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 chemical composition is altered during measurement.

• Physical Property: A property that can be measured or observed 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 of properties.

• 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). Example: salt water.

  • Heterogeneous Mixture: Does not have uniform composition; different parts can be seen. Example: sand in water.

Example: Air is a homogeneous mixture, while a salad is a heterogeneous mixture.

Summary: Pure substances have fixed composition; mixtures can vary in composition and may be uniform (homogeneous) or non-uniform (heterogeneous).

Law of Chemical Combination

Law of Conservation of Mass

The laws of chemical combination describe how substances react and combine to form new substances. The Law of Conservation of Mass is a foundational principle in chemistry.

• Law of Conservation of Mass: In any chemical reaction, the total mass of the reactants is equal to the total mass of the products formed. Mass is neither created nor destroyed during a chemical reaction.

Mathematical Expression:

Example: When hydrogen gas reacts with oxygen gas to form water: The 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 18th century and is fundamental to all chemical equations and stoichiometric calculations.