Name ________________________________________________________ Mods. ____
Chemistry in Biology Labs
Background Reading and Demonstration: Non-polar and Polar molecules
In chemistry, a molecule is defined as two or more atoms sharing electrons. Electrons
have a negative charge and orbit around the positively charged particles (protons) and
neutrons (no charge) that make up an atom’s nucleus:
A hydrogen atom looks something like this:
-
H (single atom)
+
One of the simplest molecules in nature is Hydrogen gas (H2). In this case both atoms
share their one electron with the other atom. H2 looks something like this:
+
+
-
H2 Two atoms bonded together by sharing
electrons (a molecule).
The two electrons in the diagram above are orbiting both nuclei in a figure 8 pattern at an
immense speed. Since Hydrogen atoms are the same size, each nucleus “attracts” the
electrons equally, and thus, they are shared equally. Since this is the case the charges are
equaled out around the molecule. In other words, there is no positive “end” to the
molecule, nor is there a “negative” end to the molecule. Other examples of non-polar
molecules are O2 (Oxygen) and CO2 (Carbon dioxide). Common substances that are nonpolar are fats, oils and wax.
Now, let’s consider polar molecules…
The best example of a polar molecule is water (H2O):
H
H
O
An oxygen atom is larger than hydrogen. Oxygen has 6 protons (+) and 6 electrons (-),
compared to hydrogen’s 1 proton and 1 electron. In a water molecule the shared
electrons are moving around the three nuclei at an immense speed, but in this case the
oxygen atom has a greater pull on the electrons than the hydrogen atoms do. Because of
this, the electrons orbit the oxygen atom multiple times for every one time they orbit a
hydrogen atom. Thus, the “pole” or side of the water molecule that has the oxygen atom
takes on a slight negative charge due to the fact that the negatively charged shared
electrons are found there more often. The protons (+) of the Hydrogen atoms then give
the Hydrogen side of the molecule a slight positive charge. A water molecule looks
something like this:
H
H
O
Using the red and white magnetic water molecules, bind them together in a flat line. You
will notice that the water molecules can only “bond” together in one fashion. The
Hydrogen ends (+) always touch the oxygen ends (-) of the neighboring molecules. This
is another type of bond called a hydrogen bond. These are extremely important in
biological system.
Hydrogen bonding and polarity account for many chemical properties of substances
necessary for life such as:
 Solubility
 Cohesion
 Adhesion
 Molecular shape and function (especially proteins!)
 Selective permeability of membranes
Station #1 Polarity and Solubility:
Background:
 Polar substances can only dissolve in polar substances.
 Non-polar substances will only dissolve in non-polar substances.
Materials:
1. Two Petri dishes
2. water
3. cooking oil
4. Iodine
5. Copper sulfate
Procedure:
1. In one petri dish add enough water to cover the bottom.
2. In the other petri dish add enough oil to cover the bottom.
3. Put a crystal of iodine in each dish.
4. Put a crystal of copper sulfate in each dish.
5. Fill in the data table below:
Material
Polar or Non-polar?
Oil
Water
Iodine
Copper sulfate
How do you know this?
Station #2 Polarity and Solubility II:
Study the anatomy of a gobstopper below:
“Flaked-on” wax outer covering (NON-POLAR).
Food coloring coating (POLAR).
Sugar “core” (POLAR).
Procedure:
1. Fill a petri dish halfway up with water.
2. Place four different colored gobstoppers in the petri dish in the pattern shown below:
3. Do not agitate the dish and let the gobstoppers dissolve in the water.
4. Sketch the final observed pattern in color in the space below:
Answer the following questions about the gobstoppers:
1. Explain how both polar and non-polar molecules are necessary in this experiment to
obtain the final observed pattern.
2. Explain how in this lab a liquid is contained by another liquid.
Station #3 How Strong is the Surface Tension created by Cohesion?
Background: Cohesion is where a molecule has an attraction to another molecule
identical to itself. The obvious example is water. One molecule of water is attracted to
another due to hydrogen bonding. Cohesion is a very important property of which
comes into play in many biological processes.
Note: It is important for you consider that at the surface of any volume of water there are
virtually no water molecules above its surface. This is important, as the hydrogen
bonding between the molecules that make up the surface itself are slightly stronger than
those below the surface. Thus, a volume of water has a molecular “skin” on its surface
made up of hydrogen bonds.
Materials:
1. Needle
2. Beaker
3. Candle
Procedure:
1. Fill the beaker with water.
2. Place the needle in the water so that it sinks. Do this by putting it into the water point
first (vertically). This demonstrates that the metal that the needle is made out of is
denser than water.
3. Fish the needle out of the water and dry it off.
4. Now hold the needle horizontally above the water. Gently place the needle on the
surface of the water so that it does not sink. If your fingers touch the water’s surface
you will not be able to keep the needle “afloat.”
5. If you cannot get the needle to stay on the top of the water, rub the needle with the
side of the candle before setting it on the surface of the water.
Answer the following question about the needle experiment:
1. What allows the needle to rest on the surface of the water?
2. Why would using wax (non-polar) make it easier to rest the needle on the water?
Station #4 Breaking Cohesion and Surface Tension:
Background: The large molecules that make up many detergents have regions that are
polar and other regions that are non-polar. This allows detergents to dissolve things that
are both polar and non-polar. In water, detergents surround water molecules and thus,
weaken the hydrogen bonds between them.
Materials:
1. Glass
2. Water
3. Pennies
4. Dish detergent
Procedure:
1. Fill the glass all the way to the brim with water.
2. Continue adding drops of water with an eye dropper until water is visibly over the top
of the glass as shown:
Water above top of
3. Carefully drop pennies into the water until it spills
glass
over the sides of the glass.
4.
5.
6.
7.
Carefully drop pennies into the water until it spill over the sides of the glass.
Record the number in the chart below.
Mix a tablespoon of dish detergent into another beaker of water and repeat steps 1–4.
Record your results again in the chart below.
Data and Observations:
Number of
pennies without
detergent added
Number of
pennies with
detergent added
Answer the following questions about the penny lab:
1. What happens to the surface tension of the water when detergent is added to it?
 How do you know this is true?
Station #5 Bending a Cohesive Stream of Water:
Background: By rubbing a balloon against your hair you are giving the balloon a charge.
The friction created robs electrons from your hair and adds them to the surface of the
balloon. Since electrons are negatively charged the balloon’s surface becomes negatively
charged.
Materials:
1. Burette filled with water.
2. Two beakers
3. Balloon
Procedure:
1. Set up the burette as shown in the diagram to the right.
2. Rub the balloon on your head to accumulate a charge
3. Open the stopcock at the bottom of the burette so that a stream of
water flows into beaker #1.
4. Hold the charged balloon near the stream and try to make it bend
into beaker #2.
2
1
Answer the following questions about the bending water lab:
1. Below is a diagram of negatively charged object. Using the following symbol of a
water molecule
illustrate how three molecules would orient themselves in
relation to the
negatively charged object.
-
 Explain why a water molecule would rotate in the way that you illustrated:
-
2. Basing your answer on the questions above, what caused the stream of water to bend?
3. If you were to repeat this experiment using oil rather than water, would the stream
bend next to the balloon? Why or why not?
Station #6 Adhesion; Water Along a String:
Background: Adhesion is where a molecule has an attraction to another molecule
different from itself. A good example of this is a drop of water on a window pane. Even
though gravity is pulling on the water drop it may not run down the glass. The reason is
that there is an attraction between the water molecules in the drop and the molecules in
the glass. Hydrogen bonding is again the force between the molecules. Adhesion is
another very important property which comes into play in many biological processes.
#1
Part A: Water Along a String:
Materials:
1. Dry string (about 80 cm long).
2. Two beakers
3. Water
A
B
#2
Procedure:
1. Hold the string in your fingers as shown in the diagram above.
2. Point A and point B in the diagram represent where you should hold the string.
3. Let the string “dangle” into beaker #2
4. Pour water from beaker #1 onto the dry string and observe what happens.
5. Dunk the string in the beaker of water and repeat the steps above. Observe what
happens this time.
Answer the following questions about the string lab:
1. Explain how both adhesion and cohesion are necessary for the water to travel along
the string successfully and land in beaker #2.
Station #7 Polarity and how Biological Molecules Fold:
Background: By now you understand that water is polar and oil is non-polar. In this
experiment you will explore one more facet of polar and non-polar molecules that is
extremely important in biological systems. In this activity you should remember that
graphite (pencil) is non-polar and paper is polar.
Procedure:
1. Darken one side of a piece of plain paper with graphite from a pencil. It is important
that the paper is extremely well coated with graphite.
2. Use a hole punch to create several circles of paper that are coated with graphite on
one side and are plain paper on the other.
3. Place the circles into a jar that is filled with both oil and water.
4. Shake the jar.
5. Observe the orientation of the circles after they have had a few minutes to settle.
6. In the diagram to the right, shade the color of the
disks as they appear from the top (black or white).
Answer the following questions:
1. What liquid formed the top of the column in the
container?
2. Why do the two liquids not mix?
3. Which liquid is denser?
 How do you know this?
Interface of
oil and water
4. How do you explain the pattern of the disks in relation to their polar/non-polar nature
compared to the polar/non-polar nature of the liquids they are in?
5. Proteins are very important biological molecules. The function of proteins in living
things often depends on their shape; they are often folded into intricate shapes. Proteins
are made up of amino acids which can be polar (positively or negatively charged) or nonpolar. Below is a model of a protein made up of 10 amino acids.
+
-
In the space below draw the way that this protein would fold:
Station #8 Acids, Bases and Indicators:
Background: Acids and bases are also very important in many of the biological systems
in the natural world. Acids are substances that produce a hydrogen ion (H+) and bases
are substances that produce hydroxide ions (OH-).
Hydrochloric acid (HCl) is a common acid which might look something like this:
H
l
Cl
When HCl dissociates it creates H+ions and Cl- ions:
ClH+
l
The more H ions a substance produces, the stronger and acid it is. In fact, this is the
+
origin of the shorthand “pH” which stands for Potential Hydrogen ions.
Sodium hydroxide (NaOH) is a common base which might look something like this:
Na
O
H
When NaOH dissociates it creates Na+ions and OH- ions:
O
Na+
H
You should recall that the pH scale is used to measure the strength of an acid or a base.
Remember, the pH scale is a logarithmic scale so 6 is ten times as acidic as 7. In other
words something that is 6 on the scale will produce ten times the Hydrogen ions as
something that is 7. Something that is 5 will produce ten time the Hydrogen ions as
something that is 6 but 100 times the Hydrogen ions of 7. The same is true as you move
to the right on the scale except here you are measuring the amount of Hydroxide ions
(OH-) that is being produced.
ACIDS
0
1
2
3
NEUTRAL
4
5
6
7
8
BASES
9
10
11
12
13
14
Procedure:
1. Wear goggles for this portion of the lab.
2. Put 100 mL of deionized water (pH = 7) into a beaker.
3. Turn on the magnetic stirring rod until you get a good vortex.
4. Slowly add drops of Universal Indicator until the water turns a color and stays that
color.
5. Slowly add drops of HCl until the solution changes color and stays that color.
6. Slowly add drops of NaOH until the solution changes color and stays that color.
7. Record your observations in the chart below.
Substance
Universal Indicator
Deionized water
HCl
NaOH
pH range?
Color w/Universal Indicator
Answer the following questions:
1. What color does Universal indicator turn as you add acid?
2. HCl, H2SO4 and HNO3 are all examples of acids. What do they all produce that
makes them an acid?
3. If Universal indicator could be represented in the following way 
it would it look like if it was combined with a hydrogen ion:
U
, draw what
U
, draw what
4. What color would the compound you just drew be?
5. If Universal indicator could be represented in the following way 
it would look like if it was combined with a hydroxide ion:
6. What color would the compound you just drew be?
Station #9 Exothermic Reactions:
Materials:
1. 50 mL Beaker
2. Water
3. 1 scoop of Calcium metal
4. Thermometer
Procedure:
1. You must wear goggles for this portion of the lab!!
2. Add the water to the beaker and record the temperature of the water.
3. Add the calcium chloride to the beaker of water.
4. Stir the solution gently with the thermometer.
5. Record your data in the table below.
Data and Observations:
Temperature of water before reaction
Temperature of water after reaction
Station #10 Endothermic Reactions:
Materials:
1. 50 mL Beaker
2. Water
3. 20g barium hydroxide Ba(OH) 2 .8H2O(s)
4. 10 g ammonium thiocyanate (NH2SCN)
5. Small wooden block.
Procedure:
1. You must wear goggles during this station of the lab!!
2. Put about 20g of barium hydroxide in a 50 mL beaker.
3. Add 10g of ammonium thiocyanate to the beaker.
4. Stir the two solids together with a wooden splint.
5. Place the beaker on the small wooden block with a small pool of water between the
beaker and the block.
6. After the reaction has been allowed to take place for a few minutes the beaker should
freeze the water on the block and stick the beaker in place. Do not move the beaker
as you stir the two solids together.
7. Record your results in the data table below:
Data and Observations:
Temperature of water before reaction
Temperature of water after reaction