The half-life of a radioisotope is the time required for its activity to reduce to: MCQ with Answer and Explanation

Explanation:
Amorphous solids like glass do not have a sharp melting point; they soften over a range of temperatures. Crystalline solids have sharp melting points and long-range order. Glass is an insulator.

Explanation:
Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂ (on boiling). The precipitate is calcium carbonate. Sulfates and chlorides cause permanent hardness and cannot be removed by boiling.

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.