Which of the following reactions is an oxidation-reduction reaction?
A. H₂ + Cl₂ → 2HCl
B. CaCO₃ → CaO + CO₂
C. NaOH + HCl → NaCl + H₂O
D. NaCl + AgNO₃ → AgCl + NaNO₃
Answer: Option A
Solution (By JKExamLibrary)
In H₂ + Cl₂ → 2HCl, hydrogen is oxidized (0 to +1) and chlorine is reduced (0 to -1). Thus it's a redox reaction. The others: A and C are double displacement with no oxidation state change; D is decomposition with no change (Ca +2, C +4, O -2 throughout).
Explanation:
A solution turning red litmus blue is basic, so pH > 7. Among options, pH 9 is basic. pH 2 and 5 are acidic (blue litmus turns red). pH 7 is neutral. Litmus is a natural indicator extracted from lichens. Red litmus in base remains blue, blue litmus in acid turns red. Practical: Phenolphthalein gives pink in basic solution (pH > 8.3).
Explanation:
Hydrochloric acid (HCl) is commonly used for pickling (cleaning) steel to remove rust and scale (iron oxides) before further processing like galvanizing or plating. Reaction: Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O. Sulfuric acid is also used. Nitric acid is an oxidizer and would passivate some metals. Acetic acid is too weak.
Explanation:
Mendeleev used atomic mass as basis. Anomalies like Ar (mass 40) placed before K (mass 39) occurred. These were resolved when Moseley proposed atomic number as the basis.
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