Q.Maximum amount of a solid solute that can be dissolved in a specified amount of a given liquid solvent does not depend upon ____________.
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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 …
The maximum amount of a solid solute that can dissolve in a given liquid solvent is known as its solubility. This solubility is determined by several factors.
- Temperature: For most solid solutes, solubility in a liquid solvent increases with increasing temperature, as the dissolution process is often endothermic.
- Nature of solute and solvent: The principle of "like dissolves like" dictates that polar solutes dissolve well in polar solvents, and non-polar solutes dissolve well in non-polar solvents. This highlights the dependence on the chemical nature of both components. …
The solubility of a solid in a liquid is largely unaffected by changes in pressure because solids and liquids are nearly incompressible. The correct option is (iii).
When we talk about the maximum amount of a solid solute that can dissolve in a specified amount of a liquid solvent, we are discussing its solubility - a dynamic equilibrium between dissolution and crystallisation.
Let's examine each option:
-
(i) Temperature - Dissolution of most solids is endothermic, so increasing temperature shifts equilibrium toward more dissolution (a few solids are exothermic and behave oppositely, but temperature still matters). Solubility does depend on temperature.
-
(ii) Nature of solute - 'Like dissolves like': polar solutes dissolve well in polar solvents, non-polar in non-polar. Solubility does depend on the nature of the solute. …
Concept: Factors Affecting Solubility of a Solid in a Liquid
Solubility of a solid in a liquid is defined as the maximum amount of solute that can dissolve in a given amount of solvent at a specified temperature to form a saturated solution.
Method: Factor Elimination Method
Steps:
-
Identify the system — Here, the solute is a solid and the solvent is a liquid.
-
Recall the key factors that affect solubility for this specific system:
- Temperature — For most solids, solubility increases with temperature (e.g., sugar in water). ✓ Affects solubility.
- Nature of solute — Polar solutes dissolve in polar solvents; non-polar in non-polar. ✓ Affects solubility.
- Nature of solvent — Same logic as above. ✓ Affects solubility. …
Here is the breakdown of the common mistakes students make on this specific concept, along with how to avoid them.
The Core Concept: Solubility of Solids in Liquids
The question tests your understanding of solubility — specifically, the factors that affect how much of a solid can dissolve in a liquid.
The correct answer is (iii) Pressure.
- Why? Pressure has a significant effect on the solubility of gases in liquids (Henry's Law), but it has a negligible effect on the solubility of solids and liquids in liquids. Solids and liquids are nearly incompressible, so changing pressure doesn't change the space available for solute particles to fit.
Common Mistake #1: Confusing "Solid in Liquid" with "Gas in Liquid"
The Mistake:
Students often pick Pressure as a factor that does affect solubility, because they remember the general rule: "Solubility increases with pressure." This is true, but only for gases.
How to Avoid It:
- Create a mental checklist by state of matter. When you see "solubility," immediately ask: Is the solute a solid, liquid, or gas?
- Memorise the two key rules side-by-side:
- Solids in liquids: Solubility depends on Temperature and Nature (of both solute and solvent). Pressure is NOT a factor.
- Gases in liquids: Solubility depends on Temperature (inversely) and Pressure (directly, via Henry's Law). The nature of the gas and solvent also matters.
Common Mistake #2: Forgetting that "Nature" is a Factor
The Mistake:
Some students might incorrectly eliminate options (ii) or (iv), thinking that "nature" is too vague or not a real factor. They might think only temperature matters.
How to Avoid It:
- Understand "Like Dissolves Like." The nature of the solute and solvent is the most fundamental factor. Polar solutes (like sugar) dissolve in polar solvents (like water). Non-polar solutes (like wax) dissolve in non-polar solvents (like kerosene). This is a direct dependence on nature.
- Remember the definition: Solubility is a specific property of a pair of substances. You cannot talk about the solubility of "salt" without specifying the solvent (e.g., "in water" vs. "in oil").
Common Mistake #3: Misinterpreting the Effect of Temperature
The Mistake:
A student might think temperature is not a factor because they recall that for some solids (like Ca(OH)X2 or CeX2(SOX4)X3), solubility decreases with an increase in temperature. They then incorrectly conclude temperature doesn't affect it.
How to Avoid It: …
- 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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