Pure Substance vs Compound — NTSE Trick in 59 Sec ⚡

 

❓ Question

Why are compounds called pure substances even though their properties are completely different from their constituent elements?

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✍️ Short Solution

This is a concept clarity + assertion–reason logic question.
No formula — just definition + reasoning separation 💯

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🔹 Step 1 — Pure Substance Definition (MOST IMPORTANT 💯)

👉 Pure substance ka matlab:

$$\text{Fixed composition + uniform properties}$$

👉 Is category me aate hain:

• Elements
• Compounds

📌 “Pure” ka matlab single type of particle, not “same properties”

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🔹 Step 2 — Elements vs Compounds

👉 Element:

• Sirf ek type ke atoms
• Example: H₂, O₂

👉 Compound:

• Different elements ka fixed ratio me combination
• Example:
  • H₂O
  • CO₂

📌 Ratio always fixed hota hai → isi wajah se pure substance

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🔹 Step 3 — Property Change Concept

👉 Jab compound banta hai:

$$\text{Properties of compound} \ne \text{Properties of elements}$$

Example:

• Hydrogen → highly inflammable 🔥
• Oxygen → supports combustion
• Water → neither burns nor supports burning

📌 Completely new substance banta hai ✅

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🔹 Step 4 — Assertion–Reason Link

👉 Important logic:

• Assertion (A) → Compound is a pure substance ✅
• Reason (R) → Properties change after formation ✅

👉 But:

$$\text{R is NOT the explanation of A}$$

📌 Kyunki:

• A → based on composition (fixed ratio)
• R → based on property change

👉 Dono alag concepts hain ⚠️

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🔹 Step 5 — NTSE Golden Traps

❌ Compound ko mixture samajh lena
❌ “Property change = not pure” soch lena
❌ Har correct R ko A ka explanation maan lena

⚠️ Golden Rule:

Har correct reason, correct explanation nahi hota

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✅ Final Answer

$$\boxed{\text{Both A and R are true, but R is NOT the correct explanation of A}}$$

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⭐ Golden NTSE Insight

• Pure substance ⇒ fixed composition
• Compound ⇒ chemically combined + definite ratio
• Property change ⇒ new substance formation

🧠 One-line memory:

Pure ka matlab “fixed ratio” hota hai, “same properties” nahi