LSS_Solutions Lesson

Solutions

Solids and liquids are frequently combined to make solutions. Recall that solutions are homogenous mixtures. This means that the particles are evenly mixed. In a solution, the macroscopic properties do not vary throughout the sample. This is in contrast to a heterogeneous mixture in which the macroscopic properties depend upon the location in the mixture. While solids dissolved in liquids is probably the most common type of solution, it is not the only type. Actually, every phase of matter can serve as either the solute or the solvent. We will now focus on solutions in a more in depth manner, looking at why solutions form and the formation process.

Solutions and Intermolecular Attractions

In order to fully understand why solutions form, it must be understood that nature favors disorder. Two gases will naturally mix among each other when there exists an opportunity for such mixing to take place. The statistical probability that the two gases would ever separate out completely is nearly zero. In addition to the natural tendency towards disorder, attractive forces amongst particles contribute to the formation of solutions. In gases, these forces are so small they are ignored. But, this is not the case in solids and liquids. 

To learn what causes solids and liquids to form solutions, let's first look at a common example of two chemicals that don't form a solution. It is well known that "oil and water don't mix." But why?

On the interactive below,

• Click on the play button (the triangle) to run the model of the separated oil and water and watch what happens. Keep in mind that water (blue and red) is polar and that oil (gray) is nonpolar.  
• Next, click on "Shake up the oil and water mixture". Then press play to run it again. Watch what happens to the molecules over time.

If you allowed the interactive to run long enough, you would have seen the oil and water separate again. This happens because the water molecules are more attracted to other water molecules than they are to the oil molecules. Think about this from the perspective of IMA (intermolecular attractions). The polar water molecules have a great attraction for one another, in the form of hydrogen bonding . The nonpolar oil molecules are weakly attracted to one another through London forces. There are some weak attractions between the water molecules and the oil molecules as well. These are also London forces.   So, which of these attractions is the strongest? The water's attraction to other water molecules is the strongest of these IMA. Therefore, these molecules do not form a solution. 

What about nonpolar molecules like CCl₄ and benzene (a symmetrical molecule to the right)?

The only IMA present are the London forces, (temporary induced dipoles). These are weak IMA. The forces of attraction between CCl4 molecules are about as weak as those between benzene molecules. We commonly refer to this idea as "like dissolves like".

"Like dissolves like" can be applied to solids dissolving in liquids as well. When a solid is dissolved in a liquid, the intermolecular attractions play a similar role.

Now, let's look at some examples where solutions are formed. For a solution to form, the particles must intermingle. In order for this to occur, the solute and solvent particles must have attractions for each other that are stronger than (or at least as strong as) their attraction for particles of their own kind. Look at water and methanol as an example. Both molecules are polar. Both molecules exhibit hydrogen bonding. (Remember that hydrogen bonding is an IMA, not an actual bond). If water were more attracted to another water molecule than it was to methanol then it would move the methanol molecule out of the way to experience the attraction to another water molecule. But, water is attracted to methanol almost equally as it is to other water molecules. Since their intermolecular attractions are similar, they mix freely together and are said to be miscilbe, or soluble in one another.

The process you observed above (the surrounding of the ions by water molecules) is called hydration. The generic term for hydration is solvation, referring to the surrounding of the solute ion by the solvent. In the case of hydration, the solvent is water. Nonpolar molecules are not able to undergo hydration, due to the difference in polarity, but would create a solution in a nonpolar solvent. For example, water and oil are of different polarities and therefore will not mix. They are said to be immisible.

Remember to work on the module practice problems as you complete each section of content. 

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