Solubility & Henry's Law
Master the physical chemistry of solutions. Decode the rules of dissolution, the critical $K_H$ constant traps, and the physiological realities of scuba diving and high altitudes.
Module Focus: The Dynamics of Dissolution
Solubility is the maximum amount of a substance that can be dissolved in a specified amount of solvent at a given temperature. While the universal rule "Like dissolves like" applies broadly, NEET heavily tests the specific effects of Temperature (via Le Chatelier's and Thermodynamics) and Pressure (via Henry's Law) on different phases of solutes.
1. Solubility of Solids & Liquids in Liquids
The dissolution relies on intermolecular interactions. Polar solutes dissolve in polar solvents (e.g., $NaCl$ or glucose in Water), and non-polar solutes dissolve in non-polar solvents (e.g., Naphthalene or Anthracene in Benzene). This is the "Like dissolves like" rule.
Dissolution of solids is a dynamic equilibrium. We apply Le Chatelier's Principle:
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Endothermic Process ($\Delta_{sol} H > 0$): Heat is absorbed. Increasing temperature shifts equilibrium forward.
Solubility INCREASES with Temp. -
Exothermic Process ($\Delta_{sol} H < 0$): Heat is released. Increasing temperature shifts equilibrium backward.
Solubility DECREASES with Temp.
Pressure does not have any significant effect on solubility of solids in liquids, or liquids in liquids.
2. Solubility of Gases in Liquids
Unlike solids, the solubility of gases is heavily dependent on both Pressure and Temperature. Many gases dissolve in water (e.g., $O_2$ sustains aquatic life, $CO_2$ is highly soluble).
When a gas dissolves in a liquid, its molecules are restricted to a much smaller volume. This means entropy decreases ($\Delta S < 0$).
For a process to be spontaneous, Gibbs Free Energy must be negative ($\Delta G < 0$). Since $\Delta G = \Delta H - T\Delta S$, and $-T\Delta S$ is positive, $\Delta H$ must be highly negative to make $\Delta G$ negative.
Why do aquatic species feel more comfortable in cold water rather than warm water?
Because at lower temperatures, the solubility of oxygen gas ($O_2$) in water is higher. Cold water has more dissolved oxygen available for respiration.
3. Henry's Law (Effect of Pressure)
William Henry formulated a quantitative relationship between pressure and gas solubility.
"The partial pressure of the gas in vapor phase ($p$) is proportional to the mole fraction of the gas ($x$) in the solution."
Where $K_H$ is the Henry's Law constant. Its units are the same as pressure (e.g., atm, bar, Torr) because mole fraction ($x$) is unitless.
Rearranging the formula: $x = \frac{p}{K_H}$. Therefore, at a constant pressure, higher $K_H$ means LOWER solubility.
Different gases have different $K_H$ values in the same solvent. E.g., Helium has a very high $K_H$ (low solubility), while $CO_2$ has a lower $K_H$ (high solubility).
As Temperature increases, solubility decreases. Therefore, $K_H$ increases with an increase in Temperature.
A plot of $p$ versus $x$ gives a straight line passing through the origin. The slope of this line is $K_H$.
4. Biological & Commercial Applications
To increase the solubility of $CO_2$ in soft drinks and soda water, the bottle is sealed under high pressure. When opened, pressure drops, solubility drops, and it fizzes out.
At high underwater pressure, more $N_2$ dissolves in the blood. Upon surfacing, pressure drops, and $N_2$ bubbles out in capillaries, causing painful and dangerous "Bends".
Solution: Tanks are diluted with 11.7% Helium (very low solubility / high $K_H$).
At high altitudes, the partial pressure of $O_2$ is less than at ground level. This leads to low concentrations of dissolved $O_2$ in the blood and tissues, causing weakness and inability to think clearly, a condition known as Anoxia.
NEET Grand Test: Solubility
15 High-Yield Questions testing KH formulas, thermodynamics, and physiological applications.
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