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Lesson 3: Solutions
Lesson 3: Solutions
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.
In CH 104, you encountered a classification scheme for matter that looked like this. As you can see, the term "solution" refers to a homogeneous mixture. This means a mixture with no "clumps" in it - everything is evenly dispersed, right down to the molecular scale. Most often, solutions are liquid mixtures, but this is not always the case; solutions can be solid or gaseous too.
How can you tell when a solution forms? There are some good visual indicators. The test tube on the left shows a clear colorless solution with no visible clumps. If two or more substances combine to look like that, a solution has likely formed. Conversely, the tube on the right clearly does not contain a homogeneous mixture ... it has brown clumps in it.
While solutions must always be free of visible clumps, they are not always colorless. The test tube at right shows a solution of copper (II) sulfate in water. The copper (II) ion has a blue color that it retains, even if it is completely dissolved into a solution. In general, if a substance has a particular color, it will retain that color when it forms a solution.
Similar Mixtures
Similar Mixtures
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.
In this image, the beaker on the left contains a suspension, the one in the center contains a colloid, and the one one on the right contains a true solution.
In a suspension, particles of solid are temporarily suspended in the liquid. These particles scatter light because they are relatively large: billions or trillions of atoms in size. Because they are so large, they settle over time due to gravity. In the image at right, you can see that the cloudiness in the left-hand beaker has faded because the particles 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, but shining light directly on it always reveals some cloudiness. The particles in a colloidal dispersion remain dispersed in the liquid and will not settle out.
On the other hand, a true solution does not scatter light that passes through it. Observe the difference between this image and the one above. This is because light scattering only occurs when a mixture contains particles larger than a single molecule. In a solution, there are no "clumps" larger than a molecule, so there can be no scattering of light.
In this lesson, we'll discuss the structure of solutions in greater detail. We'll learn the reasons why some mixtures of substances form solutions while others don't, as well as some chemical reactions that can occur in solution. Finally, we'll discuss the ways that solutions behave differently from pure substances in terms of their physical properties.
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