Low concentration of oxygen in the blood and tissues of people living at high altitude is due to ______. (i) l

NCERT Exemplar · Q6

Q. Low concentration of oxygen in the blood and tissues of people living at high altitude is due to ____________.

(i) low temperature

(ii) low atmospheric pressure

(iii) high atmospheric pressure

(iv) both low temperature and high atmospheric pressure

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Concept understanding — Henrys Law

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 = k_H \cdot 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)
$k_H$ = 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 $k_H$ Tells You

$k_H$ is not universal. It's different for every gas-liquid pair. For example:

CO₂ in water has a certain $k_H$
O₂ in water has a different $k_H$ (smaller — oxygen doesn't dissolve as easily)

Temperature matters too. Higher temperature means lower $k_H$ — gases become less soluble in hot liquids. That's why a warm soda goes flat faster than a cold one.

Watch out

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 = k_H \cdot x

Where:

$P$ = partial pressure of the gas above the liquid
$x$ = mole fraction of the gas dissolved in the liquid
$k_H$ = 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 $k_H$

So:

\text{Rate}_{\text{dissolve}} \propto 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 $k_H$

So:

\text{Rate}_{\text{escape}} \propto x

4. Equating the Two Rates

At equilibrium:

\text{Rate}_{\text{dissolve}} = \text{Rate}_{\text{escape}}

Therefore:

P \propto x

Introducing the proportionality constant $k_H$ :

P = k_H \cdot 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 — $k_H$ doesn't change …

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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 = k_H \cdot p_{\text{gas}}

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: $p_{O_2}=0.21\times p_{atm}$ .
Henry's Law governs oxygen dissolution in blood - lower $p_{O_2}$ means lower dissolved oxygen. …

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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 $\uparrow$ → Atmospheric Pressure $\downarrow$ .
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" …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQ
$Q. The value of Henry's law constant for some gases at 293 K is given below. Arrange the gases in the increasing order of their solubility. (He: 144.97 kbar, H2: 69.16 kbar, N2: 76.48 kbar, O2: 34.86 kbar)$
(a) He < N2 < H2 < O2
(b) O2 < H2 < N2 < He
(c) H2 < N2 < O2 < He
(d) He < O2 < N2 < H2
›Reveal solutionSolution

Henry's law: p = KH · x, so solubility x = p/KH — a larger Henry's law constant means the gas is LESS soluble.

Henry's law states that the partial pressure of a gas over a solution is proportional to its mole fraction in solution: p = KH·x. Rearranged, x = p/KH, so for the SAME partial pressure, a gas with a larger KH dissolves LESS (lower x), and a gas with a smaller KH dissolves MORE.

Given KH values (kbar) at 293 K: He = 144.97, N2 = 76.48, H2 = 69.16, O2 = 34.86.

…

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GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQ
$Q. At 298 K, which of the following gas has lowest solubility in the liquid. Gas: Ar, CO2, Methane, Vinyle Chloride KH/K bar: 40.3, 1.67, 0.413, 0.611$
(a) Methane
(b) CO2
(c) Vinyle Chloride
(d) Ar
›Reveal solutionSolution

By Henry's law, p = KH x (mole fraction dissolved); a HIGHER KH at the same partial pressure means a LOWER amount dissolved, so the gas with the highest KH is the least soluble.

Henry's law: p(gas) = KH x x(gas dissolved), where x is the mole fraction of the gas dissolved in the liquid at equilibrium with partial pressure p.

Rearranging: x = p / KH. For a given partial pressure p, a gas with a larger KH dissolves to a smaller mole fraction x - i.e., higher KH means lower solubility.

…

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GUJCET 2022Set 171 markMCQ
$K_H$
›Reveal solutionSolution

Higher T ⇒ lower gas solubility ⇒ larger $K_H$ .

Concept. Henry's law: $p = K_H \cdot x$ . Dissolving a gas is exothermic, so raising the temperature drives the gas out (solubility decreases). Since $K_H = p/x$ , a smaller mole fraction $x$ at the …
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GUJCET 2020Set 071 markMCQ
$Q. Maximum amount of a solid solute that can be dissolved in a specified amount of a given liquid solvent does not depend upon ______.$
(i) Temperature
(ii) Nature of solute
(iii) Pressure
(iv) Nature of Solvent (A)
(ii) &
(iv) (B)
(ii) (C)
(i) &
(iii) (D) (iii)
›Reveal solutionSolution

Solid-in-liquid solubility does not depend on pressure — only (iii).

Concept — factors affecting solubility. Maximum solubility of a solid solute depends on temperature, nature of solute, and nature of solvent. Pressure has negligible effect on solid–liquid systems (it …
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GUJCET 2014Set A1 markMCQ
$Q. From the given graph at constant temperature, which gas has the least solubility? [FIGURE: partial pressure vs solubility straight-line plots labelled gas-A (steepest), gas-B, gas-C, gas-D (least steep)] (A) gas - D (B) gas - B (C) gas - A (D) gas - C$
›Reveal solutionSolution
[!TLDR]

Steepest line = largest Henry's constant $K_H$ = least soluble gas = gas-A.

Concept

Henry's law states $p = K_H\,x$ , where $p$ is the partial pressure of the gas and $x$ its solubility (mole fraction in solution). On a graph of partial pressure versus solubility, the slope is $K_H$ . A gas with a higher $K_H$ dissolves less for the same partial pressure, i.e. it is less soluble.

Solution …
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Q.Low concentration of oxygen in the blood and tissues of people living at high altitude is due to ____________.

Concept understanding — Henrys Law

Henry's Law: The Physics of "Fizz"

The Intuition: Gas Wants to Dissolve

The Precise Statement

What the Constant $k_H$ Tells You

Real-Life Examples

Why this formula?

Henry's Law: Why the Formula Holds

Why This Linear Relationship Exists

1. Dynamic Equilibrium at the Interface

2. The Driving Force for Dissolution

3. The Driving Force for Escape

4. Equating the Two Rates

Why It's Linear (Not Exponential or Logarithmic)

Why altitude affects oxygen availability

Step-by-step reasoning

Concept: Partial Pressure of Oxygen & Altitude Physiology

Method: Henry's Law & Dalton's Law of Partial Pressures

Step-by-step reasoning

The Correct Answer

The Core Concept: Partial Pressure of Oxygen

Common Mistakes & How to Avoid Them

Mistake 1: Choosing "Low Temperature" (Option A)

Mistake 2: Choosing "Both Low Temperature and High Atmospheric Pressure" (Option D)

Mistake 3: Confusing "Atmospheric Pressure" with "Partial Pressure" …

Concept

Solution …

Q.Low concentration of oxygen in the blood and tissues of people living at high altitude is due to ____________.

Why altitude affects oxygen availability

Step-by-step reasoning

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