Polarity Activities
Patti Mason, Delphi High School, Indiana
Molecules with polar bonds can be polar or non-polar. If they are completely
symmetrical, the symmetry of the molecule can cancel out the polarity of the bonds. If
a molecule is polar, it may have greater attraction for other polar molecules than nonpolar molecules will. Non-polar molecules have an even distribution of charge, and will
not attract one another, and thus will mix quite well. In general, the phrase “ Like
dissolves like” describes the solubility of solutes in solvents. That is, polar solvents such
as water, will easily dissolve other polar molecules; polar solvents will also dissolve ionic
compounds because ionic compounds represent the extreme case of a polar molecule.
Non-polar solvents will generally dissolve non-polar solutes.
Part I
In this activity, you will determine the solubility of a variety of solutes in two different
solvents, water and hexane.
Obtain 7 test tubes of the same size, and place 5 mL of water in each one.
( After you have measured the first one, you may estimate the 5 mL level in the other
6 tubes.)
Test the water solubility of each of the seven solutes by adding a different solute to
each test tube. For liquid solutes, add about 20 drops. Transfer a pea-sized sample of
each solid solute using a spatula or a wood splint. Gently mix the test tube and
contents by flicking the bottom with your finger.
Record your observations in the chart: S = soluble, SS = slightly soluble, IN= insoluble.
Discard the samples into the waste container labeled “Water Waste.”
Clean and dry the test tubes and repeat using hexane as the solvent.
Dispose of the hexane and solute wastes in the container labeled “Hexane Waste.”
Solute
Water ( H2O)
Hexane (C6H14)
Urea ( CO(NH2)2 )
Iodine ( I2)
Ammonium chloride
Naphthalene ( C10H18)
Copper(II) sulfate
Ethanol ( C2H5OH)
Sodium chloride
Which solutes were more soluble in water than in hexane?
Which solutes were more soluble in hexane that in water?
What can be said about the polarity of each of the solutes?
Part II
Capillary action depends partially on the polarity of the molecules of the liquid and of
the material that forms the tube or space into which the liquid rises. A polar attraction
between the material of the tube and the molecules of a polar liquid causes the liquid to
rise into the tube. As one molecule moves up, it attracts neighboring molecules which,
in turn attract their neighbors. Once the upward attractive force is equal to the
downward force of gravity, the liquid stops rising. An extreme example of this
phenomenon occurs in giant trees like redwoods. It is estimated that these large trees
move about 2000 liters of water each day from their roots to the uppermost leaves by
capillary action.
Obtain a well-plate, eight capillary tubes, and a container of water. Place a few drops of
water in one of the wells in the plate. With one end of the tube, touch the water. The
top of the tube must be open. Leave the tip of the tube in contact with the liquid for at
least 15 seconds. Once the liquid stops rising, remove the tube from the well and
measure the height of the column of liquid. Record the value. Drain the capillary tube
by touching its tip to a paper towel. Repeat the procedure to allow liquid to rise into the
tube 2 more times. Record the heights in the table.
Repeat the procedure using different liquids in the wells, and a new capillary tube for
each liquid. (Note: the glycerol will take more than 15 seconds to reach its final height.)
Calculate the average height of the liquid columns for each substance tested.
Liquid
Water
Ethanol
Ethylene glycol
glycerol
#1 ( Height)
#2
#3
Average
Based on the heights of the liquid columns, draw a conclusion about the relative
polarity of the solvents tested.
Part III
Hydrogen bonding is due to the polarity of the oxygen-hydrogen, nitrogen-hydrogen, or
fluorine-nitrogen bonds in molecules. Hydrogen bonding is an intra-molecular bond.
That means it occurs between molecules, not within a molecule. Hydrogen bonding in
water is responsible for some of water’s interesting properties. One of these properties
is surface tension. In this activity, you will investigate hydrogen bonding and surface
tension by seeing how many drops of a liquid you can place on a penny before it runs
over.
Trial number
Number of drops
Trial #1
Trial #2
Trial #3
average
You will be given three solutions: sodium carbonate, sodium chloride and a 1% liquid
detergent sample. Your task is to design and carry out an experiment to determine their
effect on the surface tension of water.
Design a procedure which includes a control, steps that will ensure that the results are
reproducible, and predictions of the effect of each solute.
Create a table that will organize your data clearly.
Questions:
1. Compare the average number of drops placed on your penny with the results
obtained by some of the other lab groups. What might account for differences?
2. What were your predictions for the effect of each solute on the surface tension of
the water?
3. What was your control?
4. What steps did you include in your procedure to increase the reliability or
reproducibility of your results?
5. Will the addition of salts always change the surface tension of water?
6. What effect did the detergent have on the surface tension?
7. What are some practical applications of what you tested and learned?