Q.Low concentration of oxygen in the blood and tissues of people living at high altitude is due to ____________.
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Henry's Law: The Physics of "Fizz"
Imagine you open a cold bottle of soda. You hear that familiar psshhht sound. Bubbles rush out. Now think: why were those bubbles inside the bottle in the first place? The liquid wasn't boiling. The answer is Henry's Law.
The Intuition: Gas Wants to Dissolve
Gases are just molecules flying around. When a gas touches a liquid, some of those molecules get "trapped" inside the liquid — they dissolve. But here's the key: the more you push on the gas, the more of it gets forced into the liquid.
Think of a crowded bus. If you push more people toward the door (higher pressure), more people get squeezed inside. If you let the pressure off (open the bottle), people rush out. That's exactly what happens with gas and liquid.
In the soda bottle, carbon dioxide gas is pumped in at high pressure. That pressure forces a huge amount of CO₂ to dissolve into the liquid. When you open the bottle, the pressure above the liquid drops to normal air pressure. Suddenly, the liquid can't hold all that CO₂ anymore — so it escapes as bubbles. That's the fizz.
The Precise Statement
Henry's Law says:
C=kH⋅P
Where:
- C = concentration of the dissolved gas in the liquid (usually mol/L or g/L)
- P = partial pressure of that gas above the liquid (usually atm or kPa)
- kH = Henry's law constant — a number that depends on the specific gas, the liquid, and the temperature
In words: At a constant temperature, the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of that gas above the liquid.
What the Constant kH Tells You
kH is not universal. It's different for every gas-liquid pair. For example:
- CO₂ in water has a certain kH
- O₂ in water has a different kH (smaller — oxygen doesn't dissolve as easily)
Temperature matters too. Higher temperature means lower kH — gases become less soluble in hot liquids. That's why a warm soda goes flat faster than a cold one.
Henry's Law works only for dilute solutions and non-reacting gases. If the gas reacts chemically with the liquid (like HCl gas dissolving in water to form hydrochloric acid), Henry's Law does not apply — the concentration will be much higher than predicted.
Real-Life Examples
| Situation | What Henry's Law explains |
|---|---|
| Soda fizz | High pressure forces CO₂ in; releasing pressure lets it out |
Why this formula?
Henry's Law: Why the Formula Holds
Henry's Law describes the solubility of a gas in a liquid at a constant temperature. The key formula is:
P=kH⋅x
Where:
- P = partial pressure of the gas above the liquid
- x = mole fraction of the gas dissolved in the liquid
- kH = Henry's constant (depends on gas, liquid, and temperature)
Why This Linear Relationship Exists
1. Dynamic Equilibrium at the Interface
Imagine a gas above a liquid. At the molecular level:
- Gas molecules constantly strike the liquid surface and dissolve
- Dissolved molecules constantly escape back into the gas phase
At equilibrium, the rate of dissolution equals the rate of escape. This is a dynamic balance, not a static one.
2. The Driving Force for Dissolution
The rate at which gas molecules enter the liquid depends on:
- How many gas molecules hit the surface — this is proportional to the partial pressure P of the gas
- How easily they dissolve — this is captured by kH
So:
Ratedissolve∝P
3. The Driving Force for Escape
The rate at which dissolved molecules leave the liquid depends on:
- How many dissolved molecules are near the surface — this is proportional to the mole fraction x of the gas in the liquid
- How easily they escape — also captured by kH
So:
Rateescape∝x
4. Equating the Two Rates
At equilibrium:
Ratedissolve=Rateescape
Therefore:
P∝x
Introducing the proportionality constant kH:
P=kH⋅x
Why It's Linear (Not Exponential or Logarithmic)
The linearity arises because:
- No saturation effects at low concentrations — the molecules don't "crowd" each other
- Ideal behavior is assumed — gas molecules don't interact strongly with each other or with the solvent
- Temperature is constant — kH doesn't change …
Concept: Henry's Law — the solubility of a gas in a liquid is directly proportional to its partial pressure above the liquid.
At high altitude, atmospheric pressure drops significantly. Since air is roughly 21% oxygen, the partial pressure of O2 also decreases proportionally. According to Henry's Law, lower partial pressure of oxygen means less oxygen dissolves into the blood at the alveolar–capillary interface in the lungs. …
At high altitude, atmospheric pressure drops, reducing the partial pressure of oxygen and thus its solubility in blood according to Henry's Law. The answer is (ii) low atmospheric pressure.
Why altitude affects oxygen availability
Henry's Law: the solubility of a gas in a liquid is proportional to its partial pressure. When you climb a mountain, total atmospheric pressure falls - and with it, the partial pressure of oxygen in the air you breathe. Even though oxygen still makes up about 21% of the atmosphere at any altitude, the absolute partial pressure of oxygen is lower, so less oxygen dissolves into blood plasma and binds to hemoglobin - hypoxia.
C=kH⋅pgas
Step-by-step reasoning
- Atmospheric pressure decreases with altitude (roughly 1 atm at sea level, ~0.33 atm at Everest's summit).
- Partial pressure of oxygen falls proportionally: pO2=0.21×patm.
- Henry's Law governs oxygen dissolution in blood - lower pO2 means lower dissolved oxygen. …
Concept: Partial Pressure of Oxygen & Altitude Physiology
The correct answer is (ii) low atmospheric pressure.
Method: Henry's Law & Dalton's Law of Partial Pressures
Why this method?
Oxygen moves from air into blood by diffusion, which depends on the partial pressure of oxygen (PO2) in the atmosphere — not just its percentage.
Step-by-step reasoning
- Dalton's Law Total atmospheric pressure (Ptotal) is the sum of partial pressures of all gases.
PO2=Fraction of O2×Ptotal
At any altitude, the fraction of oxygen in air remains ~21%.
- Effect of altitude on Ptotal At sea level: Ptotal≈760mm Hg At high altitude (e.g., 5000 m): Ptotal≈400mm Hg So,
PO2 at sea level=0.21×760≈160mm Hg
PO2 at high altitude=0.21×400≈84mm Hg
- Henry's Law The amount of gas dissolved in blood is proportional to its partial pressure. …
Here is the breakdown of the common mistakes students make on this question, along with the correct reasoning.
The Correct Answer
The correct answer is (ii) low atmospheric pressure.
The Core Concept: Partial Pressure of Oxygen
The key to this question is understanding Henry's Law and Partial Pressure.
- Henry's Law: The amount of gas dissolved in a liquid (like blood) is directly proportional to the partial pressure of that gas in the atmosphere above the liquid.
- Partial Pressure: The pressure exerted by a single gas (like oxygen) in a mixture of gases (like air).
At high altitude, the total atmospheric pressure drops. Since oxygen makes up a constant ~21% of the air, the partial pressure of oxygen also drops significantly. This lower partial pressure means less oxygen is forced into the blood and tissues.
Common Mistakes & How to Avoid Them
Mistake 1: Choosing "Low Temperature" (Option A)
Why students choose this:
Students often think, "It's cold on a mountain, so cold air must have less oxygen." They confuse the density of air with the concentration of oxygen.
The Reality:
- Temperature affects the density of air, but the percentage of oxygen in the air remains ~21%.
- More importantly, cold temperature actually increases the solubility of gases in liquids. If temperature were the only factor, cold blood would hold more oxygen, not less.
- The primary reason for low oxygen in the blood is the low partial pressure, not the cold.
How to Avoid:
- Focus on the "why": Ask yourself: "What is the driving force that pushes oxygen into the blood?" The answer is pressure (specifically, the pressure gradient). Temperature is a secondary factor that affects solubility, not the primary driving force.
- Remember the context: The question asks about high altitude. The defining characteristic of high altitude is low atmospheric pressure, not just low temperature.
Mistake 2: Choosing "Both Low Temperature and High Atmospheric Pressure" (Option D)
Why students choose this:
This is a trap for students who are guessing or who haven't read the options carefully. They might think "high altitude = cold" and then incorrectly assume pressure must also be high.
The Reality:
- Atmospheric pressure decreases with altitude. It is never "high" at high altitude. This option contains a direct factual error.
- The combination of "low temperature" and "high pressure" is actually the condition for maximum oxygen solubility, which is the opposite of what happens at high altitude.
How to Avoid:
- Know the basic relationship: Altitude ↑ → Atmospheric Pressure ↓.
- Eliminate contradictions: As soon as you see "high atmospheric pressure" in an option about high altitude, you can immediately discard it. It violates a fundamental fact.
Mistake 3: Confusing "Atmospheric Pressure" with "Partial Pressure" …
- KEAM 2025Set pha-2025-0424F4 marksMCQQ.Which of the following gas has highest solubility in water at 298 K? (A) Formaldehyde (B) Methane (C) CO2 (D) Vinyl chloride (E) Argon
›Reveal solutionSolution
Formaldehyde (HCHO) is polar and hydrogen-bonds with water (forming its hydrate/formalin), giving it much higher solubility than the non-polar or weakly polar options.
Gas solubility in water is largest for polar molecules that can hydrogen-bond or react with water:
- Formaldehyde (HCHO) — polar carbonyl; hydrogen-bonds and hydrates readily (aqueous solution = formalin) → very high solubility. ✓
- CO2 — only moderately soluble (partly as carbonic acid). …
- KEAM 2025Set pha-2025-0424A4 marksMCQQ.N2 exerts a partial pressure of 7.648 bar when dissolved in 1 litre of water at 298 K. What is the mole fraction of N2 at same temperature? ( Henry's law constant (KH) for N2 at 298 K = 76.4 k bar) (A) 10−5 (B) 10−3 (C) 10−4 (D) 10−6 (E) 10−2
›Reveal solutionSolution
The mole fraction of N2 is 10−4.
Henry's law states that the partial pressure of a gas above a solution is proportional to its mole fraction in solution:
p=KHx.
Given p=7.648 bar and KH=76.4 k bar =76400 bar: …
- KEAM 2025Set pha-2025-0429F4 marksMCQQ.Which of the following gas has the lowest solubility in water at 298 K? (A) Argon (B) Carbon dioxide (C) Formaldehyde (D) Methane (E) Vinyl chloride
›Reveal solutionSolution
Solubility falls as the Henry's-law constant KH rises. Argon, an inert monatomic gas, has by far the largest KH (≈ 40 kbar) among these species and hence the lowest solubility in water at 298 K.
Reasoning
Approximate Henry's-law constants at 298 K (kbar; higher KH = lower solubility):
- Formaldehyde — extremely soluble (hydrates/reacts with water), very small KH
- Vinyl chloride — moderate
- CO2 ≈ 1.67 kbar
- Methane ≈ 0.4 kbar (but see below)
- Argon ≈ 40 kbar (largest) …
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