Fundamental Concepts in General Chemistry: Properties, Classification of Matter, and Laws of Chemical Combination

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 when measured.

• 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, ability to rust, and combustibility.

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, carbon dioxide).

• 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).

Type	Definition	Examples
Element	Cannot be broken down into simpler substances	Oxygen (O2), Gold (Au)
Compound	Composed of two or more elements chemically combined	Water (H2O), Carbon dioxide (CO2)
Homogeneous Mixture	Uniform composition throughout	Salt water, Air
Heterogeneous Mixture	Non-uniform composition	Sand in water, Salad

Law of Chemical Combination

The laws of chemical combination describe the fundamental principles governing how substances combine to form new compounds.

Law of Conservation of Mass

This law states that in any chemical reaction, the total mass of the reactants is equal to the total mass of the products formed.

• Statement: Mass is neither created nor destroyed during a chemical reaction.

• Mathematical Expression:

$\text{Total mass of reactants} = \text{Total mass of products}$

• 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.

Additional info: The Law of Conservation of Mass was first formulated by Antoine Lavoisier in the late 18th century and is foundational to modern chemistry.