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Mass, Volume, Alcohol, Water
Name: Megan O.
Status: student
Age: 8
Location: N/A
Country: N/A
Date: 2/10/2004
Question:
What happens to the mass and volume when you combine two equal amounts of water
and alcohol? or water and common table salt?
Replies:
Megan,
I will assume that when you say "equal amounts", you are referring to equal masses. If so the
mass of the results will equal the sum of the masses of all components. For example, 100 grams
of alcohol plus 100 grams of water will make a solution whose mass is 200 grams. The same will
be true for salt and water.
However, the volume that results from a combination of different liquids may or may not be
additive because the things dissolving may produce solutions whose volume may be slightly more
or less than the sum of the individual components. It all depends on how the dissolved
molecules fit together in the solution.
Do an experiment for yourself and see. Have an adult assist you and then mix equal volumes of
different liquids together to observe the volume of the resulting solution.
Regards,
ProfHoff 801
Mass for chemical reactions is conserved, i.e. the total mass of products equals the total mass
of reactants. The mixing examples you propose are just "simple" chemical reactions. On the
other hand, the volume of mixing two (or more) substances together is not conserved. Sometimes
the volume of the mixture is less than, some times the sum of, and other times greater than the
volume of the components. In the case of ethanol (alcohol) and water the volume of some
concentrations is less than the sum of the components. Liquid water has a somewhat "open"
structure that is broken up by the addition of ethanol so the mixture "collapses". In general
there is no good way of predicting volumes of mixing of either liquids or of liquids and
solids.
Vince Calder
Megan,
The mass of the two substances add together, however, the volumes will not. The reason is that
the sizes of the individual molecules are different enough that the smaller molecules can slip
into the spaces between the big molecules.
As a demonstration of this, get some spheres that are two different sizes (e.g., marbles and
BBs or golf balls and marbles -- you could even use gravel and sand for the demonstration).
First fill a container with the larger particles and then put some of the smaller particles
on top and shake the container. The smaller particles will slip between the larger ones and
will also fit in the container with very little increase in volume.
Greg Bradburn
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Update: June 2012
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