Evaporation and Conservation of Mass
Date: February 2009
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?
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.
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)
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
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
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.
Oklahoma State University Institute of Technology
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
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.
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.
Click here to return to the General Topics Archives
Update: June 2012