CDS (II) 2023 General Knowledge · Question 10 of 120

Correct — C, 3Fe + 4H₂O → Fe₃O₄ + 4H₂. A balanced equation must show the same number of atoms of every element on both sides, because matter is neither created nor destroyed in a chemical change. Count option (c): on the left, three iron atoms, eight hydrogen atoms and four oxygen atoms; on the right, Fe₃O₄ supplies three iron and four oxygen, and 4H₂ supplies eight hydrogen. Every column matches, so option (c) is balanced. The reaction itself is the one that happens when steam is passed over red-hot iron: the iron is oxidised to the black magnetic oxide Fe₃O₄ and hydrogen gas is given off. Note that only the coefficients in front of the formulae may be changed while balancing; the formulae themselves are fixed by the compounds, so four molecules of water must be written as 4H₂O and never as H₂O₄.

• (a)Fe + 4H₂O → Fe₃O₄ + H₂ — Only the oxygen has been dealt with. Iron stands at one on the left against three on the right, and hydrogen at eight against two. This is the half-finished line a student reaches after the first step of balancing and then mistakes for the answer.
• (b)3Fe + H₂O → Fe₃O₄ + 2H₂ — Iron balances at three on each side, but nothing else does. There is one oxygen on the left against four on the right, and two hydrogen atoms on the left against four on the right.
• (d)3Fe + 4H₂O → Fe₃O₄ + H₂ — The closest miss, and the one worth checking carefully. Iron and oxygen both balance at three and four, but the hydrogen does not — eight atoms enter in the four water molecules and only two leave in the single hydrogen molecule.

Balancing rests on the law of conservation of mass: in a chemical reaction the total mass of the products equals the total mass of the reactants, so atoms are only rearranged, never made or destroyed. The usual classroom procedure is hit-and-trial — start with the compound carrying the most atoms, balance the element that appears most often in it, work through the remaining elements one at a time, and finish by recounting every element on both sides. Only coefficients may be altered; changing a subscript would change the substance itself.

This item is a textbook worked example reproduced almost exactly, and its distractors are the intermediate lines of that worked example, which is why they look so plausible. The chapter balances Fe + H₂O → Fe₃O₄ + H₂ step by step: first the oxygen, giving Fe + 4H₂O → Fe₃O₄ + H₂; then the hydrogen, giving Fe + 4H₂O → Fe₃O₄ + 4H₂; and only then the iron, giving the finished 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Options (a) and (d) are two of those unfinished stages. The lesson is procedural rather than chemical: never stop balancing when one element has come right, and always end by recounting all three columns. Doing that here takes about fifteen seconds and cannot go wrong.

• A balanced equation has equal numbers of atoms of each element on both sides, as required by the law of conservation of mass.
• 3Fe + 4H₂O → Fe₃O₄ + 4H₂ is the reaction of red-hot iron with steam; Fe₃O₄ is magnetite, the black magnetic oxide of iron.
• While balancing, only the coefficients before a formula may be changed — never the subscripts inside it.
• The hit-and-trial method starts from the compound with the largest number of atoms and finishes with a recount of every element.
• State symbols — (s), (l), (g) and (aq) — are added after balancing and are not part of the balancing itself.

• Stopping as soon as one element balances; the distractors here are exactly those half-finished lines.
• Changing a subscript instead of a coefficient, so that 4H₂O becomes H₂O₄ and the substance changes.
• Assuming the reaction of iron with steam gives Fe₂O₃; the product is Fe₃O₄, and the difference decides the whole balance.

As a pick-the-balanced-equation item, as a fill-in-the-coefficient blank, or as a statement about what balancing conserves.

• practice — not a real PYQ

  The coefficients that balance the equation ___Al + ___O₂ → ___Al₂O₃ are respectively

  • (a)2, 3, 2
  • (b)4, 3, 2
  • (c)2, 3, 1
  • (d)4, 6, 2

  Answer(b) 4, 3, 2 — four aluminium atoms and six oxygen atoms on the left, and 2Al₂O₃ supplies exactly four aluminium and six oxygen on the right.

• practice — not a real PYQ

  A chemical equation is balanced in order to satisfy which one of the following laws?

  • (a)Law of constant proportions
  • (b)Law of conservation of mass
  • (c)Law of multiple proportions
  • (d)Avogadro's law

  Answer(b) Law of conservation of mass — atoms are only rearranged in a chemical change, so the same number of each kind must appear on both sides of the equation.