Which of the following is an example of a decomposition reaction? MCQ with Answer and Explanation

Explanation:
All matter is composed of tiny particles (atoms, molecules, or ions). This is a fundamental concept. Matter may not always be visible (e.g., air). It can change states (solid, liquid, gas). Gases have mass; for example, a filled gas cylinder weighs more than an empty one. The particulate nature of matter explains diffusion, Brownian motion, etc.

Explanation:
Dmitri Mendeleev proposed the periodic law based on atomic mass. However, anomalies arose. In 1913, Henry Moseley discovered that the atomic number (number of protons) is the fundamental property. He proposed the modern periodic law: the properties of elements are periodic functions of their atomic numbers.

Explanation:
Combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O + heat) is an exothermic reaction, releasing heat to surroundings. Melting, evaporation, and sublimation are endothermic phase changes requiring heat absorption to overcome intermolecular forces. Exothermic reactions: burning, respiration, neutralization (usually), condensation, freezing. Endothermic: photosynthesis, cooking, melting, boiling.