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: 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 (e.g., hydrogen, oxygen).

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

• Mixtures: Consist of two or more substances physically combined. They can be separated by physical means and are classified as:

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

  • Heterogeneous Mixtures: Do not have uniform composition; different components are visible (e.g., sand in water, salad).

This table summarizes the classification of matter into pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous).

Law of Chemical Combination

The laws of chemical combination describe the fundamental principles governing how substances combine to form new compounds. One of the most important is the Law of Conservation of Mass.

Law of Conservation of Mass

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

• Implication: Mass is neither created nor destroyed during a chemical reaction; it is conserved.

• 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: Other laws of chemical combination include the Law of Definite Proportions and the Law of Multiple Proportions, which further describe how elements combine in fixed ratios to form compounds.