The diffusion of gases is explained by: MCQ with Answer and Explanation

Explanation:
Silver and gold are noble metals and occur in the free (native) state in nature because they are extremely unreactive (least reactive), not highly reactive. They do not readily combine with oxygen, moisture, or acids. Thus, A is true, but R is false.

Explanation:
Oxidation is loss of electrons. Na loses an electron to become Na⁺, so it is oxidized. Options A, C, D involve gain of electrons (reduction). Oxidation also can be defined as gain of oxygen or loss of hydrogen. In the reaction, Na acts as reducing agent. Mnemonic: LEO (Loss of Electrons is Oxidation) GER (Gain of Electrons is Reduction).

Explanation:
Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂ (on boiling). The precipitate is calcium carbonate. Sulfates and chlorides cause permanent hardness and cannot be removed by boiling.