The half-life of a radioactive element is the time required for: MCQ with Answer and Explanation

Explanation:
Chlorination is the disinfection of water using chlorine or chlorine compounds to destroy pathogenic microorganisms. Chlorine reacts with water forming hypochlorous acid (HOCl) which is a strong germicide. Ozonation uses ozone; distillation involves boiling and condensation. Filtration removes particulate matter but not all microorganisms. Chlorination is cost-effective and provides residual protection, though it may produce trihalomethanes as by-products.

Explanation:
Reduction is gain of electrons or loss of oxygen. Here, CuO loses oxygen to form Cu, so CuO is reduced. H₂ gains oxygen to form H₂O, so H₂ is oxidized. This is a redox reaction. Oxidation and reduction occur simultaneously. Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons). In terms of oxygen, loss of O = reduction, gain of O = oxidation.

Explanation:
Dmitri Mendeleev formulated the Periodic Law based on atomic masses and left gaps for undiscovered elements. Moseley later modified it to atomic number. Newlands gave Law of Octaves. Lavoisier classified elements into metals and non-metals.