Given below are two statements.
One is labelled as Assertion (A) and the other is labelled as Reason

Q: Why can't we just use carbon to reduce alumina like we do for iron?

Aluminium is much more reactive than iron and has a very high affinity for oxygen. Carbon is not a strong enough reducing agent to displace aluminium from oxygen at feasible temperatures.

Q: What exactly is cryolite and why is it so important?

Cryolite is a double salt with the formula $\text{Na}_3\text{AlF}_6$. It is crucial because pure alumina has a very high melting point and is a poor conductor. Cryolite lowers the melting point of the mixture and significantly increases its electrical conductivity.

Q: How do we find the oxidation state of Al in a complex molecule like cryolite?

The sum of the oxidation states of all atoms in a neutral molecule is zero. Knowing that Na is always $+1$ and F is always $-1$, you can set up the equation: $3(+1) + \text{Al} + 6(-1) = 0$ to find that Al is $+3$.

Q: If both statements are true, why isn't the Reason the correct explanation?

Causality is key. We use electrolysis because of aluminium's high reactivity, not because its oxidation state is $+3$. The oxidation state is just a factual property of the compound, not the metallurgical justification for the process.

Q: How to confidently solve Assertion-Reason questions?

First, evaluate the truth value of each statement independently. If both are true, read the Assertion, add the word 'because', and then read the Reason. If the combined sentence makes logical, scientific sense, then it is the correct explanation.

Question

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 $	ext{Al}_2	ext{O}_3$ 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.

Options:

(A) (A) is true but (R) is false

(B) (A) is false but (R) is true.

(C) Both (A) and (R) are true and (R) is the correct explanation of (A).

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

Sigma Percentile · Accepted Answer

The extraction of metals is one of the most fascinating applications of chemistry, bridging the gap between raw earth and the advanced materials that build our modern world. In this problem, we dive into the extraction of aluminium, a metal so reactive that it defied traditional smelting methods for decades until the invention of the Hall-Heroult process.

### The Hall-Heroult Process: A Metallurgical Marvel

Aluminium is the most abundant metal in the Earth's crust, primarily found in bauxite ore. However, extracting pure aluminium from its oxide, alumina ($	ext{Al}_2	ext{O}_3$), is no walk in the park. Because aluminium is highly reactive, it forms incredibly strong bonds with oxygen. Traditional carbon reduction, which works beautifully for iron, fails miserably here because aluminium has a higher affinity for oxygen than carbon does.

The solution? **Electrolysis**. But there's a catch. Pure alumina has a staggering melting point of over $2000^\circ	ext{C}$ and is a poor conductor of electricity in its molten state. Heating it to such extreme temperatures is economically unviable.

Enter **cryolite** ($	ext{Na}_3	ext{AlF}_6$). By mixing alumina with molten cryolite, the melting point of the mixture drops dramatically to around $900^\circ	ext{C}$, and the electrical conductivity skyrockets. This molten mixture is then electrolysed in a steel tank lined with carbon (which acts as the cathode), using carbon blocks as anodes. Pure molten aluminium collects at the bottom of the tank.

### Decoding the Assertion

Let's look at the Assertion (A): *Aluminium is extracted from bauxite by the electrolysis of a molten mixture of $	ext{Al}_2	ext{O}_3$ with cryolite.*

Based on our understanding of the Hall-Heroult process, this statement is a perfect summary of the industrial extraction of aluminium. Therefore, the Assertion is absolutely **True**.

### The Chemistry of Cryolite

Now, let's examine the Reason (R): *The oxidation state of Al in cryolite is $+3$.*

To verify this, we need to calculate the oxidation state of aluminium in the cryolite molecule, $	ext{Na}_3	ext{AlF}_6$. 
We know the standard oxidation states of the other elements:
- Sodium ($	ext{Na}$) is an alkali metal, so its oxidation state is always $+1$.
- Fluorine ($	ext{F}$) is the most electronegative element, so its oxidation state is always $-1$.

Let the oxidation state of aluminium be $x$. Since the molecule is neutral, the sum of all oxidation states must equal zero:
$$3(+1) + x + 6(-1) = 0$$
$$3 + x - 6 = 0$$
$$x - 3 = 0 \implies x = +3$$

The calculation confirms that the oxidation state of aluminium in cryolite is indeed $+3$. Therefore, the Reason is also **True**.

### The Final Verdict: Causality vs. Coincidence

Here is where many students fall into a trap. We have established that both the Assertion and the Reason are true statements. But does the Reason *explain* the Assertion?

Let's test the causality: *Aluminium is extracted by electrolysis BECAUSE the oxidation state of Al in cryolite is $+3$.*

Does that make sense? Not at all! The choice of electrolysis as the extraction method is dictated by the **high reactivity** of aluminium and its strong affinity for oxygen, which makes carbon reduction impossible. The oxidation state of aluminium being $+3$ is simply a chemical fact about the cryolite molecule; it is not the driving force behind the metallurgical process chosen.

Therefore, while both statements are factually correct, the Reason is **not** the correct explanation for the Assertion. The correct choice is option (d).

Select Answer:

Visualized Solution

$Hall-Heroult Process$

$Evaluating Assertion (A)$

$Oxidation State in Cryolite$

$Calculation of x$

$Conclusion$

$Key Takeaways$

The Sigma Insight: Principles of Metallurgy and Extraction

The Hall-Heroult Process

Decoding the Assertion

The Chemistry of Cryolite

The Final Verdict

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