Some Basic Concepts of Chemistry

Physical Properties

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

• Definition: A physical property is any property that is measurable and whose value describes a state of a physical system.

• Examples: Mass, volume, density, color, melting point, boiling point.

• Application: Physical properties are used to identify and characterize substances without altering their identity.

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform it into different substances.

• Definition: A chemical property is a property that can only be observed by changing the chemical identity of a substance.

• Examples: Flammability, reactivity with acids, oxidation states, ability to rust.

• Application: Chemical properties are important for predicting how substances will react with others.

Classification of Matter

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

• Pure Substance: Matter with a fixed composition and distinct properties. Subdivided into:

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

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

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

  • Homogeneous Mixture: Composition is uniform throughout (e.g., saltwater, air).

  • Heterogeneous Mixture: Composition is not uniform; different parts can be seen (e.g., salad, sand and iron filings).

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.

• Equation:

• Application: This law is used to balance chemical equations and to predict the amounts of products and reactants in a chemical reaction.

• 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.