Mendeleev's periodic table left a gap for an element he called eka-aluminium. This element is now known as: MCQ with Answer and Explanation

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.

Explanation:
Graphite is used in electrodes (due to electrical conductivity) and as a moderator in nuclear reactors (to slow down neutrons). It is chemically stable and can withstand high temperatures. Diamond is an insulator. Fullerene is a nanomaterial. Charcoal is used as adsorbent and fuel. Graphite blocks are used in RBMK reactors (like Chernobyl) and in advanced gas-cooled reactors.

Explanation:
H₂SO₄ is used in production of fertilizers, detergents, dyes, explosives, petroleum refining, and many other chemicals. Its wide industrial use earns it the title. It's not the strongest (e.g., HClO₄ is stronger).