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Q & A: Volume of sugar dissolved in water

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Most recent answer: 08/04/2016
Q:
When forgetting to turn off heat for boiling sugar water for feeding hummingbirds, does just the water boil off so that I can re-add water to the fill line and still have the correct ratio sugar to water or does some of the sugar boil off too so that what's left is still the correct ratio?
- Gail Smith (age 70)
Sequim WA USA
A:

The sugar does not boil off, so all the sugar you added is still there. Whether you can just add more water to the same line or not depends on how you measured the sugar in the first place.

The problem, of course, is that one cup of sugar plus one cup of water does not make two cups of sugar water—the volume will be much less once the sugar and water mix. There are two effects at work here. First, a cup of sugar has a lot of empty space in between the sugar grains, and water can fill up that space (think about adding water to a full cup of sugar; you could add quite a bit before it started to overflow).

Second, when the sugar dissolves into individual molecules, the sugar and water molecules can get much closer together, further decreasing the total volume. We know how this works for sugar and water, but predicting exactly what happens when any substance dissolves into another is actually a complicated topic, and chemists are still learning how to model it and understand it.

So, if you originally measured the volume of the water with the sugar dissolved in it, you can just add water back to the original volume. If not, it will be much harder to get a predictable result.

Here's another option: instead of using volume to figure out how much water to add, you could use weight. You would need a kitchen scale to do this. I assume you know how much sugar you added, either in volume (cups) or weight (grams). If you used volume, convert it to weight: 1 cup of white sugar is about 200 grams.

Weigh the remaining solution. You'll have to pour it out of the original container to do this, unless you happen to know the weight of the pot. Since all the sugar is still there, if the solution weighs more than just the sugar, the extra weight is water. You can take that extra water weight, convert it to a volume if you want (1 cup of water is about 225 grams), and figure out how much more water you need to add to reach the desired ratio.

Rebecca H.


(published on 08/04/2016)

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