Examples

Water and Carbon Tetrachloride

Water is a common example of a polar molecular material. Carbon tetrachloride is a common example of a nonpolar molecular material. If pictures of angular and tetrahedral molecules have not already sprung into your mind, perhaps I should refresh your memory. It might also be a good idea for you to stop and take a few minutes to draw the electron dot diagrams of these compounds and explain to yourself why they are such good examples of polar and nonpolar molecular materials. Ask for help if you need it.

Let's test water and carbon tetrachloride to see if they are soluble or insoluble in one another. Write down your observations in exercise 4 in your workbook. I'll test them by putting about 1 mL of each into a test tube and then mixing them (left). If the two materials combine to form one homogeneous liquid, then they are soluble in one another. If they are insoluble in one another, then they will not combine to form one liquid, there will be two separate layers in the tube. As you can see, there are two layers (right). Polar water and nonpolar carbon tetrachloride are not soluble in one another.

Samples of water and carbon tetrachloride. [mixmol2.JPG]

Water and carbon tetrachloride mixed together. [mixmol3.JPG]

Next, let's test a few other things to see how soluble they are in water, and how soluble they are in carbon tetrachloride. Record your observations in exercise 5. You can base your observations on what you see here on the screen or by making your own observations in the lab when you are there.

Iodine

The first test is to add a crystal of iodine (I₂) to water (left) and to carbon tetrachloride (right). Both have had the same amount of time and stirring and you can see that the purple iodine crystal remains undissolved in the water but has dissolved in the carbon tetrachloride. As you record your observations note that iodine molecules are nonpolar.

Sugar

Here [top] we have water (left tube) and carbon tetrachloride (right tube) with sugar. After [bottom] equal amounts of sugar have been added to both tubes and with the same amount of time and stirring, you can see that the sugar has dissolved in the water but has not dissolved in the carbon tetrachloride. As you record your observations, note that sugar molecules contain polar angular C-O-H groups.

Sugar in water and carbon tetrachloride.

Ethanol

Here ethanol (ethyl alcohol) has been mixed with water (left tube) and with carbon tetrachloride (right tube). Note that ethanol mixes with both. Half of each ethanol molecule contains a polar C-O-H group and the other half contains essentially nonpolar C-H bonds. That gives ethanol molecules both polar and nonpolar characteristics.

Acetone

Acetone is another molecular material with both polar and nonpolar characteristics. Here acetone has been added to water (left tube) and carbon tetrachloride (right tube). As you can see, it has mixed with both the polar water molecules and with the nonpolar carbontetrachloride molecules.

More on Iodine

Next, let's extend our observations of iodine to include not only the solubility of iodine in water and carbon tetrachloride, but also in ethanol and acetone. From left to right the tubes contain water, ethanol, acetone and carbon tetrachloride. Approximately equal amounts of iodine were added to each tube and then stirred the same amount. You can see from the intensity of the colors that the amount of iodine that dissolves in each solvent varies. Nonpolar iodine is most soluble in nonpolar carbon tetrachloride and least soluble in polar water.

Water, ethanol, acetone and carbon tetrachloride with iodine.

Iodine in water, ethanol, acetone and carbon tetrachloride.

Purpose

The purpose of performing the tests above was to have you see that the "like dissolves like" rule of thumb is a crude approximation to things that actually happen. It is a valid rule, but not totally so. First of all, you cannot arbitrarily classify each and every material as being either nonpolar or polar. There are many degrees of polarity, all sorts of gradations in between the extremes of polar and nonpolar. You have seen that things such as ethanol and acetone are not only soluble in water, but also soluble in carbon tetrachloride. They are polar enough to dissolve in water, but not so polar that they won't dissolve in carbon tetrachloride. They are partly polar or slightly polar and they will dissolve in both.

When iodine was tested, you saw that it was very soluble in carbon tetrachloride and insoluble in water (very slighlty soluble if given a longer period of time). This is because iodine is nonpolar. You also saw that the iodine was less soluble in ethanol and acetone than it was in carbon tetrachloride. Ethanol and acetone are more polar than carbon tetrachloride. Iodine was more soluble in them than it was in water because they are less polar than water. In summary, the solubility of iodine decreased as the polarity of the solvent increased.

You should remember the phrase, "Like dissolves like," but also remember that it is an over-simplification of the way that chemicals actually interact with one another.

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E-mail instructor: Eden Francis

Clackamas Community College
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