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Name: Jerry Status: student Grade: 6-8 Location: IL Country: USA Date: Winter 2011-2012

Question:
After researching many sites including yours, my son attempted an experiment to determine what would evaporate quicker: tap water, bottled purified water, or a homemade solution of salt water. All of the research dictated that the tap water should evaporate first. He conducted the experiment 3 separate times and came up with different results every time. The variables for all three experiments were the same every time. He used 8 ounces of each specific type of water in the same size plastic container each time and allowed it to sit for 48 hours in a controlled area of the house (a dark dining room). He then measured the amount of water left in the container. During the first experiment the tap water evaporated the quickest followed by the salt water and lastly the bottled water. After 48 hours the results were 6 oz (tap), 6.125 oz (salt), 6.25 oz (bottled). In the second experiment the salt water evaporated the quickest then the bottled water and lastly the tap water. The results were 7oz (tap), 6.5 (salt) and 6.75 (bottled). In the third attempt the salt water evaporated quickest, followed by the tap water and lastly the bottled water. The results were 7 oz (tap), 6.75 oz (salt), and 7.125 oz (bottled). Please help explain these findings, we are at a loss trying to explain this. Unfortunately time isn't on our side as this needs to be turned in this Friday the 10 of Feb.

Replies:
Jerry,

Did your son have a hypothesis (which water did he predict would evaporate more quickly)? Were the containers steam cleaned before each experiment? Was each container exactly like the others? Were they located in the exact same place (and in the same order)? How were the measurements made? By weight or volume? What techniques were used? Were air vents in the area? Was shade an issue?

It is OK to be at a loss and unable to explain your results. One of the many great things about science is that often the results raise more interesting questions that need exploration.

Your son could say that the inconclusive results require further experimentation. This type of conclusion would be appropriate.

Les

--- Leslie Kanat, Ph.D. Professor of Geology Johnson State College


First, please allow me to applaud you and your child for attempting to do experimental science.

The inconsistency of the results indicate that there must be other factors at play. These might be called experimental errors. Experimental errors are not necessarily a bad thing. They can be due to unforeseen environmental factors. It may also be that your data indicate a flaw in the prevailing theories about evaporation rate, which would be especially exciting!

In looking over your results, the lack of consistency would seem to infer that there is some kind of experimental error. Identifying the source of the error can be difficult. Some critical areas that come to my mind that would affect results include: surface area, water temperature, humidity, air flow above the water sample. Another thing that you did not include in your write-up was the amount of salt in your salt water sample.

It is really important to control all the variables except those that we are changing and testing for. I am wondering why you though that bottled water would evaporate at a different rate than tap water? A lot of bottled water is just filtered tap water. Any differences in rate could be due to the concentration of mineral content in either the bottled or the tap water.

To control for all these factors may require equipment that is more sophisticated than the apparatus that you have. A better test would be to obtain distilled water and test that against various concentrations of salt water. To control for environmental factors, you should run multiple sets at the same time. For example, several containers of distilled water, several containers of low concentration salt water, several containers of slightly higher concentration salt water, several containers of even higher concentration salt water, several containers of even higher concentration salt water, and so forth. All the containers should remain in the same environment. The salt water should be made from table salt and distilled water. The results should be averaged and compared.

Ray Tedder, NBCT Chemistry Teacher


Jerry -

I think you should think the experiment through with this idea: A little liquid evaporates and makes water vapor. The air holds rather little of that. The water vapor has to go away somewhere for more liquid to evaporate, and the motion of the air is what moves that vapor.

In any one run of your experiment I'm not convinced that all 3 dishes experienced the same amount of airflow and heat input. I also can't know whether the relevant things were the same between one trial and the next.

The typical dish of water will evaporate faster if there is a fan blowing over it, than if the air is left to it's own spontaneous motions. There is almost always some rate of motion, due to convection or breezes or whatever. If the rate was zero, evaporation would pretty distinctly stop. When something that matters like this airflow is so subtle you do not see it, but it still matters, then mysterious things just keep on happening. It is a variable and you must eliminate or measure the variables.

If the room is totally closed, the fan will not help much. the whole room gets humid, then there is no more room in the air for more water vapor. Or maybe the walls drip if they are held cooler than the dishes. But the usual exhaust path for water vapor is out of the room through some window or door. Then the world at large can worry about getting rid of it. Someday It will condense in a cloud somewhere and rain onto the ground or into the ocean.

Evaporation requires heat input. That is _not_ the same as saying the temperature has to be higher. The water can be a little colder than the room and still keep evaporating. When it is cold, it tries to suck in heat from the surroundings. If it can get that heat, it keeps evaporating. If you had a perfect thermos bottle of water with no way for heat to get in, some evaporation would happen, and that would make the water cooler, and that would slow down the evaporation. It would keep getting cooler, and slower, until evaporation pretty much stopped. So all 3 dishes need to have the same heat-sinking to have a fair experiment. Be sure they have identical containers, and you don't have a lamp pointing right at one dish more than the others, or sunlight with moving shadows. Plastic cups are probably the best identical containers you can get. Maybe put all three on a common aluminum plate or pan, and maybe even have an opaque roof, elevated a few inches over the pan.

I think you need to do a control run. put 3 identical containers filled with the same liquid into your setup. They should go down at about the same rate, but maybe they are up to 30% different. Measure how fast or how far in a given time, for each. The ratios between them are what matters. If one has a better position than the others, it shows up clearly here. Do it one more time, and hope the ratios of rates are the same as the first time. If they are, you can start doing assorted various liquids and know you are measuring the right thing.

If you do the same 3 liquids three times, but rotate the positions so that each liquid gets to be in each position once, and then you average the rates of those three times for each liquid, that can be a pretty fair answer. But it is important to know that the relative advantages of the 3 positions are repeatable. That is what the control runs are for. I would use bottled water for control runs. It has the least solids, and solids can precipitate at the surface and affect the exposure to moving air, either by blocking or by wicking.

I would probably do this by making a big box with two 4" holes or getting a large pot and propping the lid up 1" on one side. then have a fan point at one hole of the box or tangent to the open side of the pot-lid. Keep the fan at a fixed and fairly large distance. Air is really going to swirl around inside the pot. I think that helps all 3 cups get equal air exposure. Having a fan in the room you used, next to the wall on the opposite side of the room, blowing parallel to the wall, probably would have helped. (I know, who would have guessed that...) It would make sure the air was moving at a fair (non-zero) pace, the same pace each time, and about the same speed for all three cups, if all three were the same distance from the wall they were near. You could let the liquids take turns being the upwind cup.

Jim Swenson


There are several issues that need to be resolved. The first is “units”. Unfortunately, in English units (ounces = oz) can be either volume, or weight (mass), if gravity is take into account. Although you assert that the samples were evaporated in a controlled area (a dark dining room), how do you “know” that this is a controlled area? For example, how do you “know” that the humidity was the same in the various conditions. Both temperature and humidity would have to be carefully a controlled constant. This is not so easy as it may seem. The presence or absence of light would be much less important than small changes in temperature and humidity. Small changes in the flow of air above the samples would be a much more important factor. How do you know these variables were controlled? Were the three test samples tested simultaneously or sequentially? From your description, it is not clear what the order of operations is. You did not mention the percent change of the various weight. It would be important if the relative change was 0.1% or 10.0%, because then “experimental” error would become a large factor.

Your experiment is not a loss, not at all! You are not getting the result you “expected”. That is probably your most important observation. What you “expected” is not what happened. That is far MORE IMPORTANT than the getting the “right answer”. Analyzing what may have happened is far more interesting!!

Vince Calder


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