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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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