Ask A Scientist Chemistry Archive

name         Megan O.
status       student
age          8

Question -   What happens to the mass and volume when you combine two equal amounts of water 
and alcohol?   or water and common table salt?
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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
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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
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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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