Why Does Sugar Stop Dissolving? The Liquid Is Full

Dissolving is not an unlimited process, because it is a balance between two opposing rates. Particles leave the solid and enter the liquid, and particles already in the liquid collide with the solid and rejoin it, and as the concentration in the liquid rises the second rate increases until it matches the first. At that point the amounts dissolving and rejoining are equal, so nothing further appears to happen and any additional solid sits at the bottom unchanged, while individual particles continue exchanging in both directions. That is a dynamic balance rather than a stop, which is why a saturated solution left with excess solid slowly grows larger crystals from smaller ones.

The amount a liquid will hold depends on several conditions:

• •Temperature, which raises the limit for most solids and lowers it for gases
• •The identity of both the substance and the liquid, which varies enormously
• •Pressure, which matters greatly for gases and almost nothing for solids
• •The presence of other dissolved substances, which can raise or lower it
• •Particle size and stirring, which change the speed but not the final amount
• •Acidity, which changes the limit for many substances substantially

A solution can be persuaded to hold more than its limit, which is unstable and produces a good demonstration. Heating a liquid raises the limit, so more solid dissolves, and cooling it slowly and without disturbance frequently fails to precipitate the excess, because forming the first crystal requires particles to assemble into an arrangement large enough to keep growing, which is improbable in a clean undisturbed liquid. The result holds more than it should and remains clear until something starts the process. Adding a single crystal, scratching the container or simply knocking it triggers crystallisation that races through the whole volume in seconds and releases heat, which is exactly how reusable hand warmers work.

Gases dissolve in liquids by the same balance and respond to conditions in the opposite direction, which is worth stating because it causes confusion. Heating a liquid drives dissolved gas out rather than allowing more in, which is why bubbles appear on the inside of a pan long before it boils and why boiled water tastes flat. Pressure raises the amount a liquid holds, which is the whole basis of carbonated drinks, where gas is dissolved under pressure and comes out of solution when the container is opened and the pressure drops. Deep water holds more dissolved gas than surface water for the same reason, which is why a diver ascending too fast develops bubbles in the blood.

The behaviour underlies a great deal of practical chemistry and everyday experience. Purification by recrystallisation dissolves a substance hot, filters out insoluble impurities and cools slowly, so the wanted material crystallises while soluble impurities remain dissolved, and it is the standard method for cleaning up a solid product. Sugar confectionery depends entirely on controlling this, since fudge, fondant and boiled sweets differ mainly in whether and how crystallisation is allowed to happen. Kidney stones form when urine holds more of certain substances than it should. Scale in kettles and pipes forms as heating lowers the limit for the compounds involved. And cave formations grow as water loses carbon dioxide and can no longer hold its dissolved load.

The limit is a balance between particles leaving the solid and rejoining it, so a saturated solution is exchanging continuously rather than sitting still. Temperature raises the limit for most solids and lowers it for gases. Careful cooling can hold more than the limit until a crystal, a scratch or a knock starts crystallisation racing through the volume. Recrystallisation, confectionery, kidney stones and kettle scale all follow from this.