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 during chemical reactions.

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

• Examples: Reactivity with acids, flammability, 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 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 a fixed ratio (e.g., H2O, NaCl).

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

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

  • Heterogeneous Mixtures: Do not have uniform composition (e.g., sand in water, salad).

Summary: Pure substances have constant composition, while mixtures can vary in composition and properties.

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.

Application: This law is the basis for balancing chemical equations and is essential in quantitative chemical analysis.