BrightKidz Library
Subjects
Sugar crowding the water at a crystal's surface, and a current sweeping that crowded water away Two tanks of water seen from the side, each holding the same slab of sugar along the bottom. In the still tank on the left, the sugar that has already come off has nowhere to go, so it piles up in a crowded layer pressed against the slab, two deep and almost touching. Water that full cannot take much more, so the slab has nearly stopped losing sugar, and only three specks have escaped into the clear water higher up. In the stirred tank on the right, two long arrows show the water moving steadily past, and the same twenty three specks are now spread evenly through the whole tank. The water lying against the slab is fresh and empty again. Nothing has touched the slab. What stirring changes is which water is in contact with it.

The Sugar Did Not Disappear

About 12 minutes

Put a spoon of sugar into a glass of water and stir. Within half a minute there is nothing to see. No cloud, no grains on the bottom, no film on top. It looks exactly like the water you started with.

Two things are worth asking about that. Where did the sugar go, and what did the stirring actually do.

The second question has an answer that surprises nearly everybody.

Nothing left the glass

Start with where it went. Taste the water: the sugar is still in there.

You can do better than tasting. Put the glass of water on a kitchen scale with the dry sugar sitting beside it in the spoon, and write the number down. Tip the sugar in, stir until it is gone, and read the scale again. It has not moved.

Whatever happened, it was not a disappearance. It was a change in how the sugar is arranged.

A crystal is a stack, and water takes it apart from the outside

A grain of sugar is not one lump of stuff. It is an enormous number of sugar molecules stacked in a regular pattern, each one held in place by its neighbours. That is what makes it a crystal, and it is why sugar grains have flat faces and sharp corners if you look at them closely.

Water molecules are constantly bumping into that stack. Most bumps do nothing. But a molecule sitting at the surface of the crystal is held by fewer neighbours than one buried inside, and every so often a bump is enough to knock one loose. Water molecules crowd round it, and it drifts off among them.

That is dissolving. The sugar molecules are not broken, not burnt, not turned into anything else. They are simply taken out of the stack one at a time and spread through the water, and they end up so far apart and so small that light goes straight past them. Nothing here is invisible. It is just too finely scattered to see.

Two things follow from this, and both are worth holding on to.

Only the surface can dissolve. The molecules deep inside the grain are not touching any water at all, and nothing can happen to them until the layers above them have gone.

So more surface means faster. That is the entire reason icing sugar vanishes almost instantly while a sugar cube sits there. Break the same amount of sugar into finer pieces and you have not changed how much sugar there is, only how much of it is exposed.

What is happening at one spot on the surface of a grain, in a glass nobody is stirring.

  1. Water molecules bump into the crystal thousands of times a second. Nearly all of these bumps achieve nothing.
  2. One bump lands on a molecule at the surface, which is held by fewer neighbours than the ones underneath. It comes free, and water molecules gather round it.
  3. The loosened molecule drifts a short distance away and stops there, because in still water there is nothing to carry it further. Its neighbours are doing the same. The water pressed against the crystal is filling up with sugar.
  4. Now the bumps are arriving from water that is already crowded. Sugar keeps coming off, but sugar keeps landing back on too, and the two very nearly cancel. The crystal has almost stopped shrinking, and it is not because it ran out of water.

So what does stirring do

Ask around and you will be told that stirring breaks the sugar up, or shakes the grains apart, or rubs them down.

None of that is right. A spoon is unimaginably clumsy next to a sugar molecule. Sweeping a spoon past a grain to break its molecules apart would be like sweeping a hand across a beach to break the sand into smaller sand.

Look back at stage 4 instead. The thing stopping the crystal is the crowded layer of water pressed right against it. Once that layer is full of sugar, it cannot take much more, and the crystal sits in a small pocket of sweet water going nowhere.

Stirring drags that pocket away.

That is all it does, and it is enough. Fresh water, with room in it, arrives at the surface. The bumping starts working again, more sugar comes free, and the moment the water there begins to fill up you sweep it off once more.

You are not moving the sugar. You are changing the water that is touching it.

The test that proves it

  1. Fill both glasses with the same amount of cold water. Same glasses, same water, same everything.
  2. Put one level teaspoon of sugar into each. Try not to jog them.
  3. Stir the first glass steadily. Do not touch the second at all. Note how long the stirred one takes to go clear.
  4. Now leave the second glass completely alone and go and do something else. Come back and look at it every ten minutes.
  5. Write down how long each one took.

The stirred glass takes under a minute. The still glass takes a long time, and here is the result that matters: it gets there too.

If stirring worked by breaking the sugar up, the glass nobody stirred could never clear at all, because nothing ever broke anything up in it. Instead it clears slowly, because dissolved sugar does gradually wander away from the crystal on its own. Stirring is not doing a different job from waiting. It is doing the same job, faster.

Why warm water is quicker, and where the limit is

Warm water works better for two separate reasons, and they are worth keeping apart.

Its molecules move faster, so they bump the crystal harder and more often, and knock more molecules loose every second. That is speed.

Warm water also holds more sugar in total before it fills up. That is a different thing: not how fast, but how much.

Because there is a limit. Keep adding sugar to a glass and stirring, and eventually you reach a point where no more will go in, however long you stir. Sugar sits on the bottom and stays there. The water is full, and the word for that is saturated. Stirring cannot help now, because the problem is no longer the crowded layer round the crystal. Every part of the water is crowded.

That limit is why syrup makes crystals. Dissolve as much sugar as hot water will take, then let it cool. Cool water cannot hold as much, so the sugar it can no longer keep has to come back out and rebuild itself into crystals. Rock sweets grown on a string are made exactly this way, and so is the gritty crust in the bottom of an old jar of honey.

Two identical glasses of water each get a sugar cube. One is stirred hard, the other is left alone for a whole afternoon. What happens?

What to try next

Race a sugar cube against the same amount of granulated sugar, in two glasses of cold water, neither of them stirred. You have taken stirring out of it entirely, so the only difference left is how much surface each one has.

Then try to guess, before you start, roughly how much longer the cube will take. Being wrong about that is useful. It tells you something about how much of a cube is buried where the water cannot reach.