# How does volume contraction in solvent mixing work?

I recall seeing chemistry demonstrations where a significant amount of a solute is dissolved in a solvent and the solution volume barely changes. Questions regarding this:

1) How is this possible?

2) What solvent/solute combinations best demonstrate this effect?

3) Is it possible to wind up with less solution, in other words Is it possible to have Volume(solution) < Volume(solvent) + Volume(solute) ?

Yes this happens fairly regularly. It is a deviation from ideal behavior associated with the property of partial molar volume. For an ideal solution, the total volume always equals the summed volumes of the pure components, but ideal behavior neglects several important facts about real molecules, to wit, that they may have different shapes, and may also experience stabilizing or destabilizing interactions with each other.

It's fairly easy to see how molecular shapes and sizes could affect this property. Macroscopic objects experience similar effects. Suppose I have a bucket "full" of golf balls and a bucket full of sand, if I work carefully, I can pour quite a lot of sand into the bucket of golf balls because the smaller size of the sand grains allows them to fit into the gaps between the golf balls. Another example: Suppose you have a 10 liter bucket of golf balls and a 10 liter bucket of tiny cubes packed tightly together. If I then try to pour those two buckets into a 20 liter container simultaneously, it almost certainly will not be able to fit all of the balls and cubes, because they will not pack as efficiently in the 20 L container, costing some space. These are extreme examples, but illustrate one way that different shapes can play a role.

Differences in intermolecular interactions can play an even bigger role. If you are dissolving salt in water, the individual ions making up the salt crystal can be solvated very efficiently by the polar water molecules, which experience a strong attractive interaction with the ions. (Recall that the O and H-atoms in water have partial negative and positive charges, respectively.) This results in the solution having a significantly smaller volume than expected based on the summed volumes of the pure salt and water used to make the solution.

Partial molar volume is not too hard to measure experimentally. For something like aqueous sodium chloride it can be determined in a fairly straightforward way from precise measurements of the densities of a series of solutions with increasing concentrations. However it is worth noting that the volume and mass measurements used for the calculation must be very precise, yielding measured densities with 5 or more significant digits. This requires careful use of volumetric glassware and precision balances, and the temperature of the solutions must be kept constant throughout.

For purposes of demonstrating the qualitative effect, you could just use water and ethanol (or isopropyl alcohol), which also have a significant negative excess volume. (see the first graph on the wikipedia page for partial molar volume for details).

For comparison you could use mixtures of benzene and toluene, which behave almost ideally with respect to volume effects.

• You mention dissolving salt in water. What particular salt would provide the most dramatic effects? – docscience Jan 9 '15 at 15:46
• The most easily observable effects from commonly available ingredients would probably be mixing water and an alcohol, like ethanol or isopropanol. You could do a demo with a couple of graduated cylinders. 50 mL each of water and ethanol should have a negative excess volume of a few mL for sure. As far as which salt is concerned, I don't really know what would give the biggest effect there, but NaCl certainly works. It's easier to measure the volumes liquid samples – dtmoore1971 Jan 9 '15 at 16:57

1.Volumes do not add when you mix things together because the intermolecular forces of a mixture are different than the ones in the pure substances. The way this is handled is by using a property of mixtures called the "partial molar volume."

2.There are a couple of reasons why volumes of solution are not additive. One important one is that some chemical reaction occurs. The second is especially important if one component is water. Liquid water has an open structure, that is, the water molecules even in the liquid form molecule sized structures of lower density than the mean. The addition of ionic and/or polar solutes can "collapse" this structure resulting in an increased density. To complicate matters, the solute may also have structure that may be smaller than or greater than the average density of the solid. This will result in an expansion (or contraction) when the solute is dissolved.

There are no good theoretical models, of a general nature, to predict or estimate whether a solvent/solute combination will have a positive or negative "excess" volume of solution.

3.When $50\mathrm{mL}$ of water and $50\mathrm{mL}$ of ethanol are combined in a $100\mathrm{mL}$ graduated cylinder, the resulting mixture has a volume of $\approx 97\mathrm{mL}$. Intermolecular interactions between water molecules in a pure sample involve hydrogen bonding, though the details of these complex interactions are not completely understood. Some models predict transient ice-like structures within liquid water, where the local density of the sample is temporarily diminished. Ethanol and water molecules are also attracted to each other through hydrogen bonding. The ethanol likely interrupts the transient ice-like structures present in liquid water, resulting in a slight contraction of the sample.