Solubility
Quick Notes
- Solubility: Maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature and pressure.
- Solid in liquid: Solubility often increases with temperature; depends on intermolecular forces.
- Gas in liquid: Solubility decreases with temperature and increases with pressure.
- Henry’s Law: p = KH × x
- p = partial pressure of the gas
- x = mole fraction of the gas in solution
- KH = Henry’s law constant
- Applications: Fizzy drinks, scuba diving, altitude adaptation.
Full Notes
Solubility determines how much of one substance (solute) can dissolve in another (solvent). It is influenced by the chemical nature of both substances, temperature, and pressure. This property is critical in natural processes (like gas exchange in the environment) and industrial applications (such as pharmaceuticals and beverage production).
Solubility of Solid in a Liquid
When a solid dissolves in a liquid, a dynamic equilibrium is established between dissolved and undissolved particles. The amount of solute at saturation defines the solubility.
For example:
When iodine (I2) is added to water, only a small amount dissolves. As this happens, an equilibrium is set up between the undissolved solid and the iodine molecules in solution:
I2(s) ⇌ I2(aq)
Initially, solid iodine dissolves into water. Over time, some of the dissolved iodine also starts to come out of solution and re-form the solid. At equilibrium, these two opposing processes — dissolution and precipitation — occur at the same rate, so the concentration of dissolved iodine remains constant, even though the process is still ongoing at the molecular level.
Types of solutions:
Saturated solution:
Cannot dissolve more solute at a given temperature and pressure.
Unsaturated solution:
Can dissolve more solute at the same temperature.
Solubility = concentration of solute in a saturated solution.
Factors affecting solubility of solids:
- Effect of Temperature:
- Governed by Le Chatelier’s Principle:
- If dissolution is endothermic (ΔHsol > 0): Solubility increases with temperature.
- If dissolution is exothermic (ΔHsol < 0): Solubility decreases with temperature.
- Effect of Pressure: Pressure has negligible effect on solubility of solids in liquids. Reason: Solids and liquids are incompressible.
Solubility of Gas in a Liquid
Gases dissolve in liquids depending on the gas's nature, the liquid, and external conditions such as temperature and pressure.
Solubility of gases increases with an increase in pressure and a decrease in temperature.
Effect of Pressure:
The effect of pressure on gas solubility is explained by Henry’s Law: “The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas over the solution.”
- Higher KH means lower solubility
- Different gases have different KH values at the same temperature.
Henry’s law constant can be found by plotting how the mole fraction of a gas changes with partial pressure. It is the gradient of the straight line graph.
Effect of Temperature:
Solubility of gases decreases with increase in temperature.
This is because dissolution is exothermic (like condensation), hence solubility drops as temperature rises (Le Chatelier’s Principle).
Applications of Henry’s Law (Expanded)
Example Soft Drinks and Soda Bottles
Carbon dioxide (CO2) is dissolved in soft drinks under high pressure. According to Henry’s Law, the higher the pressure of CO2 gas above the liquid, the more CO2 dissolves in the liquid. When the bottle is sealed, the gas remains dissolved. On opening the bottle, pressure is released, causing CO2 to come out of solution rapidly—seen as fizzing or bubbling.
Example Scuba Diving and the Bends
Underwater, divers are exposed to high external pressure, which increases the solubility of gases like nitrogen (N2) in their blood. As per Henry’s Law, more nitrogen dissolves in the blood at high pressure. Upon rapid ascent, the pressure decreases suddenly, and the dissolved nitrogen comes out of solution in the form of bubbles in the bloodstream. These nitrogen bubbles can block capillaries and cause a painful and potentially dangerous condition called decompression sickness or "the bends". To avoid this, divers ascend slowly to allow gradual release of nitrogen and use special gas mixtures (e.g. heliox) containing helium, nitrogen, and oxygen—helium is less soluble and causes fewer problems than nitrogen.
Example High Altitudes and Anoxia
At high altitudes (e.g., in mountains or during air travel), atmospheric pressure is lower than at sea level. According to Henry’s Law, the solubility of oxygen (O2) in blood decreases at lower pressure. This leads to lower oxygen availability to body tissues, a condition called anoxia. Symptoms include fatigue, dizziness, confusion, and breathlessness. This is why climbers and people living at high altitudes may need time to acclimatize or use supplemental oxygen.
At 298 K, Henry’s law constant for oxygen in water is 1.20 × 105 atm. Calculate the mole fraction of oxygen in water at 1 atm pressure.
- Write Henry’s law: p = KH × x so x = p / KH
- Substitute values: x = 1.0 / (1.20 × 105)
- Calculate: x = 8.33 × 10−6
Summary
- Solubility is the concentration of solute in a saturated solution at specific temperature and pressure.
- For solids in liquids temperature can increase or decrease solubility depending on the sign of ΔHsol.
- Gases are more soluble at higher pressure and lower temperature.
- Henry’s law relates gas solubility to partial pressure with p = KH × x.
- Real-world applications include carbonated drinks scuba diving safety and high-altitude physiology.