Q.Match the compounds/elements given in Column I with uses given in Column II.
Column I (Compound/element):

The key idea is Alloy Composition — recognising the specific industrial roles of lanthanoids and their alloys.

Reasoning:

1. Lanthanoid oxide (e.g., $\ce{Eu2O3}$, $\ce{Y2O3}$) acts as a phosphor and is used in television screens → (b).
2. Lanthanoid (the pure metal, e.g. Ce/La) is added directly to molten iron/steel as a deoxidiser and desulphuriser → used in production of iron alloys → (a).
3. Misch metal is, by definition, the alloy of a lanthanoid metal (~95%) with iron (~5%) → its composition is exactly (d) Lanthanoid metal + iron. …

Why this approach works

Lanthanoids (the 15 elements from La to Lu) are not just a list of obscure metals — they have distinct chemical and physical properties that translate directly into specific industrial uses. The trick in matching questions is to connect what a substance does (its function) to why it is used (its property). For example, misch metal sparks when struck because cerium is highly reactive with oxygen — that’s why it goes into lighter flints, not because it’s particularly strong or cheap.

Let’s walk through each item in Column I and find its natural home in Column II.

1. Lanthanoid oxide (i)

   Lanthanoid oxides like $Eu_2O_3$ and $Y_2O_3$ are excellent phosphors — they absorb energy (from an electron beam or X-rays) and re-emit it as visible light. This makes them ideal for coating the inside of television screens (cathode-ray tubes) and X-ray screens (where they convert X-rays into visible light for film or detectors).

   Looking at Column II, two options fit: (b) Television screen and (f) In X-ray screen. Since the question lists (i) as a single entry, we need the most direct match. In standard textbooks, lanthanoid oxides are specifically cited for TV screens (the red phosphor is often $Y_2O_3:Eu^{3+}$). So (i) → (b).

2. Lanthanoid (ii)

   This entry asks for the pure lanthanoid metal’s own industrial use, not a restatement of its composition. A pure lanthanoid metal (like cerium or lanthanum) is added directly to molten iron/steel as a deoxidiser and desulphuriser, improving the alloy’s quality. Option (a), “Production of iron alloy”, is exactly this use. So (ii) → (a).

3. Misch metal (iii)

   Misch metal is an alloy of about 50% cerium, 25% lanthanum, and smaller amounts of other lanthanoids, plus iron (~5%, with traces of S, C, Ca, Al). That composition — a lanthanoid metal combined with iron — is exactly what option (d), “Lanthanoid metal + iron”, describes. So (iii) → (d). (Misch metal’s well-known sparking/pyrophoric property is what makes it a component of the separate magnesium-based alloy in item (iv) below — that use is not what Column II asks for here.)

4. Magnesium based alloy is constituent of (iv) …

Method: Direct Matching by Functional Application

This method works by identifying the key property or application of each substance and linking it to its most well-known use.

Steps:

1. Identify the core material in each Column I entry.
2. Recall its primary industrial or commercial application from standard chemistry.
3. Match it to the most specific use in Column II.

Step-by-step matching:

(i) Lanthanoid oxide

• Key property: Acts as a phosphor — absorbs energy from an electron beam and re-emits it as visible light.
• Use: Coating for television screens (e.g., $\ce{Y2O3:Eu^{3+}}$ red phosphor).
• Match: → (b) Television screen

(ii) Lanthanoid

• Key property: The pure lanthanoid metal (Ce, La) is added directly to molten iron/steel as a deoxidiser and desulphuriser.
• Use: Production of iron alloy (improves steel quality).
• Match: → (a) Production of iron alloy

(iii) Misch metal

• Key property: By definition, misch metal IS the alloy of a lanthanoid metal (~95%) with iron (~5%, plus traces of S, C, Ca, Al).
• Use: This composition is exactly what Column II option (d) describes.
• Match: → (d) Lanthanoid metal + iron

(iv) Magnesium based alloy is constituent of

• Key property: Lightweight, strong alloy. …

Here are the common mistakes students make when matching alloy/composition concepts like this, along with how to avoid each.

Mistake 1: Confusing "Misch Metal" with a Pure Lanthanoid

• The Mistake: Students often think Misch metal is a lanthanoid element, or match it directly with "Bullets" (confusing it with the magnesium alloy that actually goes into bullets).
• Why it happens: The name "Misch metal" sounds like a single metal, and students forget its actual composition.
• How to Avoid:
  • Memorize the definition: Misch metal IS, by definition, the alloy of a lanthanoid metal (~95%, mainly Ce, La, Nd) with iron (~5%, plus traces of S, C, Ca, Al) — this composition is exactly what Column II describes as "Lanthanoid metal + iron."
  • Distinguish from the pure metal's own use: A separate magnesium-based alloy CONTAINING misch metal is pyrophoric and goes into bullets and lighter flints (see Mistake 5) — that is not misch metal's own Column II match.
  • Correct Match: (iii) Misch metal → (d) Lanthanoid metal + iron.

Mistake 2: Forgetting the Specific Use of Lanthanoid Oxides in Screens

• The Mistake: Students match "Lanthanoid oxide" with "Petroleum cracking" or "Production of iron alloy" because they recall oxides are used in industry.
• Why it happens: The term "oxide" is too general. Students don't specify which oxide or which screen.
• How to Avoid:
  • Connect the specific property: Lanthanoid oxides (like $\ce{Eu2O3}$ and $\ce{Y2O3}$) are excellent phosphors. They emit visible light when struck by electrons or X-rays.
  • Two key applications:
    1. Television screen (CRT): Red phosphor.
    2. X-ray screen: Intensifying screens.
  • Correct Match: (i) Lanthanoid oxide → (b) Television screen (or (f) In X-ray screen, depending on the specific exam's pairing). In this list, (b) is the primary match.

Mistake 3: Misinterpreting "Mixed Oxides" as a Single Compound

• The Mistake: Students think "mixed oxides" is the same as "lanthanoid oxide" and match it to the same use (TV screen).
• Why it happens: The wording is similar, and students don't distinguish between a pure oxide and a mixture of oxides.
• How to Avoid:
  • Understand the context: "Mixed oxides of lanthanoids" refers to a catalyst mixture (e.g., in zeolites).
  • Key use: These are used as cracking catalysts in petroleum refining to break large hydrocarbon molecules into smaller ones (gasoline).
  • Correct Match: (v) Mixed oxides of lanthanoids are employed → (c) Petroleum cracking.

Mistake 4: Confusing "Lanthanoid" (Element) with the Alloy It Forms

• The Mistake: Students match "Lanthanoid" (a single element) with "Lanthanoid metal + iron" (thinking Column I is just describing its own composition) instead of its actual industrial use.
• Why it happens: They assume Column I's "Lanthanoid" entry must map to the compositional description in Column II, rather than to what the pure metal is actually USED for.
• How to Avoid: …