Basic Concepts of Chemistry: Properties, Classification of Matter, and Laws of Chemical Combination

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: Properties such as mass, volume, and density that do not alter the identity of the substance when measured.

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

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform its chemical identity.

• Definition: Properties that can only be observed during a chemical reaction, resulting in the formation of new substances.

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

Classification of Matter

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

• Pure Substance: Has a fixed composition and distinct properties. Can be further classified as:

  • Element: A substance made of only one kind of atom (e.g., O2, Fe).

  • Compound: A substance composed of two or more elements chemically combined in fixed proportions (e.g., H2O, NaCl).

• Mixture: Consists of two or more substances physically combined. Can be further classified as:

  • Homogeneous Mixture: Has uniform composition throughout (e.g., salt water, air).

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

Type	Subcategories	Examples
Pure Substance	Element, Compound	O2, H2O
Mixture	Homogeneous, Heterogeneous	Air, Salad

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.

• Statement: Mass is neither created nor destroyed during a chemical reaction.

• Equation:

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

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