Lesson 3: Solutions - Mixed Molecules ( & Ions) in Motion

When materials are mixed they may or may not form solutions. In this lesson we will look at what combinations form solutions, why they do, and what makes solutions so special.

In doing so, we will consider topics such as molecular polarity, solubility rules, dynamic equilibrium, electrolytes, changes in freezing and boiling points, and osmosis.

Introduction | Similar Materials: Solutions, Colloids, & Suspensions

Introduction

I would like you to think for a moment about the sea.

Think about what's in it.

One of the things you should know about the sea is that it contains dissolved salts. It also has a lot of suspended solids in it. It also has a variety of living things, most of which need oxygen. The sea contains dissolved gases such as oxygen and nitrogen. Of course, it also has a few oil slicks and some natural tars. In all, it contains quite a variety of things.

How would you go about classifying the ocean? What kind of material would it be?

Although classifying the ocean may not be a particularly useful thing to do, what would you come up with? Well, when you look at the ocean and everything that's in it, you'd have to classify it as a heterogeneous mixture, because of the things which are in it but not dissolved: the suspended silt and sand, the fish, the whales, the kelp and so forth. If you filter out all of these things and the other solids, what is left is a solution. Why a solution? A solution because it is homogeneous, but with various materials dissolved in it. If you evaporate away the water, a salt residue remains. Even though the filtered seawater is homogeneous, you can separate one component from the others.

Classification Scheme

The first thing in this lesson is to try to identify just what a solution is and put it in the context of some other things we have studied. Let's go back to the classification scheme that we dealt with months ago. (There is also a diagram of it in example 1 in your workbook.) Solutions are right in the middle of the diagram. They are neither heterogeneous nor pure.

Classification flow chart. [1flowch5.gif]

Solutions are homogeneous mixtures. (Note that I did not say homogeneous liquid mixture. A solution can be a solid, a liquid or a gas. Air, for example, is a gaseous solution (or a homogeneous mixture) of different gases. Metal alloys are examples solutions of solid materials. Some minerals are solid solutions.)

We will be dealing mostly with liquid solutions although many of the things that are dissolved in these liquid solutions may have been solids before they were dissolved. Actually we will be dealing primarily with aqueous solutions, things dissolved in water.

Well, then, why all this about the ocean? Primarily to bring out a few basic ideas.

One of those ideas is that some things will dissolve in water, such as salt. We want to know about the nature or the properties of those things that will allow them to dissolve in water.

Another basic idea is that there are some things that will not dissolve in water, such as oil. What is the nature (what are the properties) of those materials that keeps them from dissolving in water?

Those are some of the things that we will be pursuing in this lesson. Here are some more.

What happens to materials when they dissolve? Salt is a white crystalline solid. When you add it to water, it just seems to disappear. How can it do that? What is happening?
And what about this, anhydrous cupric sulfate? Look what happens when it dissolves in water.
What about the water? When salt is added, it conducts electricity better, boils at a higher temperature and freezes at a lower temperature. There are other changes that we will deal with as we go through this lesson on solutions.

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Solutions, Colloids, & Suspensions

We need to distinguish between solutions and other similar materials such as pure liquids, colloidal dispersions and suspensions.

Components of Solutions

Every solution has at least two components. The components of a solution are the solute and the solvent. The solvent is the primary component. The solute is the material that is dissolved in the solvent. For example, in salt water, salt is the solute and water is the solvent. Often there is more than one solute in a solution and sometimes more than one solvent.

Pure Liquid

At the observable level, the distinction between solutions and pure liquids is this. The components of a solution can be separated from one another by simple phase change operations like evaporation or distillation. On the other hand, a pure liquid will go through those same kinds of changes, without being separated into components.

Suspensions and Colloidal Dispersions

There are two other kinds of mixtures that are somewhat similar to solutions that you need to know about. They are called suspensions and colloidal dispersions. Here are some examples of dispersions and suspensions. Look at these samples and write down physical descriptions of them in exercise 2 in your workbook.


suspension	colloidal dispersion	solution

In a suspension, the particles which are temporarily suspended in the liquid are large enough to collectively make the material appear cloudy. They will settle out after a while. You can now see that after several minutes, the particles in the suspension have settled to the bottom.
In a colloidal dispersion, we have very small particles spread throughout the liquid which are large enough to reflect light, but not large enough to be seen individually. A colloidal dispersion may look either clear or cloudy in ordinary room light. The particles in a colloidal dispersion remain dispersed in the liquid and will not settle out.
A solution, on the other hand, will appear clear even when a light is shown through it.