15. Some carbonated beverages are made by forcing carbon dioxide gas into a beverage solution.
When a bottle of one kind of carbonated beverage is first opened, the beverage has a pH value of 3.
After the beverage bottle is left open for several hours, the hydronium ion concentration in the
beverage solution decreases to 1/100th of the original concentration. What is the new pH of the
beverage solution
a. 5
b. 10
c. 3
d. 6
Original pH 3 means [H₃O⁺] = 10⁻³ M from dissolved CO₂ forming H₂CO₃ → H⁺ + HCO₃⁻. After hours open, CO₂ escapes reducing [H₃O⁺] to 1/100th = 10⁻⁵ M, so new pH = -log(10⁻⁵) = 5.
🥤 Carbonic Acid Fizz Chemistry
Carbonated beverages force CO₂ under pressure: CO₂(g) + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (pKa₁ ≈ 6.3), giving acidic pH 3-4. Opening releases pressure, shifting equilibrium left per Le Chatelier, expelling CO₂ and dropping [H⁺] dramatically. 1/100th change = 2 pH unit increase since ΔpH = -log(1/100) = 2.
📊 Option Analysis
- a. 5: ✅ Correct. pH 3 → [H⁺] = 10⁻³ M; 1/100th = 10⁻⁵ M → pH 5 exactly.
- b. 10: ❌ Wrong. Would need [H⁺] = 10⁻¹⁰ M (1/10¹⁰th original), impossible from CO₂ loss.
- c. 3: ❌ No change. Assumes no CO₂ escape, contradicts hours-open scenario.
- d. 6: ❌ Close but wrong. 1/1000th reduction (10⁻⁶ M) needed; question specifies 1/100th.
🔬 CSIR NET pH Change Table
| Stage | [CO₂] | [H₃O⁺] | pH | Fizz Level |
|---|---|---|---|---|
| Bottled | High (pressurized) | 10⁻³ M | 3 | Bubbly |
| Just opened | Partial escape | ~10⁻³ M | 3 | Peak fizz |
| Hours open | Near zero | 10⁻⁵ M | 5 | Flat |
| Neutral water | None | 10⁻⁷ M | 7 | No acidity |
🎯 CSIR NET Key Takeaway
CSIR NET biochemistry tests pH-log relationships and weak acid equilibria like H₂CO₃ in beverages/enzymes. Key: [H⁺] ×100 → pH -2 units
