Isotopes are used in which of the following applications? MCQ with Answer and Explanation

Explanation:
Calorific value is the energy released per unit mass or volume of fuel. Common units: kJ/kg, kJ/g, J/g, kcal/kg, Btu/lb. It measures fuel efficiency. Hydrogen has about 141.9 MJ/kg (higher), petrol ~47 MJ/kg. Temperature (°C) is not a unit for calorific value. Pressure and concentration are not appropriate.

Explanation:
J.J. Thomson discovered the electron through cathode ray tube experiments in 1897 and measured the e/m ratio by applying electric and magnetic fields. Rutherford discovered the nucleus via the gold foil experiment. Chadwick discovered the neutron (1932). Bohr proposed the planetary model of the atom with quantized orbits. Thomson's model (plum pudding) was later superseded. Exam memory: Thomson = electron (e/m); Rutherford = nucleus; Chadwick = neutron; Bohr = energy levels.

Explanation:
Hard water contains dissolved Ca²⁺ and Mg²⁺ ions which react with soap to form insoluble scum, preventing lather. Soft water lathers readily. Distilled water is pure (soft). Rainwater is naturally soft but may dissolve CO₂ and become slightly acidic.