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 substances.

• 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 only by changing the chemical composition of a substance.

• Examples: Reactivity with acids, flammability, oxidation states, and ability to rust or tarnish.

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: Substances with a fixed composition and distinct properties. They can be further divided into:

  • Elements: Substances that cannot be broken down into simpler substances by chemical means (e.g., hydrogen, oxygen).

  • Compounds: Substances composed of two or more elements chemically combined in a fixed ratio (e.g., water, sodium chloride).

• Mixtures: Combinations of two or more substances where each retains its own properties. Mixtures can be:

  • Homogeneous Mixtures: Also known as solutions; the composition is uniform throughout (e.g., saltwater, air).

  • Heterogeneous Mixtures: The composition is not uniform; different parts can be seen (e.g., sand in water, salad).

Summary: Pure substances have a fixed composition, while mixtures contain two or more substances physically combined.

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 equals the total mass of products.

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

• Mathematical Expression:

$$ \text{Total mass of reactants} = \text{Total mass of products} $$

• Example: When hydrogen reacts with oxygen to form water: $$ 2H_2 + O_2 \rightarrow 2H_2O $$ The combined mass of hydrogen and oxygen before the reaction 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.