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steelmarket's avatar

Chemistry Q: Will adding a little bit of soap to water make it evaporate slower or faster?

Asked by steelmarket (3598points) January 20th, 2009

Or have no effect at all?

Observing members: 0 Composing members: 0

10 Answers

richardhenry's avatar

Don’t quote me, but I think it evaporates more slowly? Someone else will correct me or explain better.

cage's avatar

I don’t see why it would have an effect.
I mean, they’re two separate things right.
If you burnt the water off, it you’d be left with a soapy substance. (or the other way round)

I think the difference would be negligible.

critter1982's avatar

Refer to Raoult’s law. This says that the evaporation of a solution is based on each chemical component and the composition of each. This is why salt water evaporates slower. I’m not positive about soapy water, but I would assume that it would be slower simply because soap itself takes longer to evaporate.

Harp's avatar

Soap is a surfactant, and so will tend to form an extremely thin fatty layer at the interface between water and air. This will act as a barrier to evaporation, though only a slight one.

LostInParadise's avatar

Even a small amount of soap is very effective in breaking water’s surface tension. My guess would be that this causes a reduction in the energy required to make it evaporate and so increases the evaporation rate.

Lightlyseared's avatar

Why don’t you carry out the experiment and tell us. That way we wouldn’t need to guess. You could set up a web cam and we could all watch online in real time.

Harp's avatar

It has even been proposed to weaken hurricanes by releasing surfactants into the ocean in the storm’s path to reduce the evaporation that fuels the convection currents.

steelmarket's avatar

I recently purchased an evaporator humidifier, and I got to wondering if a drop of dishwashing soap in the water reservoir might help the evaporator pads soak up the water. This triggered my question here.

OK, I’ve heard from two chemists now on this issue (one in Fluther, one outside). The consensus seems to be that a small amount of soap will not affect the evaporation rate. Soap reduces the surface tension of water, allowing the water to clean better. “But”, according to one chemist, “the surface tension of a liquid does not affect the evaporation rate.” A lot of soap would just build up in the works in the machine, so I am not going to try it.

LostInParadise's avatar

Harp, as usual, got it right. There would be a reduction in the evaporation rate, but it would probably not be noticeable.

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