Question Traps
1. TITLE
The Common Ion Solubility Reduction trap NEET uses every year — Don't fall for it
2. THE TRAP PATTERN
THE TRAP:
NEET loves to test the Common Ion Effect on Solubility by:
Asking what happens to the solubility of a sparingly soluble salt when a strong electrolyte containing a common ion is added to the solution.
What it looks like: A simple question about adding a new, highly soluble salt to an existing saturated solution, sometimes tricking students into thinking it won't affect the original salt or that $K_{sp}$ remaining constant means solubility stays constant.
What it's actually testing: Le Chatelier's Principle and how the Common Ion Effect shifts the equilibrium backwards to form a precipitate, thereby decreasing solubility.
3. EXAMPLE TRAP QUESTION
TRAP QUESTION:
Adding $\text{AgNO}_3$ to a saturated solution of $\text{AgCl}$ causes the solubility of $\text{AgCl}$ to:
A) Increase
B) Decrease
C) Remain the same
D) Become zero
70% students choose (C). The correct answer is (B).
4. WHY THE TRAP WORKS
- What assumption do students make? Students assume that since the Solubility Product Constant ($K_{sp}$) only depends on temperature, the solubility must also remain strictly constant. They confuse the constant $K_{sp}$ with the variable solubility ($S$).
- What do they skip reading? They miss that adding $\text{AgNO}3$ introduces an excess of $\text{Ag}^+$ ions (a common ion), which increases the Ionic Product ($Q_{sp}$) so that $Q{sp} > K_{sp}${sp} > K_{sp}$.
- What hidden condition changes everything? To re-establish equilibrium and bring $Q_{sp}$ back down to equal $K_{sp}$, the system must apply Le Chatelier's principle and shift the reaction backward to consume the excess $\text{Ag}^+$. This forces solid $\text{AgCl}$ to precipitate out, meaning its overall solubility decreases.
5. HOW TO SPOT THIS TRAP
RED FLAGS TO WATCH FOR:
* When question says "Adding [Salt A] to a saturated solution of [Salt B]" → Check for a hidden common ion shared between the two compounds.
* If a $Q_{sp} > K_{sp}$ scenario is given → Don't assume the solid will dissolve; it will precipitate.
* Keywords that signal this trap: "sparingly soluble", "adding", "solubility of", "precipitate".
6. THE CORRECT APPROACH
Step-by-step:
1. First, identify the ions produced by both the sparingly soluble salt and the added strong electrolyte.
2. Check if a "common ion" applies (e.g., both produce $\text{Ag}^+$ or $\text{Cl}^-$).
3. Only then apply Le Chatelier's principle: adding a common ion increases the product-side concentration, shifting the equilibrium to the left (towards the solid reactant).
4. Verify by concluding that this backward shift creates more precipitate, effectively decreasing the solubility of the sparingly soluble salt.
7. SIMILAR TRAPS IN THIS CHAPTER
SAME TRAP, DIFFERENT QUESTIONS:
| Trap Type | What they ask | What they actually want |
|---|---|---|
| Common ion in weak acids | Adding $\text{CH}_3\text{COONa}$ to $\text{CH}_3\text{COOH}$ | Tests that the common $\text{CH}_3\text{COO}^-$ suppresses the ionization of the weak acid, pushing equilibrium back and decreasing $[\text{H}^+]$. |
| Gas common ion | Passing $\text{HCl}$ gas through saturated $\text{NaCl}$ | Tests that the common $\text{Cl}^-$ ion from the gas forces $\text{NaCl}$ to precipitate out, which is a method used to purify salt. |
| Precipitation prediction | Mixing two solutions and asking if a precipitate forms | Tests calculating the instantaneous ionic product ($Q_{sp}$) and verifying if $Q_{sp} > K_{sp}$ to confirm precipitation will occur. |
8. PRACTICE IDENTIFYING TRAPS
TRAP DETECTION PRACTICE:
Q1: Passing $\text{HCl}$ gas through a saturated solution of common salt ($\text{NaCl}$) results in the precipitation of pure $\text{NaCl}$. Why?
What's the trap? [Answer hidden below]
Q2: Calculate the molar solubility of $\text{Ni(OH)}_2$ in $0.10\text{ M NaOH}$.
What's the trap? [Answer hidden below]
Answers:
Q1 Trap: The trap is assuming $\text{HCl}$ gas makes the solution more acidic and dissolves the solid. The real test is the common ion effect: the $\text{Cl}^-$ from $\text{HCl}$ drastically increases the $\text{Cl}^-$ concentration, shifting the $\text{NaCl}$ equilibrium to the solid side and causing precipitation.
Q2 Trap: Students might try to calculate the solubility in pure water using just $K_{sp} = 4S^3$. The trap is missing the common $\text{OH}^-$ ion from the strong base $\text{NaOH}$, which must be added to the equilibrium equation (as $0.10 + 2S$), drastically reducing the solubility of $\text{Ni(OH)}_2$.
9. NCERT VS NEET GAP
"NCERT teaches the Common Ion Effect simply. NEET adds these twists:"
- Twist 1: Quantitative Common Ion Calculations. NEET won't just ask if solubility decreases; they will ask you to calculate the exact new molar solubility of a salt in the presence of a common ion by setting up an equation where the common ion concentration is predominantly determined by the strong electrolyte.
- Twist 2: Cross-Phase Common Ions. NEET tests the concept using different phases, such as passing a gas ($\text{HCl}$) through a liquid solution ($\text{NaCl}$) to cause a solid precipitate, testing if you realize the phase doesn't stop the common ion effect from applying.
10. CTA
Master all traps in this chapter → Equilibrium Guide
Trap-based questions test → [Tricky MCQs: 6-Equilibrium]