Some Basic Concepts of Chemistry

Physical and Chemical Properties

Understanding the properties of substances is fundamental in chemistry. These properties are broadly classified into physical properties and chemical properties.

• 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 a 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 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: Physical combinations of two or more substances where each retains its own properties. Mixtures are classified as:

  • Homogeneous Mixtures: Have uniform composition throughout (also called solutions; e.g., salt water).

  • Heterogeneous Mixtures: Do 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

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

Law of Conservation of Mass

This law states that for any chemical change, the total mass of the reactants is always equal to the total mass of the products formed. In other words, 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 gas reacts with oxygen gas to form water:

  \[ 2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l) \]

  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 18th century and is foundational to all chemical calculations.