What if sugar would not dissolve?

Put a spoon of sugar in water. Stir, stir, stir!

The sugar seems to vanish. But the water tastes sweet. What if sugar would not dissolve?

Drop a spoonful of sugar into a glass of water and stir. The white grains swirl, shrink and then seem to vanish, but the water is still sweet, so the sugar must still be there.

Where did it go? This lab is a thought experiment that asks what if sugar would not dissolve. To answer it, we need to zoom in on what dissolving really means, and then zoom back out to sweet drinks and sparkling sugar crystals.

Britannica describes sucrose as colorless, sweet-tasting crystals that dissolve in water. Ordinary table sugar is sucrose, C₁₂H₂₂O₁₁, found naturally in sugarcane, sugar beets, maple sap, dates and honey.

This thought experiment asks what would change if sugar were insoluble. It uses the question to explore what happens at the molecular level when a solid dissolves, why there is a limit to how much will dissolve, and how dissolved sugar can be coaxed back into crystals.

Sugar is made of tiny, tiny bits. They are much too small to see.

In water, the bits spread out everywhere. That's why every sip tastes sweet.

Sugar is made of molecules, tiny particles far too small to see. A grain of sugar is a huge pile of them stuck together. In water, the sugar molecules break away from the grain and spread out evenly among the water molecules.

The mixture is now a solution. The water is called the solvent and the sugar is the solute. Even though sugar molecules are heavier than water molecules, they don't sink back to the bottom. Stirring speeds things up, but the sugar would spread out eventually even without it.

Table sugar is a covalent solid of sucrose molecules. When it dissolves, its molecules become uniformly distributed among water molecules; the subscript (aq) marks them as individually dispersed solutes in an aqueous solution, each surrounded by solvent.

Although sucrose molecules are heavier than water molecules, gravity does not make them settle out. Dissolution is a spontaneous process: stirring speeds it but is not required, and an increase in matter dispersal always accompanies forming a solution (OpenStax Chemistry 2e).

Pretend sugar would not dissolve. You stir and stir. The sugar just sinks to the bottom.

Your drink might be gritty, like sand in water!

Imagine stirring sugar into lemonade and watching it swirl around and then settle on the bottom like sand. Every sip might be gritty, and the sweetness might stay stuck at the bottom of the glass.

Candy might change too, because candy makers start by dissolving sugar in water. And sugar crystals couldn't grow from a sugary solution if there were no solution to start with. These are ideas, worked out from how dissolving works today.

Insoluble sugar would behave like sand in water: a suspension that settles, not a solution. Sweetened drinks, syrups and much of confectionery, which depends on dissolving sucrose and controlling its recrystallization, would not work as they do.

These consequences are reasoned speculation. Real sucrose is highly soluble, and its solubility rises with temperature, which is exactly what makes syrups and crystal growing possible.

You can't dissolve sugar forever. At last some sugar stays at the bottom.

Warm water can hold more sugar than cold water.

Keep adding sugar and stirring, and at some point the extra sugar won't dissolve, no matter how long you stir. The water is saturated: it's holding all the sugar it can at that temperature. The amount it can hold is called solubility.

Warmer water can hold more dissolved sugar. When very sugary hot water cools, it ends up holding more sugar than it normally could. That is called supersaturated, and the extra sugar is ready to come back out as crystals.

Add sugar to water until undissolved sugar remains however long you stir: the concentration is then the solubility, and the solution is saturated. That limit depends on the relative strengths of solute-solute, solute-solvent and solvent-solvent attractions.

For most solids, solubility increases with temperature (with exceptions). A solution saturated while hot and cooled without precipitating becomes supersaturated, a nonequilibrium state. Adding a seed crystal or agitating it can start precipitation of the excess.

Sugar grains look like tiny cubes. Candy makers cook sugar and water into syrup.

As it cools, sugar can grow back into crystals. That's how rock candy is made!

Look closely at dry sugar and you'll see little cube-like crystals. Under a microscope, they're actually long and slanted at both ends.

Rock candy is made by dissolving lots of sugar in hot water, about three times as much sugar as water, then letting it cool with a string or stick that has a few sugar grains on it. The extra sugar comes out of the solution and builds crystals on the string. Other candies, like lollipops and caramels, are made smooth by stopping crystals from forming.

Confectionery splits into crystalline candies such as fudge and fondant and noncrystalline ones such as lollipops, taffy and caramels. Cooked sugar syrup cools into an unstable supersaturated state that crystallizes at the least provocation, so noncrystalline recipes add fructose and glucose (from acid inversion or corn syrup) or butter to get in the way of sucrose molecules locking together.

Rock candy exploits the opposite: a 3:1 sugar-to-water mixture dissolved with heat becomes supersaturated as it cools, and sugar crystallizes on seeded strings or sticks.