JEE Main 2021 · 31 August · morning · Single correct · +4 / −1

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Aluminium is extracted from bauxite by the electrolysis of molten mixture of Al₂O₃ with cryolite.
Reason (R) : The oxidation state of Al in cryolite is +3.
In the light of the above statements, choose the most appropriate answer from the options given below :

  1. (a) (A) is true but (R) is false.
  2. (b) (A) is false but (R) is true.
  3. (c) Both (A) and (R) are correct and (R) is the correct explanation of (A)
  4. (d) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
Answer and solution

Answer (d) Both (A) and (R) are correct but (R) is not the correct explanation of (A)

Solution

Chemistry ma’am · 4 steps

Both statements are true, but the +3 state of Al in cryolite says nothing about why cryolite is added.

  1. Check the assertion. In the Hall–Héroult process, purified Al₂O₃ from bauxite is mixed with cryolite and electrolysed in the molten state. Aluminium collects at the cathode, so (A) is true.
  2. Check the reason. Cryolite is Na₃AlF₆. With Na at +1 and F at -1, 3(+1) + x + 6(-1) = 0 gives x = +3. So (R) is true.
  3. Why cryolite is used. Pure alumina melts at about 2323 K, which is far too hot to run a cell economically. Cryolite lowers the melting point of the mixture to about 1140 K and makes the melt conduct electricity better. The oxidation state of Al in cryolite plays no part in either effect.
  4. So. (A) and (R) are both correct, but (R) does not explain (A). The answer is (d).

The idea to keep. In assertion–reason questions, judge each statement alone, then ask if the reason is the real cause. Cryolite lowers the melting point and raises conductivity; oxidation state has nothing to do with it.

Answer from the NTA final answer key. Solution by PrepWiser.

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Asked about 80 times in JEE Main since 2020. This question is from 2021.