Some Basic Concepts of Chemistry

Physical and Chemical Properties

Understanding the properties of substances is fundamental in chemistry. These properties are classified as either physical or chemical, depending on whether the substance's composition is altered during measurement.

• Physical Property: A property that can be measured without changing the chemical composition of a substance. Examples include mass, volume, and density.

• Chemical Property: A property that can only be evaluated by changing the chemical identity of the substance. Examples include flammability, reactivity with acids, and oxidation states.

Example: Measuring the boiling point of water is a physical property, while observing iron rusting (reacting with oxygen) is a chemical property.

Classification of Matter

Matter is anything that has mass and occupies space. It can be classified based on its composition and uniformity.

• Pure Substance: Matter with a fixed composition and distinct properties. Pure substances are further divided into:

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

  • Compound: A substance composed of two or more elements chemically combined in a fixed ratio (e.g., H2O, NaCl).

• Mixture: A combination of two or more substances in which each retains its own identity. Mixtures are classified as:

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

  • Heterogeneous Mixture: Does not have uniform composition (e.g., sand in water).

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

Law of Chemical Combination

Chemical reactions are governed by fundamental laws. One of the most important is the Law of Conservation of Mass.

Law of Conservation of Mass

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

• Mathematical Expression:

• Implication: Mass is neither created nor destroyed during a chemical reaction; it is conserved.

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: Other laws of chemical combination include the Law of Definite Proportions and the Law of Multiple Proportions, which further describe how elements combine to form compounds.