According to the passage, the natural passivization layer on

[sook65]

This is question is from the 5th CAT exam (part of which pasted below). Why would D be the correct answer? It seems like for D to be correct, one would have to make the assumption that the result of passivization (a swift, impermeable, seal) are a "chemical advantage," when nothing in the passage indicates that is necessarily the case. In fact, the same paragraph in the passage explains that aluminum's properties make it "difficult" and "expensive" to separate by "traditional chemical means." This seems to suggest that option E is be more correct, which talks directly about how traditional chemical processes are not inexpensive.

Any thoughts would be much appreciated.


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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.

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According to the passage, the natural passivization layer on the surface of aluminum metal

A. causes atmospheric oxygen to react chemically with the metal

B. is less impenetrable than typical iron oxide films formed on pure iron

C. lowers the utility of aluminum in its uncontaminated state

D. provides aluminum with a chemical advantage, relative to other metals

E. precludes the inexpensive purification of the metal by traditional chemical processes

[jlucero]

First off, E has a causation problem. I agree with your statement: aluminum's properties make it "difficult" and "expensive" to separate by "traditional chemical means." But passivization isn't a property of aluminum, and there's no connection saying that passivization is what makes separating aluminum difficult and expensive.

That leaves us with D, the best available answer choice. With the tone of the argument being positive for aluminum (the extreme reactivity of aluminum helps protect its modern, ubiquitous manifestations, such as aluminum foil) and talks about the differences, both positive and negative, of other metals (many other metals, such as iron, are less reactive than aluminum, but their superficial oxides do not form as swiftly, completely, or impermeably), it's not a stretch to say that aluminum has an advantage and a disadvantage.

[JbhB682]

Please confirm if B is wrong for the following reason

-- Per the passage (marked in red), oxide of aluminum prevents further corrosion
-- Per the passage (green), aluminium oxide is much faster formed compared to iron oxide

Hence corrosion in general on iron happens faster because of the rate of oxide formation is slower in iron

But that doesn't mean necessarily the oxides of iron vs. the oxide of aluminium have different penetrability rates



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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.

For several decades after its discovery, aluminum was considered a precious metal and was more costly than gold or platinum, not because of any fundamental scarcity, but because of its elevated cost of production. The price of aluminum suddenly plummeted in 1886, however, when two 23-year-old inventors independently developed an electrolytic process of separating pure aluminum from a bath of molten aluminum salts, primarily cryolite. Cryolite itself is rare enough that synthetic salts eventually replaced it as the solution medium.

[JbhB682]

Hi - on D specifically

Isnt the word (Chemical advantage) too general ?

Chemical advantage can technically be applied to anything ....there may be other chemical properties not discussed in the passage at all

I thought this answer choice meant "looking at all chemical properties" ....Aluminum s more advantageous

Please tell me why is my thinking wrong

[Sage Pearce-Higgins]

Sure, there may be other chemical advantages (or disadvantages), but we're only interested in the ones stated in the passage. Be careful that you're actually answering the 'according to the passage...' question, and not a general question about aluminium.

We're told that "passivization" 'prevents further corrosion' of aluminium, and that other metals don't have this property. This constitutes a chemical advantage.

[JbhB682]

Hi Sage - could you eliminate D because of the following logic

Background :

a) Passage states, separation on Amalgam (a.k.a bauxite ore) can occur
b) Passage states, this separation is very expensive
c) The passivization layer is formed NOT on Amalgam but on pure Aluminium
d) Once the Pure Aluminium element reacts with oxygen, then only does this passivization layer form

Hence the passage does not even discuss anything about passivization layer being the problem during separation

The separation process occurs on Amalgams (where the passivization layer may or may not be formed at all)

Please let me know if my logic is accurate on this

Thank you

[Sage Pearce-Higgins]

No need to repost the problem and someone else's question.

I'm having trouble understanding your question: are you discussing answer D or some different answer choice?

[JbhB682]

Sorry i meant E - Can i eliminate E in the question because per the passage

The separation process occurs on Amalgams (where the passivization layer may or may not be formed at all)
Passivization per the passage has to do with Aluminium oxide and nothing to do with Amalgam

[Sage Pearce-Higgins]

Please stop reposting the message above - it simply clutters the forums.

Yes, that logic works. The passage makes no connection between the 'passivization layer' and the separation process. Hence we can eliminate answer E.