Properties of Solutions

A solubility curve is a graph of how many grams of a solute dissolve in 100 grams of water at each temperature. For most solids, the curve rises as temperature rises, which is why sugar disappears fast into hot tea: the hot water can hold far more dissolved sugar than cold water can, and the added heat also speeds up mixing. Drop the same sugar into iced tea and the curve's limit at that low temperature is much smaller, so solid sugar is left sitting on the bottom no matter how long you stir.

Reading the curve tells you the solution's state. If the dissolved amount is below the curve's value at that temperature, the solution is unsaturated and could still take more solute. If it sits exactly at the curve, it is saturated: the liquid holds the maximum, and any extra solute added stays solid. If a solution somehow holds more than the curve allows, usually by dissolving solute hot and cooling it carefully, it is supersaturated, an unstable state that crashes back to solid with the slightest disturbance.

Gases break the solid pattern: their solubility in water falls as temperature rises, because faster-moving molecules escape the liquid surface more easily, which is why a warmed soda fizzes and goes flat faster. Dissolved particles of any kind also change the solvent's other properties: they lower vapor pressure, which raises the boiling point above 100 degrees Celsius and lowers the freezing point below 0. These shifts connect solubility to the broader idea that a solution's physical behavior depends on how much solute is mixed in, not just what the solute is.