Solubility of Gas in Liquids — Revision Notes

⚡ 30-Second Revision

Henry's Law: — $P = K_H x$ (Partial pressure $propto$ Mole fraction in solution)
$K_H$: — Henry's Law constant; higher $K_H$ means lower solubility.
Temperature Effect: — Gas solubility generally decreases with increasing temperature (exothermic process).
Pressure Effect: — Gas solubility increases with increasing partial pressure of the gas.
Nature of Gas/Solvent: — 'Like dissolves like'; reactive gases (e.g., $ext{NH}_3$ in $ext{H}_2\text{O}$) show high solubility.
Units: — Ensure consistency (e.g., atm, bar, Pa) for $P$ and $K_H$.

2-Minute Revision

The solubility of a gas in a liquid is governed by three main factors: pressure, temperature, and the nature of the gas and solvent. Henry's Law is paramount, stating that the partial pressure of a gas ($P$) above a liquid is directly proportional to its mole fraction ($x$) in the solution, given by $P = K_H x$.

Remember, a higher Henry's Law constant ($K_H$) signifies lower solubility. For example, gases with high $K_H$ like $ext{He}$ are sparingly soluble. Temperature has an inverse effect: as temperature increases, gas solubility generally decreases because the dissolution process is typically exothermic.

This is why aquatic life thrives in colder water with more dissolved oxygen. Finally, the 'like dissolves like' principle applies; polar gases are more soluble in polar solvents, and gases that chemically react with the solvent (e.

g., $ext{NH}_3$ in water) show exceptionally high solubility, often beyond what Henry's Law predicts for physical dissolution. Be prepared for numerical problems on Henry's Law and conceptual questions on these factors and their real-world implications.

5-Minute Revision

Let's quickly recap the solubility of gases in liquids, a key topic for NEET. At its core, it's about how much gas can dissolve in a liquid under specific conditions. The most crucial quantitative relationship is Henry's Law: $P = K_H x$.

Here, $P$ is the partial pressure of the gas above the liquid, $x$ is its mole fraction in the solution, and $K_H$ is Henry's Law constant. A critical point to remember is the inverse relationship: a *higher* value of $K_H$ means *lower* solubility for that gas in that solvent at that temperature.

For instance, if you're given $K_H$ values for different gases, the one with the largest $K_H$ will be the least soluble.

Temperature's effect is also vital. Unlike most solids, gas solubility *decreases* as temperature increases. This is because the dissolution of a gas is usually an exothermic process. According to Le Chatelier's Principle, adding heat (increasing temperature) shifts the equilibrium to favor the reverse, endothermic process, which is the gas escaping from the liquid.

Think of a warm soda going flat faster than a cold one, or fish struggling in thermally polluted water due to less dissolved oxygen.

Finally, the nature of the gas and solvent matters. The 'like dissolves like' rule holds: polar gases (like $ext{HCl}$, $ext{NH}_3$) are more soluble in polar solvents (like water) due to stronger intermolecular forces or even chemical reactions.

Non-polar gases ($ext{O}_2$, $ext{N}_2$) are less soluble in water. Gases that chemically react with the solvent, such as $ext{NH}_3$ forming $ext{NH}_4\text{OH}$ in water, exhibit exceptionally high solubility.

Be ready for numerical problems applying $P = K_H x$ (ensure unit consistency!) and conceptual questions on temperature effects and real-world applications like deep-sea diving (decompression sickness) or high-altitude sickness (anoxia).

Practice converting pressure units and interpreting $K_H$ values correctly.

Prelims Revision Notes

Solubility of Gas in Liquids: NEET Quick Notes

1. Definition: Maximum amount of gas that dissolves in a given liquid at specific T & P.

2. Henry's Law:

* Statement: At constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid surface. * Formula: $P = K_H cdot x$ * $P$: Partial pressure of the gas (e.

g., atm, bar) * $x$: Mole fraction of the gas in the solution (dimensionless) * $K_H$: Henry's Law constant (same units as $P$) * **Interpretation of $K_H$:** A *higher* $K_H$ value means *lower* solubility of the gas.

* Limitations: Applies to dilute solutions, low pressures, non-reacting gases, and non-dissociating/associating gases. * Applications: * Carbonated drinks: $ext{CO}_2$ dissolved under high pressure; escapes when pressure released.

* Deep-sea diving (Bends): High pressure increases $ext{N}_2$ solubility in blood; rapid ascent causes $ext{N}_2$ bubbles. * High altitude (Anoxia): Low $P_{\text{O}_2}$ leads to low $ext{O}_2$ solubility in blood.

3. Effect of Temperature:

* General Rule: Solubility of gases in liquids *decreases* with an *increase* in temperature. * Reason: Dissolution of gas is typically an *exothermic* process ($ext{Gas(g)} + \text{Liquid(l)} \rightleftharpoons \text{Solution(aq)} + \text{Heat}$). By Le Chatelier's Principle, increasing temperature shifts equilibrium to the left (desorption). * Implication: Aquatic life prefers colder water due to higher dissolved oxygen levels.

4. Effect of Nature of Gas and Solvent:

* 'Like dissolves like': Polar gases ($ext{HCl}$, $ext{NH}_3$) are more soluble in polar solvents ($ext{H}_2\text{O}$). Non-polar gases ($ext{O}_2$, $ext{N}_2$) are less soluble in polar solvents.

* Molecular Size: Larger gas molecules tend to be more soluble due to stronger London dispersion forces. * Chemical Reactivity: Gases that react with the solvent (e.g., $ext{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4\text{OH}$) show exceptionally *high* solubility, as the reaction removes the dissolved gas, driving the equilibrium forward.

5. Key Problem-Solving Tips:

* Always ensure consistent units for pressure ($P$) and Henry's constant ($K_H$). * Remember the inverse relationship between $K_H$ and solubility. * Distinguish between physical dissolution (Henry's Law) and chemical reaction leading to high 'solubility'.

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Pressure: Increases solubility
Temperature: Decreases solubility
Nature: 'Like dissolves like' (polarity, reactivity)
Henry's Law: $P = K_H x$
KH: Higher $K_H$ means lower solubility