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: 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, color, and state of matter.

Chemical Properties

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

• Definition: Properties that can only be observed or measured by changing the chemical identity of a substance.

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

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., O2, Fe).

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

• Mixtures: Combinations of two or more substances that retain their individual properties. Mixtures can be:

  • Homogeneous Mixtures (Solutions): Have uniform composition throughout (e.g., salt water, air).

  • Heterogeneous Mixtures: Do not have uniform composition; different parts can be seen (e.g., sand in water, salad).

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

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.

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

• Mathematical Expression:

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

• 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.