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Q & A: dripping faster when hotter

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Most recent answer: 03/17/2013
When I heat water, it drips faster than it did when it was cold. This is because the molecule move faster. When I heated a Propel Zero water and when I heated Kool-Aid which is made mostly of water, they did not drip faster, they dripped slower. Why? Is is beacause there is salt in both or because they both have vitamin C in them?
- Drew (age 9)
Laurel, MD, USA
Drew- That's a great question. It's certainly true that water will drip faster when it's hotter. In the hotter water the molecules shake loose from each other more easily, so it's less sticky. We say that its "viscosity" goes down.

We aren't sure why the viscosities of your Kool Aid and Propel Zero don't also go down when heated. The sugars, including maltodextrin, in Kool Aid increase the viscosity, and that extra viscosity also goes down as they're heated. There are a few salts and some citric acid in the PZ and those should slightly increase its viscosity, but again heating should reduce the viscosity.

Here's our best guess. Maybe as you heated the liquids, some of the water evaporated. That would leave the ones with sugar more sugary and viscous than they started out. That might be a strong enough effect to beat the viscosity reduction from heating, so the viscosity would go up instead of down. It's a bit surprising that even the PZ viscosity went up, since it doesn't sound like it has much in it to increase its viscosity.

There are ways to check this idea. How did you actually do the experiment? What did you drip the liquid through? More importantly, how did you heat it? Could water have evaporated? Here's a test you could run. Let the heated liquid cool back down. You can test it to see if it's denser and also if it's more viscous than the starting liquid. That would tell you that some of the water evaporated. Please let us know how this turns out.

Erik and Mike W.

(published on 03/17/2013)

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