$Cu$ and $HNO_{3}$ are not suitable for preparing hydrogen gas because of their

Analyze the question: We want to know why copper (Cu) and nitric acid (HNO₃) are a poor choice of reactants for making hydrogen gas in the laboratory.

Concept: Hydrogen is prepared by reacting a metal with a dilute acid. The metal must be above hydrogen in the reactivity series so that it can displace H₂ from the acid.

• Copper is below hydrogen in the reactivity series (K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au). It is not reactive enough to displace hydrogen from dilute acids.
• Nitric acid is a strong oxidizing agent. When it reacts with metals, the nitrate ion is reduced instead of H⁺. This gives nitrogen oxides (NO, NO₂) and water, not hydrogen. For example:

Cu + 4HNO₃(conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O

3Cu + 8HNO₃(dilute) → 3Cu(NO₃)₂ + 2NO + 4H₂O

Any hydrogen that forms is oxidized to water by the acid.

Checking the options:

• A. reactivity and oxidation respectively: Cu has low reactivity, and HNO₃ is an oxidizing agent. This fits.
• B. Conductivity and corrosiveness do not explain why no hydrogen forms.
• C. Melting point is irrelevant, and HNO₃ acts as an oxidizing agent, not a reducing one.
• D. Electronegativity and solubility are not the reasons.

Answer: A. Cu is too unreactive (it lies below hydrogen in the reactivity series), and HNO₃ is a strong oxidizing agent that produces nitrogen oxides instead of hydrogen.

Exam tip: For hydrogen preparation, remember "a metal above H plus a dilute non-oxidizing acid (HCl or H₂SO₄)". Zinc with dilute HCl is the standard choice. Whenever you see HNO₃, think "oxidizing agent, no hydrogen".