Some Basic Concepts of Chemistry

Physical Properties

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

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

• Examples: Mass, volume, density, melting point, boiling point, color, and state of matter.

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform it into different substances. These properties are observed only during a chemical reaction.

• Definition: Properties that can be evaluated at the cost of matter itself, i.e., the substance is changed into a new substance.

• Examples: Flammability, reactivity with acids or bases, oxidation states, and toxicity.

Classification of Matter

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

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

  • Elements: Substances that cannot be broken down into simpler substances by chemical means. Examples: Hydrogen, Oxygen, Gold.

  • Compounds: Substances composed of two or more elements chemically combined in a fixed ratio. Examples: Water (H2O), Sodium chloride (NaCl).

• Mixtures: Combinations of two or more substances that retain their individual properties and can be separated by physical means. Mixtures are classified as:

  • Homogeneous Mixtures: Have uniform composition throughout. Examples: Salt solution, air.

  • Heterogeneous Mixtures: Do not have uniform composition. Examples: Sand in water, salad.

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. The total mass of reactants is always equal to the total mass of products formed.

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

Additional info: The law of conservation of mass is foundational for balancing chemical equations and understanding stoichiometry in chemical reactions.