter.

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, and ability to rust or tarnish. bnm bmbkm

Classification of Matter

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

• Pure Substance: Matter with a fixed composition and distinct properties. It can be further divided into:

  • Element: A substance that cannot be broken down into simpler substances by chemical means (e.g., hydrogen, oxygen).

  • Compound: A substance composed of two or more elements chemically combined in a fixed ratio (e.g., water, sodium chloride).

• Mixture: A combination of two or more substances in which each retains its own identity and properties. Mixtures can be:

  • Homogeneous Mixture: Has a uniform composition throughout (also called a solution; e.g., salt water, air).

  • Heterogeneous Mixture: Does not have a uniform composition; different parts can be seen (e.g., sand in water, salad).

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:

  • 2 g of hydrogen + 16 g of oxygen → 18 g of water

  • Total mass before reaction = 18 g; total mass after reaction = 18 g

Additional info: The law of conservation of mass is foundational for balancing chemical equations and understanding stoichiometry in chemical reactions.