Lesson 2: 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 we will consider what we call physical changes. After that 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 | Separations Based on Physical Changes | Chemical Decomposition

Physical Changes

Single Phase Changes

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 also called crystallization, a second meaning for that word.

Multiple Phase Changes

In addition to these single phase changes, you can have a string of changes, one after another. There are three of these multiple phase changes that are very important and you should know them by name.

One is distillation, changing a liquid to a gas and back to a liquid.	distillation LIQUID GAS LIQUID
The second is sublimation, which you already know means changing a solid to a gas. The term also applies to a double process in which a solid is changed to a gas and then back to a solid.	sublimation SOLID GAS SOLID
The third is recrystallization, in which a solid is changed to a liquid and then back to a solid. You will see another meaning for this term soon.	recrystallization SOLID LIQUID SOLID

Dissolution and Crystallization

Another type of phase change is dissolving or dissolution. This involves a material (usually a solid) seemingly disappearing when placed into a liquid. Sugar or salt dissolving in water are examples. The condition of the sugar and salt have definitely changed--they are no longer homogeneous, opaque, white solids. But they continue to exist as sugar and salt. (This can be verified by taste, in these cases.) They have changed to a different condition or phase, but they remain the same materials. Also, the water remains water even though the taste of sugar or salt has been mixed in with it.	dissolution SOLID SOLUTION
The reverse of this process (evaporating away the water to retrieve the sugar or salt) is called crystallization (another meaning for that term).	crystallization SOLUTION SOLID
Combining both processes (solid to solution back to solid) is a second type of recrystallization.	recrystallization SOLID SOLUTION SOLID

 

Recap That gives you quite a few terms with which you may or may not be familiar. Take a moment to review them and be sure you know their meaning before continuing.

vaporization condensation distillation crystallization recrystallization sublimation dissolution

 

Top of page

Separations Based on Physical Changes

One of the important aspects of phase changes is that they can be used to separate materials from one another. For example, salt and water can be separated by evaporation. Salt and sand can be separated by dissolution. Also, when coffee is made, the color, flavor and aroma are separated from the grounds by dissolution. Similarly, when clothes are washed, most of the dirt and stains are separated from the fabric by dissolution or similar processes.

Another use to which these separations can be put is to classify materials as being mixtures or pure substances. Materials which can be separated into components by these phase changes are called mixtures. Materials which cannot be separated into components by these phase changes are called pure substances. We will deal with this aspect of classifying materials more thoroughly later in this lesson.

Now let’s take a closer look at several of the methods used for separation.

Distillation

First, let’s consider distillation. When saltwater goes through the distillation process it loses its salty taste. Left behind in this process is a white solid; a separation has occurred. The water and the salt have been separated from one another. Sometimes people say that the water has been purified because the salt has been removed from it. Actually, the water has been separated from the salt. The term "separation" applies to both the water and the salt; each has been separated from the other. The term "distilled" only applies to the water because only the water went through the process.

Evaporation alone could be used for separation if we did not care about recovering the water. If only the salt is desired, evaporation is the simplest method of separation. If you want to keep the water as well, or if your purpose is to purify the water, then a more complex setup--one for distillation--is necessary.

Here you see a common distillation apparatus.
At the left side of this apparatus, a homogeneous liquid is being heated to cause evaporation. The vapor then leaves the container through a side arm. In some cases it's desirable to measure the temperature of the vapor that's being formed. In such cases a thermometer is placed in the flask as it is here.
The vapor that is formed from the heating goes into a second "container." This "container" is referred to as a condenser because this is where condensation from the vapor phase to the liquid phase takes place. This condenser has two compartments: there's a center portion where the vapor is condensed, and there is an outer portion called a jacket which contains cooling water. The cooling water from the tap goes in the bottom end of the condenser. It goes through the jacket and cools the vapor coming through the center of the condenser and then leaves at the top end.
The cooled liquid then leaves the right hand side of the condenser and goes into the adapter which channels the drops into some kind of container which will hold the purified liquid. This particular equipment is not necessary for distillation, many different set-ups can be used; it just happens to be convenient for chemists to use this kind of equipment.

Sublimation

Distillation is not the only process which can be used to separate materials. There are many others. As mentioned before, some solids can change to a gas when heated. When this happens, we say that the solid sublimes. Dry ice is one example of a solid that sublimes at a very low temperature. It does, however, require quite a bit of work to get low enough temperature to convert the gas back into solid form.

Another example of a solid that sublimes is iodine. There is a small crystal of iodine in the test tube shown below to the left. Iodine needs to be heated just slightly above room temperature to sublime (in the center pictures below) and will change back to the solid form when it cools back down to room temperature. It is hard to see but iodine has crystallized in the cooler upper right portion of the test tube shown below on the right. The overall process of a solid to gas to solid is often referred to as sublimation. Thus, the term, sublimation, applies to two different phenomena. One of those is changing from a solid to a gas. The other is changing from a solid to a gas and back to a solid.

Sublimation, like distillation, can be used to purify substances. That is because the process can separate materials that will sublime at a certain temperature from those other materials that will not sublime at that temperature. The material that goes through the phase change process, the sublimation in this case, is purified because it is separated from materials that were formerly mixed with it.

Dissolution and Recrystallization

Dissolution and crystallization or recrystallization are often used to separate materials. If sand and salt are mixed together, they can be separated because salt can dissolve in water and sand cannot. Salt can then be crystallized from the water by evaporating away some or all of the water.

Filtration

The task of separating sand from saltwater can be accomplished by decanting or filtering. Decanting means pouring off the liquid and leaving the solid behind. Decanters were developed to keep the sediment in wine from being poured into glasses along with the wine.

Filtration usually involves pouring a liquid and a solid into a paper cone. The liquid passes through the paper and the solid does not, thus separating the two. Filtration is commonly coupled with other processes such as dissolving to accomplish separations.

Top of page

Chemical Decomposition

The kinds of changes you've just been working with--such as solid to liquid, liquid to gas, these kinds of things--are generally referred to as physical changes. The same material is present, but the condition changes. There are other changes or processes in which the material itself is changed, not just the condition of the material. These changes that go beyond just simply changing phase or mixing and separating materials from one another are called chemical reactions. In chemical reactions the materials themselves change. In doing so they sometimes give off energy and sometimes they require energy. Let's take a look at one example now.

Electrolysis of Water

Let's consider a specific example: water. Pure water is a material which can go through distillation without a change in it properties. That is, it is water before and it is water afterwards. If it started as impure water, as it goes through the process it is separated from the impurities and becomes pure water. You can distill pure water over and over again; you can freeze it; you can thaw it; and it remains water throughout all those phase changes. It can, however, be separated into two components by a process known as electrolysis. This process will decompose water into two components, both are gases. Neither of the gases can be decomposed further - although they will react with other chemicals - and so those gases are elements. You'll have a chance to experiment with the electrolysis of water in Lesson 3.

In the next section, we'll spend a bit more time discussing the classification of materials (homogeneous mixtures, heterogeneous mixtures, compounds, and elements) and how different types of changes can be used to distinguish one type of material from another.

Top of page