Sodium carbonate is a salt of: MCQ with Answer and Explanation

Explanation:
LPG (propane/butane) is odorless. A small amount of strong-smelling mercaptan (ethanethiol) is added to help detect gas leaks, preventing accidents. Natural gas also has odorant. Mercaptans contain sulfur, and their smell is detectable at very low concentrations. Calorific value, temperature, performance are unaffected. Safety is primary.

Explanation:
Robert Millikan, along with Harvey Fletcher, performed the famous oil drop experiment in 1909. By observing the motion of tiny charged oil droplets in an electric field, Millikan accurately determined the elementary charge of a single electron to be approximately 1.592 × 10^-19 coulombs (later refined to 1.602 × 10^-19 C).

Explanation:
Rhizobium bacteria form root nodules in leguminous plants (peas, beans, clover) and fix atmospheric N₂ into ammonia, providing nitrogen to the plant, while the plant supplies carbohydrates. Azotobacter and Azospirillum are free-living nitrogen fixers. Pseudomonas is a general soil bacterium. Symbiotic association is mutualistic. Inoculating seeds with Rhizobium culture increases soil fertility.