Solubility is the largest amount of a substance that dissolves at a given temperature. For solids it depends mainly on temperature. For gases, Henry’s law states that solubility is proportional to the gas pressure.
Add sugar to water and stir. After a certain amount, extra sugar stays undissolved at the bottom. The solubility of a substance is the maximum amount that dissolves in a given amount of solvent at a given temperature. For sugar in water at 20°C, it is about 200 g per 100 g of water.
When undissolved solid is present, two processes continue at once. Solid dissolves, and dissolved solute crystallises back onto the solid. At the solubility limit the two rates are equal, so the amount dissolved stays constant. This is a dynamic equilibrium, and the solution is saturated. A solution with less solute than this is unsaturated.
Solute + Solvent ⇌ Solution
Which substances dissolve in which follows a general rule: like dissolves like. Polar and ionic solutes dissolve well in polar solvents; sodium chloride dissolves in water. Non-polar solutes dissolve well in non-polar solvents; naphthalene dissolves in benzene. A solute dissolves when the attractions it forms with solvent molecules are comparable to the attractions broken in the solute and in the solvent.
The saturated solution is in equilibrium, so Le Chatelier’s principle applies: when a condition is changed, the equilibrium shifts in the direction that partly counteracts the change.
If dissolving takes in heat (endothermic), raising the temperature shifts the equilibrium towards dissolving, and solubility increases. If dissolving releases heat (exothermic), raising the temperature shifts it the other way, and solubility decreases. For most ionic solids in water, dissolving is endothermic, so they are more soluble when hot.
Pressure has almost no effect on the solubility of a solid in a liquid. Solids and liquids are nearly incompressible, so increasing the pressure hardly changes them.
Dissolving potassium nitrate in water is endothermic. Using Le Chatelier’s principle, predict whether it is more soluble in hot or cold water.
Hot water.
Heat is taken in when the solid dissolves, so the equilibrium can be written: solute + solvent + heat ⇌ solution. Raising the temperature supplies heat. The equilibrium shifts to the right, which uses up some of that heat. More solid dissolves.
Measured values agree. About 32 g of potassium nitrate dissolves in 100 g of water at 20°C, and about 110 g at 60°C.
For a gas dissolving in a liquid, pressure has a large effect. Increasing the pressure of a gas above a liquid increases the number of gas molecules striking the liquid surface per second. More of them enter the liquid, and at equilibrium more gas is dissolved.
Soft drinks are bottled with carbon dioxide at a pressure of a few atmospheres, which dissolves a large amount of CO₂. When the bottle is opened, the pressure above the liquid falls to 1 atmosphere. The solution now contains more CO₂ than it can hold at that pressure, and the excess leaves as bubbles.
Henry’s law states: at constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. A common form is:
p = KH x
p is the partial pressure of the gas. x is its mole fraction in the solution, a measure of how much has dissolved. KH is Henry’s law constant, which depends on the gas, the solvent and the temperature. A graph of p against x is a straight line through the origin. For a given pressure, a larger KH gives a smaller x, so a larger KH means a less soluble gas.
Henry’s law describes gases that do not react with the solvent. It does not apply to ammonia or hydrogen chloride in water, which react to form ions.
A warm bottle of soda releases gas more vigorously when opened than a cold one. Explain using the effects of pressure and temperature on gas solubility.
Pressure: in the sealed bottle, the CO₂ pressure is high, so by Henry’s law a large amount is dissolved. Opening the bottle lowers the pressure, so the solution now holds more CO₂ than it can at equilibrium, and gas comes out.
Temperature: dissolving a gas in water is exothermic. Raising the temperature therefore shifts the equilibrium away from dissolving, so gases are less soluble in warm water. KH is larger at higher temperature.
In a warm bottle, the equilibrium amount of dissolved CO₂ at 1 atmosphere is smaller, so the excess that must leave is larger and leaves faster. The same temperature effect means cold water holds more dissolved oxygen than warm water, which matters for fish and other aquatic animals.
1A saturated solution is one in which…
2The rule “like dissolves like” means…
3Most ionic solids are more soluble in hot water because their dissolving is…
4Henry’s law states that, at constant temperature, the solubility of a gas in a liquid is proportional to…
5Why are soft drinks bottled under high CO₂ pressure?
6Gases are generally less soluble in warm water than in cold water. One consequence is that…
Next: vapour pressure above a solution, and Raoult’s law.
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