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Name: Marjean
Status: educator
Grade: 4-5
Country: USA
Date: February 2009

Question:
I teach a matter unit in 5th grade science. One of the experiments students participate in is dissolving salt in water as an example of a reversible physical change. To help students understand conservation of mass, they weigh the cup and the dry table salt before adding distilled water. In theory, after all the water evaporates, the salt in the cup should weigh the same as it did before the dissolving took place...right? It never does! It weighs more than when we started every time! If anything, it should weigh less, since it is quite easy to lose some of the salt from the cup as it is moved from the window to the desk. I am truly puzzled as to why it would weigh more. Can you help me answer this question?



Replies:
My guess is that the salt is retaining moisture and not as dry as it was when you first placed it into the cup. The extra weight you are measuring is water that was not in the cup during the initial weight.

If the last part of your experiment was to heat the cups in even a moderately warm oven (safe enough for elementary kids) I think you'd see better results. Another thing that might help would be to spread the salt out rather than having it in a cup. A baking tray could hold salt but have more of it open to the air, which would help for more moisture to be removed.

Ian Farrell HALOSOURCE, Inc.


Marjean,

From your description I take it that the salts are dried by letting the open containers sit by the window. The problems that could occur with this process is that (1) the salts have not thoroughly dried and the additional mass you have observed is from water that have not been completely removed, (2) that salts did dry completely but not they have absorbed moisture from the air and a hydrate complex has formed where the water molecules are now chemically bound to the salts.

If you have access to an oven and a desiccator, a good approach would be to dry the salts in a drying oven (at approximately 130deg-C for several hours) and then allowed to cool in a desiccator (a chamber with some drying agent, like Dririte [copper sulfate with a blue dye that indicates it is still working], that keeps the environment dry).

If this is not possible, there are alternative ways to present the conservation of mass. For example, you could place some vinegar in an Erlenmeyer flask, some baking soda inside a rubber balloon, attach the balloon to the Erlenmeyer flask and weigh it. Then, by tilting the balloon so that the baking soda drops into the vinegar, a reaction happens. The balloon will expand because of the produced CO2 and students can see bubbling - a chemical reaction has happened. But the mass before and after the reaction should remain the same. . . . later on, you can even use this apparatus to show diffusion as the mass will go down when the CO2 starts to escape.

Hope this helped.

Greg (Roberto Gregorius)


MJ,

The formation of hydrates is considered a chemical reaction. For example:

NaCl(s) + 2H2O(g) --> NaCl.2H2O(s)

where the chemical and physical properties of the NaCl(s) differs from that of the NaCl.2H2O(s) complex.

This can be more readily shown in hydrates that differ in color with the anhydrous salt. CuSO4.5H2O is blue, heating it will cause the water to be released and form the grey-white CuSO4. If you heat the salt inside a test tube and with a bunsen burner, you can actually see the water condense at the top of the test tube.

Greg (Roberto Gregorius)


The Fleischmann/Pons Corollary to Murphy's Law says: A lab that worked perfectly when you did it will never come out right for the students.

Seriously, if you start with commercial salt, say from the store, the salt is most likely very dry that is, water that salt normally absorbs has been driven off in the processing. Salt is hydroscopic; it will absorb water into its crystal structure if given a chance. That is why salt in places with high humidity, like the seashore, clumps up and does not pour well. That is why people put rice grains in their shakers.

The salt that remains in the cups after evaporation probably contains absorbed water. This water that was not in the original salt, may well be the source of your increased weight.

Try the following:

a) Give students salt that has already been through the cycle of dissolving and drying out. This salt should have the extra absorbed water in it and should give more consistent results.

b) You could also try heating the dried salt to drive off more water although that would need explanation and might confuse you students.

c) Replace the salt with something else.

Best of luck.

Bob Avakian
Oklahoma State University Institute of Technology
Okmulgee, Oklahoma


Hi Marjean,

There may be a few things at play. You do not give details about how you dry the salt, but there is a possibility that your salt after drying has higher water content than when you started. Did you ever notice when you open a new container of salt the grains are all free flowing, but after a time you get some clumps? That is because water vapor from the air helps bind the salt crystals together, and the weight of the salt would be slightly higher. If you add water to the containers in your experiment a second time, and dry it a second time, does it go back to the second initial weight, or is it even higher?

If it is even higher on the third weighing, then you may have dust or other material falling into your containers. A test of that would be to weigh an empty container left out in the same place at the same times. If its weight also rises then that is at least part of the change.

You can use this as a teaching lesson to your students about the scientific method. Have them explore hypotheses about why the salt would weigh more, and have them devise ways to test those hypotheses.

Don Yee


Well, it is good you are starting with distilled water. Have you done a control in which they have a cup with only water in it, and measure its mass before and after the water evaporates?

Other possibilities include alkaline compounds in the water that scavenge carbon dioxide from the air. Another possibility is that the salt crystals that form upon evaporation of the water contain water included in them, making their mass seemingly greater than before.

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


Critical question -- did you use "table salt"? Table salt, in contrast to "pure" salt contains a small amount of silicon dioxide, to prevent clumping. So the silicon dioxide gives a "false" positive weight.

This may not explain your "too massive" weight, but it certainly needs to be checked. Nonetheless, the discrepancy is an important lesson -- when the result does not match the prediction, what is going on? I commend you for not ignoring it.

Vince Calder



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