RC CAT 6 - "What can be most logically inferred... iron?"

[gtckim]

Hi - this is a RC question from MGMAT CAT 6. Here is the question, with the relevant paragraph:

"The most common metal in the Earth’s crust, aluminum (or aluminium) was not discovered until 1825 because its isolated state is so reactive that free nuggets or flakes of the metal are never found in nature; rather, the metal is typically found as part of an amalgam, most commonly bauxite ore. Moreover, elemental aluminum is extremely difficult—and expensive—to separate from its ores by traditional chemical means. Indeed, the extreme reactivity of aluminum helps protect its modern, ubiquitous manifestations, such as aluminum foil. The surface of pure aluminum instantly combines with atmospheric oxygen to form a thin but robust “passivization” seal of aluminum oxide that prevents further corrosion. Many other metals, such as iron, are less reactive than aluminum, but their superficial oxides do not form as swiftly, completely, or impermeably."

What can be most logically inferred from the passage about iron?
A) It corrodes more quickly than aluminum.
B) Its oxides form more slowly and robustly than those of aluminum.
C) It is cheaper to isolate from its ores by traditional chemical means than aluminum.
D) It is more susceptible to passivization than is aluminum.
E) It is more commonly found in its isolated, elemental state.

I understand the correct answer choice (A), but I am having a hard time explaining why answer choice (E) is invalid. The passage begins by saying that aluminum is rarely found in its isolated state due to its reactivity. Then the paragraph explains that its reactivity actually gives the aluminum protective properties, which apparently protect it from corrosion. There is a logic gap here that I am having difficulty resolving: if pure aluminum is ultimately protected by the consequences of its reactivity (i.e., "passivization" seal), then why is independent aluminum so rare in nature? If such protective barriers protect it from corrosion, wouldn't aluminum be more prevalent in its pure form in nature? I'm not trying to argue the facts and science behind this, I understand this information is part of the premise. However, I'd appreciate some input on the gap I described above.

I chose answer choice (E) initially because the passage clearly states that aluminum is never found in its isolated state due to its reactivity. Additionally, it states iron is less reactive. I used these two points to draw the conclusion that iron must be more commonly found in its isolated state. Am I thinking too much into this? Is (E) eliminated simply because the passage makes no statements regarding iron's state in nature (i.e., out of scope answer choice)?

Thank you in advance!

[tim]

If you read the explanation given in the test, it should answer most of your questions. The problem is that you're assuming if aluminum is never found in its elemental state than iron is more frequently found. You didn't consider the possibility that perhaps iron is never found in its elemental state either.

[sahilmalhotra01]

Hi Expert,

I have a doubt regarding the option choice A.

Option choice A states that "Iron corrodes more quickly than Aluminum"

There are two aspects to be considered

[list=]speed of corrosion[/list]

[list=]amount of of corrosion[/list]

Speed of corrosion is the speed with which oxides are formed.

but their superficial oxides do not form as swiftly

Oxides of Aluminum form more swiftly than oxides of other metals.

Amount of corrosion

Since Aluminum form impermeable layers, it corrodes less compared with Iron.

So the option choice would have been correct if it says It corrodes more than aluminum

The word quickly talk about the rate of corrosion. The rate of corrosion of Aluminum is more than rate of corrosion of Iron.

Kindly help me understand how the official answer is correct.

Thanks

[Sage Pearce-Higgins]

Good detailed thinking! I agree that there's a little vagueness in this question. We are certainly being asked about the speed of corrosion and you're right that aluminium oxidizes more quickly than iron. However, this constitutes only a tiny bit of corrosion (a "thin" seal). However, after that, aluminium does not corrode at all ("prevents further corrosion"), therefore its speed of corrosion is 0. Further, the text states that the "extreme reactivity of aluminum helps protect" it and that it has a "robust" seal. If we understand the meaning of 'corrode' as 'damage' (https://en.oxforddictionaries.com/definition/corrode), then we can see that iron is more easily damaged than aluminium.

[JbhB682]

Why is option (B) wrong ?

[Sage Pearce-Higgins]

Here's the explanation that we provide - the issue is about the word 'robust'. Please let me know if this doesn't clear things up.

(B) We can indeed conclude that iron oxides form more slowly (“not as swiftly”), but we cannot conclude that iron oxides form more robustly than aluminum oxides. In fact, we are told that aluminum oxide forms a “thin but robust… seal,” while iron oxides, among others, “do not form as… completely, or impermeably.” If the iron oxides are less complete and less impermeable, then we cannot infer that they are more robust; if anything, we should assume that they are less robust.