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 in a non-destructive manner.

• Definition: Properties that can be measured without altering the chemical identity of the substance.

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

• Application: Determining the density of a liquid by measuring its mass and volume.

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform it into different substances. These properties are only observed during a chemical reaction.

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

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

• Application: Iron's tendency to rust when exposed to oxygen and moisture is a chemical property.

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: Uniform composition throughout (also called solutions; e.g., salt water, air).

  • Heterogeneous Mixtures: Non-uniform composition; different parts can be seen (e.g., sand in water, salad).

Summary: Pure substances have constant composition, while mixtures can vary in composition and can be separated by physical means.

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 the active reactants is always equal to the mass of the products formed.

• Mathematical Expression:

• Example: When hydrogen gas reacts with oxygen gas to form water, 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, ensuring that the number of atoms of each element is the same on both sides of the equation.