Types of Change and Separation

In Lesson 1, we dealt with observations of single static items. Now, let's take a look at change. The reason for dealing with change is, of course, that without change there wouldn't be any chemistry. There wouldn't be things happening that we could observe. In fact, without change we wouldn't be able to observe.

First, in this Lesson we will consider what we call physical changes. In Lesson 3 we will consider what we call chemical changes. The distinction between these two types of change lies in answering this question. Does the observed change result from a change in condition of the same material or from a new material with different properties than the material you started with?

More examples will follow, but let me give you one of each for now. If you evaporate gasoline, there is a change from liquid to vapor, but you still have gasoline. It smells the same and it will still burn. That is a physical change. If you burn gasoline, there is again a change from liquid to vapor, but you no longer have gasoline. It smells different and it will no longer burn. That is a chemical change.

Later in the course you will have more criteria on which to base such a decision, but for now you will have to base such decisions on what you are able to observe.

So what kinds of changes can be observed? Well, they are the same as the properties themselves: phases, colors, clarity, homogeneity, temperatures. For example we can observe solids, liquids, and gases. Therefore, we can also observe the change from solid to liquid or liquid to gas or solid to gas. We can observe that something is blue or yellow or colorless, and we can also observe that something changes from colorless to pink or that something blue and something yellow, when mixed together, become green. We can observe that something is cool or hot. We can also observe that it changes from hot to cool.

Physical Changes

Simple Phase Changes

You should already be familiar with the three phases of matter: solid, liquid, and gas. Moving between these phases is a very common way for matter to change.

Let’s focus on phase changes that involve materials changing from one state of matter to another. These phase changes can be brought about by heating or cooling. The terms used to identify the various changes are shown on this diagram. If you are already familiar with this terminology feel free to move on to the next topic.

Phase changes can be brought about by heating or cooling. For example if water (a liquid) is heated enough, it evaporates and becomes water vapor or steam (a gas). This kind of phase change--liquid to gas--is called evaporation or vaporization.

Water vapor can in turn be cooled to form liquid water. This kind of phase change--from gas to liquid--is referred to as condensation.

If you were to continue to cool down the water and change it from a liquid to a solid, you would have another kind of phase change that is called freezing or crystallization.

If you were to then take a solid and warm it up to change it into a liquid, that change is called melting. (It is also sometimes called fusion.)

Another change that can occur for some solids is to change directly into a gas instead of changing into a liquid, and that is called sublimation.

The reverse of this process can also occur. Some gases can be cooled down and changed directly into a solid. That process is called deposition.

Separations using Phase Changes

Chemists have specific terms they use for some processes involving more complex or specific types of phase changes.

In addition to the solid, liquid, and gas phases you can find most pure substances in, substances can also form solutions, where one substance is dissolved in another. We will learn more about solutions in later lessons, but for now, you should be similar with the terms dissolution, which means the process of forming a solution, and recrystallization, which refers to the process of recovering a substance from a solution. Recrystallization, like distillation, is a very important chemical lab technique.