What causes boiling water to boil more violently when I pour granulated sugar into it?

Question

I brought water in a cup to a boil in a microwave, then pour granulated sugar in to it. (It's to make a solution for hummingbird feeders.) The water, lightly boiling when removed from the microwave, violently churns when I slowly pour granulated white sugar into it.

Am I right in assuming this effect is due to the water being superheated, and the sudden disruption of the sugar being added causes the bulk of the water to boil? Is it air being purged out between the small crystals of the sugar, since the vapor pressure of the water is above the partial pressure of the air? Or something else? Or a combination of effects?

Answer 1

You've taken the first step in a good scientific inquiry. You observed a phenomenon and began trying to explain it. Now you can work toward an answer. You could see if sugar (glucose? sucrose?) is a molecule similar to salt for elevating the boiling point of water. (It depresses the freezing point like salt does.) You could try measuring water just as it boils, and then with the same instrument, measure sugar-water just as it boils, and see if it really is elevated. You can learn more about the subject by reading. The melting point of glucose (C6 H12 O6) is 113 degrees Centigrade, higher than the boiling point of water.

Then you can formulate theories as possible explanations. You've started doing that. Then you can investigate those. Here's another. Perhaps dissolving sugar is an endothermic reaction. Mix some sugar and water which are both at the same temperature. Does the temperature of the mixture change?

It takes a certain amount of energy to raise the temperature of water. Look up the definition of calorie. It takes additional energy to convert it from its liquid state to its gaseous state.

It often takes some discontinuity for a supercooled solution to freeze or form crystals, or for a superheated solution to boil. The bottom of the pot has such discontinuities, and particularly irregular parts of the pot are where boiling likely starts. When you add granulated sugar, you are adding lots of discontinuities. What happens when you add dissolved sugar instead of granulated sugar?

Keep good records, sign and date your notes and get them witnessed periodically. They could be your first steps toward a very good science fair project or perhaps a bigger discovery and a patent. Then, tell the world I helped you along. That would REALLY make my day!

Answer 2

Yes. It was super heated. This is a major danger of heating pure water in a microwave. There's very little convection or anything else to disrupt the water, also the temperature is more uniform whereas in a pot the surface of the pot on the burner is much hotter then the water and therefore the water directly against it is hotter. Therefore it heats up to much higher then the boiling point. This is a very unstable state however and any slight jar or irritation, like sugar, causes it to burst out of liquid form into steam quite violently.

Sugar poured into 100C water would have the effect of stopping the boil because it raises the boiling point.

Answer 3

Although superheating is possible, and may be contributing, I don't think it is the primary effect.

The boiling point of water is affected by dissolved materials in the water. Sugar water will actually boil at a lower temperature than pure or purer water. When the sugar is added, the mixture is now above its boiling point (kind of like superheating but with a different mechanism).

The effect is also seen with freezing point, which is why we put salt in our ice cream freezer ice, to make a material that is melting at a colder temperature than water, which can then freeze icecream which freezes below 32F (because of the sugars etc)

Answer 4

Some dumb answers here.

For a start, adding sugar raises the boiling point.
And sugar has positive heat of solution, so adding it lowers the temperature.

Which leaves your original hypothesis. Sugar crystal give nucleation points for boiling in the superheated water. The same thing does not happen with water from a freshly boiled kettle.

Answer 5

I did an experiment on that in 6th grade. The sugar is easily dissolved by the super-heated water, and it causes the water to boil.....it could be a combination of effects... I agree with the dude: the sugar could decrease the boiling point of water, which would make sense, since the water is already boiling.

Answer 6

Water and sugar boil at a "Higher" temperature.
There is a heat from solution. The sugar dissolving adds heat to the water. If the water is at boiling temperature the extra heat is added to the latent heat changing water to gas.
Try this carefully in a sink, with glasses. To hot, not boiling, water and add lye. This can boil over. This is definitely not from superheated. This is why one adds acid to water and not the other way around.

Answer 7

Some things raise the boiling point and some things lower it.
When you put the sugar in it obviously lowers the boiling point.
The water then boils vigorously because it is now above the boiling point and must release the extra heat.

Also some things (probably not sure) actually releases heat when mixed with water. One example is concentrated acid.

Answer 8

Salt raises the boiling point. It does not make water boil faster, it actually takes longer for it to boil if you add salt. Salt will allow water to boil at a higher temperature, producing more heat to cook food faster.

Answer 9

I believe that dissolving sugar in water lowers the boiling point of the water. It's been a long time since freshman chemistry...

Answer 10

I don't know about sugar but salt raises the boiling point of the water so it seems to boil harder. Maybe it's the same for sugar...