Hydrogen: JEE Advanced PYQ, 2007–2026

Answer: (A)

Hydrogen peroxide is unstable. Light, heat and metal ions such as iron or copper catalyse its decomposition into water and oxygen. It is therefore stored in dark coloured bottles, in cool conditions, and often with a stabilising agent. Associate H₂O₂ storage with protection from light, heat and metal contamination.

Answer: (D)

Dihydrogen is liberated from alkalis only by metals whose oxides and hydroxides are amphoteric. Zn + 2NaOH → Na₂ZnO₂ + H₂. Aluminium behaves similarly, forming an aluminate and H₂. Mg, Fe, Cu and Ag are not amphoteric and do not react with aqueous NaOH.

Answer: (A)

Saline hydrides such as CaH₂ are ionic solids containing the hydride ion H⁻. When molten they conduct electricity. At the anode (positive electrode) the anion is oxidised: 2H⁻ → H₂ + 2e⁻, so H₂ is evolved there. At the cathode, Ca²⁺ + 2e⁻ → Ca. The evolution of H₂ at the anode is the classic evidence for the H⁻ ion.

Answer: (C)

Hydrogen bonding between water molecules is directional and creates a tetrahedral arrangement in ice. This arrangement leaves open spaces, so ice occupies more volume per mole than liquid water and has a lower density. On melting, some of the hydrogen-bonded lattice collapses, so water molecules pack more closely and the liquid is denser. This is why ice floats.

Answer: (B)

Saline hydrides such as CaH₂ contain H⁻ ions, and H⁻ reacts with water to give H₂. Molar mass of CaH₂ = 40 + 2 = 42 g/mol. Moles of CaH₂ = 4.2/42 = 0.1 mol. From the equation, 1 mol of CaH₂ gives 2 mol of H₂, so 0.2 mol of H₂ is formed. Volume = 0.2 × 22.4 = 4.48 L.

Answer: (D)

'5.6 volume' means 1 L of the solution releases 5.6 L of O₂ at STP. Moles of O₂ = 5.6/22.4 = 0.25 mol. From 2H₂O₂ → 2H₂O + O₂, moles of H₂O₂ = 2 × 0.25 = 0.5 mol in 1 L, so the molarity is 0.5 M. As a check, 11.2 L of O₂ comes from 1 mol of H₂O₂, and 5.6/11.2 = 0.5.

Answer: (A)

At red heat, steam reacts with carbon in coke: C + H2O → CO + H2. The product mixture is called water gas, consisting mainly of carbon monoxide and dihydrogen. It is a fuel and an important source of H2 for synthesis.

Answer: (A)

Count the valence electrons against the bonds needed. In B₂H₆, boron has 3 valence electrons and cannot complete normal bonds to all its neighbours, so it is electron-deficient and uses banana (3c-2e) bonds. In CH₄, carbon's 4 electrons and 4 hydrogens give exactly four 2c-2e bonds, so it is electron-precise. In NH₃, nitrogen's 5 valence electrons give three bonds and leave one lone pair, so it is electron-rich. The order is therefore electron-deficient, electron-precise, electron-rich.

Answer: (B)

H₂O₂ acts as a reducing agent when oxygen in it is oxidised from –1 to 0, giving O₂. This happens with strong oxidants such as acidified KMnO₄, and in basic medium with I₂ or similar. In the Fe²⁺, Mn²⁺ and PbS reactions the oxygen goes from –1 to –2, so H₂O₂ is the oxidising agent. Only the permanganate reaction releases O₂ from H₂O₂ itself.

Answer: (C)

Hydrogen has three isotopes: protium (¹H, no neutron), deuterium (²H, one neutron) and tritium (³H, two neutrons). Protium and deuterium are stable. Tritium is radioactive, emitting low-energy beta particles with a half-life of about 12.3 years. Recall the isotope name and its neutron count to answer.