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
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.
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
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.
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 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
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.
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
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