Which of the following is an allotropic form of carbon used in lead pencils? MCQ with Answer and Explanation

Explanation:
Boyle's law: P₁V₁ = P₂V₂. Given P₁=1.0 atm, V₁=2.0 L, P₂=4.0 atm. V₂ = (1.0 × 2.0) / 4.0 = 0.5 L. Volume decreases as pressure increases, because gas is compressible. The product PV is constant for a fixed mass of gas at constant T. Scuba divers experience this; as they dive deeper, pressure increases, reducing air volume in lungs.

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.

Explanation:
Reactivity series orders metals according to their ease of oxidation—i.e., losing electrons to form cations. At the top (K, Na) are strong reducing agents, reacting vigorously with water and acids; at the bottom (Au, Pt) are noble metals, resistant to corrosion. The series helps predict displacement reactions and extraction methods. Mnemonic: 'Please Stop Calling Me A Cute Zebra...' etc.