When iron is exposed to moist air, it gradually rusts. This is due to the formation of?
hydrate iron (III) oxide
anhydrous iron (III) oxide
anhydrous iron (II) oxide
hydrate iron (II) oxide
Explanation
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The question says: “When iron is exposed to moist air, it gradually rusts. This is due to the formation of …”
Let’s start by understanding what’s happening here.
Iron rusts. That much you might have seen. Like, if you leave a nail or an old spoon outside and it gets reddish-brown flaky stuff on it, that is rust. Rust is just a chemical reaction. But what kind of chemical reaction?
Rust happens when iron reacts with oxygen in the presence of water. You need both. If it’s dry air with no water, iron barely rusts. If it’s water without air, iron corrodes very slowly. So moisture is essential.
Now, the rust that forms is not just pure iron oxide. It’s a special kind of iron oxide that has water molecules incorporated into it. Chemists call this a hydrate. Hydrate just means water molecules are part of the solid compound.
That’s why you see the word “hydrate” in some of the options.
Next, let’s look at the Roman numerals. This always confuses people, but it’s not hard.
Iron can exist in two main forms in oxides: Fe²⁺ (iron II) or Fe³⁺ (iron III).
Fe²⁺ is iron with a +2 charge, Fe³⁺ is iron with a +3 charge.
Rust is Fe³⁺, not Fe²⁺. So anything that says iron (II) oxide is wrong. That immediately eliminates option C and D. Don’t even think about them. They are iron (II) oxides, which are not rust.
Now we are left with:
Option A: hydrated iron (III) oxide
Option B: anhydrous iron (III) oxide
What’s the difference?
Hydrated means it contains water molecules. Rust forms in moist air, so it definitely contains water. This is the one we want.
Anhydrous means it has no water. If iron formed iron (III) oxide without water, it would just be a dry powder. That’s not what rust looks like. Rust is flaky, soft, and reddish-brown, and that is because of the water inside it.
So the correct answer is A) hydrated iron (III) oxide.
Let’s quickly break down why the others are wrong:
B) anhydrous iron (III) oxide – This is just dry Fe₂O₃. This is what you get if you heat iron strongly in oxygen. It’s not rust. Rust needs water, so this is wrong.
C) anhydrous iron (II) oxide – FeO without water. Wrong because it’s Fe²⁺, not Fe³⁺, and it’s anhydrous. Not rust.
D) hydrated iron (II) oxide – Fe²⁺ with water. Still wrong because rust is Fe³⁺.
Here’s an easy way to remember it for JAMB:
Rust = reddish-brown flaky thing you see on wet iron = hydrate + iron(III) oxide.
A little story might help it stick: imagine leaving a nail outside when it rains. Slowly it gets reddish and flaky. Those flakes are basically iron combining with oxygen and water from the air. That’s hydrated iron (III) oxide.
Yes formation of Hydrated iron III oxide
Written as Fe2O3 Or still Fe2O3.3H2O ( showing it is hydrated)
