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Name: Xandria Status: student Grade: K-3 Country: USA Date: Spring 2012

Question:
I am asking on behalf of my 2nd grader. She is doing a Science Experiment on the rate of evaporation of different liquids (water, salt water, alcohol, vinegar, and bleach). Can you have more liquid than what you originally started with? She discovered that droplets of liquid were "stuck" on the side of the flasks and when she measured the liquid, they were more than 50 ml.

Process: She put 50 ml of liquid into flasks. Water was placed in both a flask and beaker...just as a side experiment about the container shape. She placed the liquids in a room with an ambient temperature that fluctuated between 68-75 degrees Fahrenheit. Unfortunately, we did not measure the humidity (in reading the archives, it has a huge effect on evaporation). She turned on a work lamp for one hour a day for 10 days. The temperature in the room would get up to 98 degrees Fahrenheit.

At the end of the 8 days, the liquids in the flasks measured more than the 50 ml that she started off with. Is that possible? Of course, the water in the beaker evaporated the fastest and her project was a "failure" because the liquids in the flasks did not evaporate.

Replies:
Some substances can pick up moisture from the air; they are known as "hygroscopic." I'd be surprised if this occurs with the dilute materials you've named, though.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


Xandria - I am not sure I would consider that a failure. The relative rates seem roughly reasonable to me, interpreting them thus: - the beaker has large are exposed to air and a very short column of air above it, so it is easily accessible to air circulation, and it evaporated at a reasonable pace. - the flask has a smaller area and a taller column, choking off most of the air circulation which carries water vapor out of the container, and it evaporated at a much slower pace.

I would not expect water gain, but it is barely "possible". Dew and wetness on glasses of cold drinks are your prototypical water gain. Is there anything (like a metal table-top) that could funnel coolness into the flask, resulting in occasional wetness outside as well as inside? Was there occasionally wetness under the flask, at some times of the day?

I think you could simply look up the local relative humidity for the past week or month in weather reports for your location. To gain water would require humidity greater than,say, 80%. Greater than 90% would make slight gains rather plausible.

If the work lamp was close to the flask and beaker, radiating significant heat on them both, the change should have been a net loss for both. If heat energy goes into a water container it has a strong tendency to leave by evaporation of water in preference to cooling by air convection. Sunlight in a window can have a similar effect, which can be much larger than the expected loss rate predicted from temperature and air circulation. In fact, an old scientific method for measuring sunlight is to put large pans of water out in the open and measure the water loss rate. Unsurprisingly it is called "pan evaporation".

To see for sure you might run the experiment for yourself, perhaps in a more secure location. It is possible that very small losses were more than compensated by somebody's small addition. Not much you can do about past events like that.

Bear in mind that evaporation does not depend only on ambient temperature. it also depends on exposed area, air circulation rate (i.e. a fan speeds things up), and on net heat flux into the container. Also, thin films of oily impurities on the surface of the water can slow evaporation down, and crusty salt-like deposits on the walls of the container over the edge of the liquid can speed things up by drawing water up and out to cover larger area of air-exposure.

For all those reasons, evaporation experiments can often be a bit unpredictable. Admitting that is something that will normally be in the conclusions of many such experiments.

Jim Swenson


This outcome is possible because you measured it.

To get accurate results you must control all of the parameters that effect or might effect the outcome. The parameters of an evaporation experiment are Quantity of water before the experiment process Quantity of water after the experiment process Humidity of the air over the liquids Temperature of the liquids Temperature of the air Air flow over the samples Surface area between the liquid and the air..

I do not want to bias a future outcome, but you should get less liquid after an evaporation process. Try using flat pans rather than beakers. You also might try using more liquid. 50 ml is only 0.2 of a cup. Try using a half (0.5) cup (118.3 ml) or 100 ml so that minor inconsistencies will not significantly impact your outcome. You may also be able to accelerate the time for your experiment by using uniformly warmer air and greater airflow over the samples.

Sincere regards, Mike Stewart


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