Q.State Henry's law and mention some important applications.
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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 the partial pressure of the gas above the liquid at a constant temperature.
Essential reasoning:
- Mathematically, p=KH⋅x, where p is the partial pressure of the gas, x is its mole fraction in the solution, and KH is Henry's constant (specific to the gas–solvent pair at a given temperature).
- A higher KH means lower solubility at a given pressure; a lower KH means higher solubility.
Important applications:
- Carbonated beverages: Bottled under high CO2 pressure to increase solubility; opening the bottle lowers pressure, releasing excess gas as bubbles.
- Deep-sea diving: Divers breathe compressed air; increased pressure dissolves more N2 in blood. Rapid ascent causes decompression sickness (bends) as N2 bubbles form — avoided by diluting the breathing mixture with helium, which is much less soluble in blood than nitrogen (helium's KH is higher — 144.97 kbar vs 76.48 kbar for N2 — and a higher KH means lower solubility). …
Henry's law states that at constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. Its key applications include carbonated beverages, deep-sea diving, and the respiration of oxygen in blood.
Understanding Henry's Law
Henry's law is a fundamental principle in physical chemistry that describes how gases dissolve in liquids. The intuition is simple: when you increase the pressure of a gas above a liquid, more gas molecules are forced into the liquid phase. Think of it like squeezing more people into a room — the harder you push (higher pressure), the more people (gas molecules) can fit.
p=kH⋅x
where p is the partial pressure of the gas above the liquid, x is the mole fraction of the gas in the liquid, and kH is Henry's law constant (which depends on the gas-liquid pair and temperature).
The constant kH is unique for each gas-liquid system. A higher kH means the gas is less soluble (you need more pressure to dissolve the same amount), while a lower kH means the gas dissolves more readily.
Henry's law applies only to dilute solutions and when the gas does not chemically react with the solvent. For example, ammonia (NH3) reacts with water, so Henry's law does not hold for it — you need to account for the chemical equilibrium as well.
Step-by-Step Derivation and Explanation
- The basic relationship At a fixed temperature, the amount of gas dissolved in a liquid is directly proportional to the pressure of that gas above the liquid. Mathematically:
C=kH⋅p
where C is the concentration of the dissolved gas (often in mol/L or mole fraction), p is the partial pressure, and kH is Henry's constant. This is the most common form used in problems.
-
Why temperature matters
Henry's constant kH is temperature-dependent. As temperature increases, gas solubility generally decreases (think of a warm soda going flat faster). This is because dissolution of a gas is usually exothermic — heat drives the gas out of solution.
-
The mole fraction form
For precise work, especially in mixtures, we use mole fraction:
p=kH⋅x
Here x is the mole fraction of the gas in the liquid. This form is useful when dealing with multiple gases or when the solvent itself is volatile.
Important Applications
Henry's law explains several everyday phenomena and has critical industrial and biological applications:
-
Carbonated beverages
Soda and beer are bottled under high CO2 pressure. When you open the bottle, pressure drops suddenly, and CO2 comes out of solution — that's the fizz. The law predicts exactly how much gas remains dissolved at any given pressure.
-
Deep-sea diving
Divers breathe compressed air at depth. Under high pressure, nitrogen dissolves in the blood. If the diver ascends too quickly, the sudden pressure drop causes nitrogen to form bubbles in the bloodstream — this is decompression sickness ("the bends"). Henry's law governs this process, and divers must ascend slowly to allow nitrogen to leave the blood gradually. …
Henry's Law — Concept & Method
1. State Henry's Law
Henry's Law: At a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid surface.
Mathematically:
p=KH⋅x
Where:
- p = partial pressure of the gas above the solution
- x = mole fraction of the gas in the solution
- KH = Henry's constant (depends on the gas, liquid, and temperature)
2. Important Applications
| Application | Explanation |
|---|---|
| Carbonated beverages | CO₂ is bottled under high pressure. When opened, pressure drops → gas escapes → fizz. |
| Deep-sea diving | At high pressure, more N₂ dissolves in blood. Rapid ascent causes decompression sickness ("the bends"). |
| Oxygen therapy | Higher partial pressure of O₂ increases its solubility in blood for patients with respiratory issues. |
| Aquatic life | Oxygen dissolves in water according to Henry's law — essential for fish survival. |
3. One Clear Solution Method
Method: Direct Substitution in Henry's Law
Concept: When a gas dissolves in a liquid, the mole fraction of the dissolved gas is directly proportional to its partial pressure. Use the formula p=KH⋅x to find any one unknown.
Steps:
-
Identify the given quantities — partial pressure (p), Henry's constant (KH), or mole fraction (x).
-
Write Henry's law equation:
p=KH⋅x
-
Substitute the known values into the equation.
-
Solve for the unknown — rearrange if needed:
- To find x: x=KHp
- To find p: p=KH⋅x
-
Check units — ensure pressure units match (atm, bar, or Pa) and KH has consistent units.
Example Problem …
Henry's Law: Common Mistakes & How to Avoid Them
What Henry's Law Actually Says
Henry's Law: At constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid.
p=KH⋅x
Where:
- p = partial pressure of the gas above the liquid
- x = mole fraction of the gas in the solution
- KH = Henry's law constant (depends on gas-solvent pair and temperature)
🚩 Mistake #1: Writing the Formula Backwards
The error: Students write x=KH⋅p or p=KHx, confusing which variable is proportional to which.
Why it happens: Memorizing without understanding the physical meaning.
How to avoid: Remember the physical picture:
- Higher pressure above the liquid → more gas molecules forced into solution → higher solubility
- So pressure (p) and solubility (x) move in the same direction
- The constant KH connects them: p=KH⋅x
Exam tip: If you forget, think: "Pressure pushes gas in" → pressure is on the left side of the equation.
🚩 Mistake #2: Confusing Henry's Constant Units
The error: Using wrong units for KH or not specifying them in answers.
Why it happens: KH can be expressed in different forms (atm, bar, Pa, etc.)
How to avoid: Always check:
- If p is in atm → KH has units of atm
- KH is not dimensionless — it has pressure units
- Common values: KH for O2 in water ≈ 4.3×104 atm at 25°C
Remember: KH is large for less soluble gases (more pressure needed to dissolve the same amount).
🚩 Mistake #3: Forgetting Temperature Dependence
The error: Stating Henry's law without mentioning "at constant temperature."
Why it happens: Students treat it as an absolute law.
How to avoid:
- Always write: "At constant temperature..."
- Know that KH increases with temperature (solubility decreases as temperature rises)
- This is why cold drinks go flat faster when warm
🚩 Mistake #4: Applying Henry's Law to Wrong Gases
The error: Applying Henry's law to gases that react chemically with the solvent.
Why it happens: Not checking if the gas-solvent interaction is purely physical.
How to avoid: Henry's law applies only to gases that:
- Do not react chemically with the solvent
- Form dilute solutions
- Are at moderate pressures (not too high)
Examples where it fails:
- NH3 in water (reacts to form NH4OH)
- HCl in water (dissociates completely)
Borderline case — do NOT list this as a failure: CO2 in water reacts only slightly (a small fraction forms carbonic acid), so Henry's law still describes its solubility well — that is exactly why carbonated drinks are the flagship Henry's-law application, not an exception.
🚩 Mistake #5: Listing Wrong Applications
The error: Giving vague or incorrect applications.
How to avoid: Memorize these exam-favourite applications:
| Application | Explanation |
|-------------|-------------| …
Showing the 12 most recent of 16 on this concept.
- KCET 2025Set D-41 markMCQQ.Variation of solubility with temperature T for a gas in liquid is shown by the following graphs. The correct representation is: (A)
(B) 
(C) 
(D) 
›Reveal solutionSolution
Henry's law makes gas solubility fall as temperature rises — a straight line sloping DOWNWARD, not up, flat, or a hump.
Why solubility decreases with temperature for a gas. Dissolving a gas in a liquid is generally exothermic. As temperature rises, the dissolved gas molecules gain kinetic energy and increasingly escape back into the gas phase, so less gas stays dissolved at equilibrium — the everyday example is a fizzy drink going flat faster when warm. …
- KCET 2025Set D-41 markMCQQ.If N2 gas is bubbled through water at 293 K, how many moles of N2 gas would dissolve in 1 litre of water? Assume that N2 exerts a partial pressure of 0.987 bar. [Given KH for N2 at 293 K is 76.48 K bar] (A) 0.716×10−3 (B) 7.16×10−5 (C) 7.16×10−4 (D) 7.16×10−3
›Reveal solutionSolution
Use Henry's law to get the mole fraction of dissolved N2, then convert that mole fraction to moles using the ≈55.5 mol of water in 1 litre.
Step 1 — Henry's law.
The solubility of a gas in a liquid at a given temperature is proportional to its partial pressure above the liquid:
p=KH⋅x
where x is the mole fraction of the gas in solution and KH is the Henry's-law constant. Rearranged:
x=KHp
Step 2 — Substitute (watch the units on KH).
The constant is given as 76.48 Kbar, i.e. 76.48 kilobar =76.48×103 bar=76,480 bar (this is the standard NCERT value for N2 at 293 K).
xN2=76,480 bar0.987 bar=1.29×10−5
A very small number — nitrogen is only sparingly soluble in water, as expected.
Step 3 — Moles of water in 1 litre.
nH2O=18 gmol−11000 g=55.5 mol
Step 4 — Convert mole fraction to moles of N2.
By definition
xN2=nN2+nH2OnN2≈nH2OnN2
The approximation is excellent because nN2⋘nH2O (we are about to find nN2∼10−4 against 55.5). Hence
nN2=xN2×nH2O=(1.29×10−5)(55.5)
nN2=7.16×10−4 mol
Step 5 — Check the options. …
- COMEDK 2025Set 2025-E1 markMCQQ.The ratio of N2 and O2 gases in the atmosphere is 4:1. The ratio of the mole fractions of the dissolved gases N2 : O2 in rain water will be approximately ........ (At 293 K, KH for Nitrogen and Oxygen in kbar units are 76.48 and 34.86 respectively.) (A) 3:1 (B) 2:1 (C) 4:1 (D) 1:4
›Reveal solutionSolution
Henry’s law says the mole fraction of a dissolved gas is proportional to its partial pressure times its Henry’s constant. Using the given atmospheric ratio and Henry’s constants, the dissolved N₂:O₂ ratio comes out to about 2:1, so option (B) is correct.
Concept & Intuition
Rainwater is in contact with air, so gases dissolve according to Henry’s law:
xgas=KHpgas
where xgas is the mole fraction in the liquid, pgas is the partial pressure in the gas phase, and KH is Henry’s constant (here given in kbar). The atmospheric ratio N₂:O₂ is 4:1 by volume, which means the partial pressures are in the same ratio (since total pressure ≈ 1 atm, but we only need the ratio). The dissolved ratio is not simply 4:1 because O₂ dissolves more readily (lower KH). We must compute the ratio of p/KH for each gas.
Step-by-step
- Set up partial pressures In dry air, N₂ and O₂ are in volume ratio 4:1. Since partial pressure is proportional to mole fraction in the gas,
pN2:pO2=4:1
Let pO2=P, then pN2=4P.
- Apply Henry’s law for each gas Henry’s law:
xN2=KH,N2pN2,xO2=KH,O2pO2
Given KH,N2=76.48 kbar and KH,O2=34.86 kbar.
- Find the ratio of dissolved mole fractions
xO2xN2=pO2/KH,O2pN2/KH,N2=pO2pN2⋅KH,N2KH,O2
Substitute the partial pressure ratio 4/1:
- COMEDK 2024Set 2024-E1 markMCQQ.Study the graph between partial pressure and mole fraction of some gases and arrange the gases P, Q, R and S dissolved in H2O, in the decreasing order of their KH values. (A) S > P > R > Q (B) R > Q > P > S (C) P > R > S > Q (D) Q > R > P > S
›Reveal solutionSolution
Henry's constant equals the slope/intercept of the partial-pressure line; the steepest line (S) has the highest KH and the flattest (Q) the lowest, giving S>P>R>Q.
Henry's law: p=KH⋅xgas. On a partial-pressure vs mole-fraction plot the line for a gas has slope KH; the higher its pressure-axis position/steepness, the larger KH.
From the graph the lines, ranked by steepness / pressure-axis intercept (highest→lowest), are: …
- COMEDK 2024Set 2024-M1 markMCQQ.KH for O2 at 293 K is 34.86 kbar. What should be the partial pressure of O2 gas so that it has a solubility of 0.08 g/L in water at 293 K ? (Density of solution =1 g/ml) (A) 156.8 × 10−5 bar (B) 15680 bar (C) 156.8 bar (D) 1.569 bar
›Reveal solutionSolution
Convert the solubility to a mole fraction of O2, then apply Henry's law p=KHx to get p≈1.569bar.
Moles in 1 L of solution (≈ 1 L water, density 1g/mL):
nO2=320.08=2.5×10−3mol,nwater=181000=55.56mol.
Mole fraction of O2 (its own amount is negligible in the denominator):
x=55.562.5×10−3=4.5×10−5. …
- KCET 2023Set D-21 markMCQQ.A 30% solution of hydrogen peroxide is (A) '30 volume' hydrogen peroxide (B) '10 volume' hydrogen peroxide (C) '50 volume' hydrogen peroxide (D) '100 volume' hydrogen peroxide
›Reveal solutionSolution
Convert the 30% strength into moles of H2O2 per 100 mL, use 2H2O2→2H2O+O2 to get the oxygen volume at STP, and express it per mL of solution — that number is the "volume strength".
1. What "volume strength" means
A hydrogen-peroxide solution labelled 'x volume' liberates x mL of O2 at STP from 1 mL of the solution on complete decomposition. So the whole problem is: how much O2 does 1 mL of a 30% solution give?
2. Decomposition stoichiometry
2H2O2⟶2H2O+O2
2 mol H2O2 (i.e. 2×34=68 g) give 1 mol O2 = 22.4 L at STP.
3. Take 100 mL of the solution
30% strength ⇒ 30 g of H2O2 in 100 mL of solution.
n(H2O2)=3430=0.882 mol
n(O2)=20.882=0.441 mol
V(O2)=0.441×22.4=9.88 L=9882 mL (at STP)
4. Per mL of solution …
- COMEDK 2023Set 2023-E1 markMCQQ.Choose the incorrect statement: (A) Higher the KH value for a gas at a given pressure, higher is its solubility in that solvent. (B) KH value for a gas present in a given solvent depends on the nature of solute and solvent. (C) KH value is temperature dependent. (D) KH value changes with change in the partial pressure of the gas.
›Reveal solutionSolution
Statement (A) is incorrect: a higher Henry's constant KH means LOWER solubility, not higher.
Henry's law: p=KH⋅x, where x is the mole fraction of dissolved gas. Rearranging, x=p/KH, so for a fixed partial pressure, a larger KH gives a smaller x (lower solubility).
Evaluating:
- (A) "Higher KH ⇒ higher solubility" — INCORRECT; it is the reverse (higher KH ⇒ lower solubility). This is the classic wrong statement being tested.
- (B) KH depends on the nature of gas and solvent — correct.
- (C) KH is temperature dependent (it increases with temperature) — correct. …
- COMEDK 2023Set 2023-M1 markMCQQ.Which of the following is incorrect regarding Henry's law? (A) Gas reacts with solvent chemically. (B) Pressure and concentrations are not too high. (C) Temperature is not too low. (D) Gas does not change its molecular state in solution i.e., neither dissociates nor associates.
›Reveal solutionSolution
Henry's law holds only when the dissolved gas physically dissolves without chemical reaction, at moderate pressure/concentration and not-too-low temperature, and without changing its molecular state (no dissociation/association). The statement that the gas reacts with solvent chemically violates the law, so it is the incorrect one.
Henry's law (p=KH⋅x) applies under the following conditions:
- Pressure and concentration are not too high — (B) is a valid condition.
- Temperature is not too low — (C) is a valid condition.
- The gas does not change its molecular state in solution, i.e. it neither dissociates nor associates — (D) is a valid condition. …
- KCET 2022Set B-31 markMCQQ.Which property of CO2 makes it biologically and geo-chemically important? (A) Its low solubility in water (B) Its high compressibility (C) Its acidic nature (D) Its colourless and odourless nature
›Reveal solutionSolution
The textbook-intended property is CO2's low solubility in water, not its acidic character on its own — that low solubility is exactly what makes the bicarbonate buffer system (blood pH regulation, ocean carbon cycling) possible.
Why not the other options. CO2 IS colourless/odourless and does have some compressibility, but neither of those properties explains its biological/geochemical significance. "Acidic nature" (forming carbonic acid) is real, but it's a consequence of how CO2 behaves in water, not the root property being tested here. …
- KCET 2022Set B-31 markMCQQ.Solubility of a gas in a liquid increases with (A) increase of P and decrease of T (B) decrease of P and decrease of T (C) increase of P and increase of T (D) decrease of P and increase of T
›Reveal solutionSolution
Henry's law makes gas solubility rise with pressure, and because dissolution of a gas is exothermic, Le Chatelier's principle makes it rise as temperature falls.
Step 1 — The pressure dependence: Henry's law.
p=KH⋅x
where p is the partial pressure of the gas above the solution, x its mole fraction in solution, and KH the Henry's-law constant. Rearranged:
x=KHp
So solubility x is directly proportional to pressure. Physically: higher pressure means more gas molecules striking the liquid surface per second, so more of them get captured until a new equilibrium is reached.
⇒ Increase P ⇒ increase solubility.
(Everyday proof: a soda bottle is sealed under high CO2 pressure; the instant you open it and the pressure drops, dissolved CO2 fizzes out.)
Step 2 — The temperature dependence: Le Chatelier.
Dissolution of a gas in a liquid is an exothermic process, because the gas molecules lose their kinetic freedom and are stabilised by solvent interactions:
Gas+Solvent⇌Solution+Heat(ΔH<0) …
- COMEDK 2022Set 20221 markMCQQ.Which of the following does not affect solubility of a gas in liquid? (A) Nature of gas and liquid (B) Pressure (C) Concentration (D) Temperature
›Reveal solutionSolution
'Concentration' is not an independent variable here - the concentration of the dissolved gas IS the solubility (the quantity being measured), not a factor that determines it. So concentration does not affect the solubility of a gas in a liquid.
Concept: Solubility of a gas in a liquid.
The factors that govern how much gas dissolves in a liquid are:
- Nature of the gas and of the solvent (like dissolves like; CO2 is far more soluble in water than O2 because it reacts/interacts with water).
- Pressure of the gas above the liquid - Henry's law, p = K_H * x, so solubility rises with partial pressure. …
- KCET 2021Set B-21 markMCQQ.Henry’s law constant for the solubility of N2 gas in water at 298 K is 1.0×105 atm. The mole fraction of N2 in air is 0.8. The number of moles of N2 from air dissolved in 10 moles of water at 298 K and 5 atm pressure is (A) 4.0×10−4 (B) 4.0×10−5 (C) 5.0×10−4 (D) 4.0×10−6
›Reveal solutionSolution
Get the partial pressure of N2 from Dalton's law, convert it to a mole fraction with Henry's law, then convert that mole fraction into moles dissolved in 10 mol of water.
Step 1 — Partial pressure of N2 (Dalton's law).
Henry's law uses the partial pressure of the gas, not the total pressure:
pN2=yN2×Ptotal=0.8×5=4 atm.
Step 2 — Henry's law.
pN2=KHxN2⟹xN2=KHpN2=1.0×1054=4×10−5.
The large KH tells us N2 is only sparingly soluble — the tiny mole fraction is expected.
Step 3 — Convert mole fraction to moles.
xN2=nN2+nH2OnN2≈nH2OnN2(since nN2⋘nH2O) …
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