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.

• Definition: A physical property is any property that can be measured without altering the identity or composition of the substance.

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

• Application: Measuring the density of water does not change its chemical structure.

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform it into different substances.

• Definition: A chemical property is a property that can only be observed by changing the chemical identity of a substance.

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

• Application: The ability of iron to rust when exposed to oxygen is a chemical property.

Classification of Matter

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

• 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., hydrogen, oxygen).

  • Compounds: Substances composed of two or more elements chemically combined in fixed proportions (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).

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

Diagram Description: A flowchart showing the classification of matter: Matter → Pure Substance (Element, Compound) and Mixture (Homogeneous, Heterogeneous).

Law of Chemical Combination

The laws of chemical combination describe the fundamental principles governing how substances combine to form new compounds.

Law of Conservation of Mass

This law states that mass is neither created nor destroyed in a chemical reaction.

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

• Mathematical Expression:

• Example: When hydrogen reacts with oxygen to form water, 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 in stoichiometry and is used to balance chemical equations.