Some Basic Concepts of Chemistry

Physical Properties

Physical properties are characteristics of a substance that can be measured or observed without changing its chemical composition. These properties help in identifying and describing substances.

• Definition: Properties that can be measured without altering the chemical identity of the substance.

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

Chemical Properties

Chemical properties describe the ability of a substance to undergo chemical changes and transform into different substances. These properties are observed during chemical reactions.

• Definition: Properties that can be evaluated only by changing the chemical composition of matter.

• Examples: Reactivity with acids, flammability, oxidation, corrosiveness.

Classification of Matter

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

• Pure Substances: Have a fixed composition and distinct properties. They can be further classified as:

  • Element: A substance made up of only one kind of atom. Example: Oxygen (O2)

  • Compound: A substance composed of two or more elements chemically combined in fixed proportions. Example: Water (H2O)

• Mixtures: Consist of two or more substances physically combined. They can be classified as:

  • Homogeneous Mixture: Has uniform composition throughout. Example: Salt solution

  • Heterogeneous Mixture: Has non-uniform composition. Example: Sand and iron filings

Law of Chemical Combination

Law of Conservation of Mass

The law of conservation of mass states that in any chemical reaction, the total mass of the reactants is always equal to the total mass of the products formed. This law is fundamental to chemical equations and stoichiometry.

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

• Mathematical Expression:

• Example: When hydrogen reacts with oxygen to form water: The mass of hydrogen and oxygen used equals the mass of water produced.