Separations Based on Physical Changes

Lesson 2

Separations Based on Physical Changes

Introduction

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, which is very unscientific. 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.

Crystal of iodine before heating [1sublim1.JPG (8202 bytes)]

Crystal of iodine being heated [1sublim2.JPG (8430 bytes)]

Vapors of iodine from continued heating [1sublim3.JPG (8277 bytes)]

Crystals of iodine forming after cooling [1sublim5.JPG (8186 bytes)]

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.