NCERT Line-by-Line
1. TITLE
NCERT says 'the mass of a gas dissolved... is proportional to the pressure' — Here's how NEET tests it
2. THE NCERT STATEMENT
NCERT EXACT LINE:
"This equilibrium is governed by Henry's law, which states that the mass of a gas dissolved in a given mass of a solvent at any temperature is proportional to the pressure of the gas above the solvent."
— NCERT Class 11, Chapter 6, Page 171
3. WHY THIS LINE MATTERS
- How many times has NEET tested this? Henry's Law and its practical application (the soda bottle) are frequently tested, both as direct factual statements and assertion-reasoning questions regarding physical equilibrium.
- What makes this statement tricky? Students often confuse the behavior of gases dissolving in liquids with solids dissolving in liquids. The roles of pressure and temperature here are very specific.
- What do students typically misremember? Students often forget the exact temperature relation mentioned in the very next line—that gas solubility decreases with an increase in temperature.
4. MCQ (Directly from this NCERT line)
According to NCERT, the equilibrium of gases dissolving in liquids (such as $CO_2$ in soda water) is characterized by:
A) The mass of a gas dissolved in a given mass of a solvent is proportional to the pressure of the gas above the solvent.
B) The mass of a gas dissolved in a given mass of a solvent is inversely proportional to the pressure of the gas above the solvent.
C) The amount of dissolved gas increases with an increase in temperature.
D) The concentration of the gas in the liquid remains constant regardless of the pressure above the solvent.
5. ANSWER + LINE BREAKDOWN
Correct Answer: A
Break down the NCERT line:
- "mass of a gas dissolved" means the concentration or solubility of the gas in the liquid phase.
- "proportional to the pressure" means that high pressure forces more gas into the liquid. This is exactly why soda bottles are sealed under high pressure to keep the $CO_2$ highly soluble.
- The key word is "proportional" because it establishes the direct relationship that drives the "fizzing" phenomenon. As soon as you open the bottle, the pressure drops to atmospheric levels, breaking the equilibrium, and causing the gas to rapidly escape (fizz) to reach a new, lower concentration.
6. HOW NEET TWISTS THIS
Show 3 ways NEET can ask the same fact:
- As direct question: "According to Henry's law, how does sealing a soft drink under extremely high pressure affect the $CO_2$ gas concentration in the liquid?" (Answer: It increases).
- As assertion-reason: Assertion: A soda water bottle fizzes when opened. Reason: The solubility of a gas in a liquid decreases because the partial pressure of the gas drops when exposed to the atmosphere.
- As except question: "All of the following are true regarding the dissolution of gases in liquids EXCEPT: A) It is governed by Henry's law B) Solubility decreases with increased temperature C) Soda bottles are sealed under high pressure D) Concentration of the gas in liquid is inversely proportional to pressure." (Answer: D)
7. MEMORY LOCK
MEMORIZE:
Higher pressure = Higher gas solubility. Higher temperature = Lower gas solubility.
Mnemonic: High-Pro (High Pressure = Proportional Solubility). Think: Fizz drops when Pressure drops!
8. RELATED NCERT LINES
Also remember these from the same page:
- "This amount decreases with increase of temperature."
- "For dissolution of gases in liquids, the concentration of a gas in liquid is proportional to the pressure (concentration) of the gas over the liquid."
9. NCERT REFERENCE
NCERT Class 11, Chapter 6, Page 171, Paragraph 3 (under "Gases in liquids")
10. CTA
All important NCERT lines from this chapter → Equilibrium Guide