Boiling the Sugar Syrup

When sugar crystals get into contact with enough water, they break down. The sugar dissolves. However, it only does so, if there is enough water. If the sugar solution has not yet been saturated.

But what is ‘enough’ water?

That depends. More specifically, it depends on the temperature of the water.

Hot = Faster movement of molecules

When you increase the temperature of something, you’re increasing the speed at which molecules move. The warmer something is, the faster the molecules move. They can vibrate, shake, or move through spaces. The hotter it is, the more energy they contain, the faster they go.

So, when you heat up water, the water molecules in that water will start moving more quickly. If sugar is dissolved in that water, those molecules will move more quickly as well.

Have you ever tried to add sugar to a cold cup of tea? And did you also notice it dissolved a lot more slowly than when that same sugar was added to a hot cup of tea? The heat helps the molecules move, and the crystals disintegrate more quickly.

Sugar dissolves more quickly in HOT than in COLD water.

Heat = Higher concentration

But that’s not all. Sugar doesn’t just dissolve faster in hot water. You can also dissolve more sugar overall in warmer water.

Recall that are room temperature you can dissolve about 200g of sugar in 100 ml of water?

That is a lot. But, when you heat the water, you can increase that by a lot! In boiling water, you can dissolve up to 500g of sugar per 100ml of water. That’s a lot of sugar.

Cooling down – supersaturating

How much sugar you can dissolve in water depends on the temperature. But, keep in mind that at some point, you have to cool the sugar solution down again. Upon cooling down, the solubility of sugar decreases again. A boiling sugar solution cooled down at room temperature can again only contain 200g of sugar per 100 ml.

However, the sugar won’t immediately not be dissolved anymore. Just because the amount of sugar dissolved in water isn’t stable over the long run, doesn’t mean it will leave the water immediately. In some cases it can actually be stable for a long period of time (more on that later). When a sugar solution contains more dissolved sugar than is energetically stable, we call that sugar solution: supersaturated.

What is the opposite of sugar dissolving?

Do you know what the opposite process of dissolving sugar is?

Remember that when dissolving sugar in water, sugar crystals break down. When the opposite happens, sugar crystals are formed. At high concentrations of sugar, when there’s too much sugar in water, it can start to crystallize again.

The individual sugar molecules will find each other again. During crystallization they again form this highly ordered tight structure, to form a crystal.

We will come back to crystallization and how to control this process, in week 3 of this course. For now, let’s have a closer look at sugar solubility and concentrations.

References

Numbers for solubility obtained from: Chest of Books, Sucrose Solubility, link