Solubility curves are graphical representations that plot the maximum mass of solute (y-axis) against the temperature of the solvent (x-axis).
Each point along the curve represents a perfectly saturated solution at that specific temperature; points below the curve indicate an unsaturated solution.
If a hot saturated solution is cooled, the solubility decreases, and the excess solute can no longer remain dissolved, causing it to precipitate or form crystals.
To calculate the mass of crystals formed during cooling, subtract the solubility value at the lower temperature from the solubility value at the higher temperature, then adjust for the actual mass of solvent used.
To determine solubility experimentally, one must first prepare a saturated solution by adding solute to a solvent at a controlled temperature until undissolved solid remains at the bottom.
A known mass of the clear, saturated liquid is transferred to a pre-weighed evaporating basin, ensuring no undissolved solid is accidentally moved.
The solvent is then carefully evaporated through gentle heating until only the dry solute remains. Rapid heating should be avoided to prevent "spitting," which causes loss of the solid product.
The technique of heating to constant mass is employed: the basin is repeatedly heated, cooled, and weighed until two consecutive measurements are identical, confirming all moisture has been removed.
| Feature | Solubility of Solids | Solubility of Gases |
|---|---|---|
| Temperature Effect | Increases with Temperature | Decreases with Temperature |
| Pressure Effect | Negligible change | Increases with Pressure |
| Measurement | g per 100g solvent | Depends on P and T |
Unit Awareness: Always verify the units on solubility graphs; if a question asks for 50g of water but the graph is for 100g, you must divide the solubility value by two.
Crystal Formation: When calculating crystals formed, the formula is: .
Precise Terminology: Use terms like 'saturated', 'solute', and 'precipitate' correctly. Avoid saying a substance 'disappears' when it dissolves; instead, state that it forms a clear solution.
Reasoning Checks: If you calculate a value, ask yourself if it makes sense. For instance, if temperature increases for a solid, the solubility you find should almost always be higher than the initial value.
Solvent vs. Solution Mass: A common error is using the mass of the entire solution in solubility calculations instead of just the mass of the solvent (water). Solubility is always relative to the solvent mass.
Incomplete Evaporation: Failing to heat to constant mass results in an overestimation of solute mass, as trapped water molecules add to the weight of the supposedly 'dry' solid.
The 'Disappearing' Solid: Students often think an insoluble substance might dissolve if given enough time; however, solubility is a physical limit. Stirring or waiting will not dissolve an insoluble substance beyond its saturation point.
