Assertion (A): The mass of an atom is concentrated in the nucleus. Reason (R): Protons and neutrons are present inside the nucleus. MCQ with Answer and Explanation

Explanation:
Complete combustion of ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat. In sufficient oxygen, carbon burns to CO₂, hydrogen to H₂O. Incomplete combustion (lack of O₂) produces CO and soot (C). Ethanol is a biofuel, reducing CO₂ emissions if derived from biomass. It has high octane number and is blended with petrol (gasohol). CO₂ is a greenhouse gas, but bioethanol is carbon-neutral over cycle.

Explanation:
Carbon's ability to form bonds with other carbon atoms (catenation) and its tetravalency (four valence electrons) allow it to form long chains, branches, and rings, leading to millions of organic compounds. The C-C bond is strong (bond energy ~348 kJ/mol). Silicon also shows catenation but to much lesser extent. Carbon also forms multiple bonds, enhancing diversity.

Explanation:
Ozone (O₃) is a powerful disinfectant that decomposes to oxygen, leaving no toxic residues or taste. Chlorination can form carcinogenic trihalomethanes (THMs) with organic matter. However, ozone is more expensive, must be generated on-site, and leaves no residual protection in distribution system, often requiring a small chlorine dose. Ozonation also removes color and odor. Both kill bacteria and viruses.