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Chemistry

Assertion (A): Hydrogen gas is liberated when metals like Zn, Al, Pb react with caustic alkalies.

Reason (R): Alkalies are soluble in water.

  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false but R is true.

Analytical Chemistry

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Answer

Both A and R are true but R is not the correct explanation of A.

Explanation — Certain metals like zinc, aluminium and lead react with hot concentrated caustic alkalis (NaOH, KOH) to give the corresponding soluble salt and liberate hydrogen.

Zn+2NaOHhot and conc.⟶Na2ZnO2sodium zincatecolourless+H2↑\text{Zn} + \underset{\text{hot and conc.}}{2\text{NaOH}} \longrightarrow \underset{\underset{\text{colourless}}{\text{sodium zincate}}}{\text{Na}2\text{ZnO}2} + \text{H}_2↑

Zn+2KOHhot and conc.⟶K2ZnO2potassium zincatecolourless+H2↑\text{Zn} + \underset{\text{hot and conc.}}{2\text{KOH}} \longrightarrow \underset{\underset{\text{colourless}}{\text{potassium zincate}}}{\text{K}2\text{ZnO}2} + \text{H}_2↑

Hence the assertion (A) is true.

Alkalis are bases that are soluble in water. When dissolved in water, alkalis release hydroxide ions (OH⁻), making the solution basic (pH > 7). Hence reason (R) is true but it does not support assertion (A).

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