Water Properties
Organisms are composed of from forty to fifty percent water to over ninety
percent water. In addition, most of the important chemical reactions of life
take place in water. Because of these and other reasons, there is no doubt
that water is essential for life as we know it. Ultimately, water’s importance
can be attributed to its chemical structure. Water’s chemical structure
gives it some important properties that help make life possible.
Engage questions
1. How does water rise from the roots of a redwood tree to the very top?
Several forces collaborate to make this possible. Adhesion of water to the wood and
cohesion of water molecules with one another play a critical role in the climbing
process. The continual flow of water into the roots and evaporation of water from the
leaves maintain an upward flow.
2. How do insects walk on water?
Due to cohesion of the water molecules, the water surface is firm enough to support the
insect's weight.
3. Why does ice float rather than sink?
Most elements or molecules are closer together in the solid state than in the liquid state;
therefore, the solid is more dense and falls to the bottom of its corresponding liquid.
Water is different. In water, the molecules actually move apart as freezing occurs, to
form a regular latticework. of H2O molecules. As a consequence, ice is less dense than
water and floats on top of it, a characteristic very important for the maintenance of life
on earth.
4. Why do people become seriously ill, or die, if they go without liquid for a
week or so?
The human body carefully maintains a particular salt and sugar content in the blood and
lymph , keeping the concentrations of these substances within a very narrow range. When
humans are deprived of water, this balance becomes increasingly difficult to maintain.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
5. How would life in a lake be affected if ice sank and lakes froze from the
bottom up?
Many organisms survive the winters in the water under the ice. If lakes froze from the
bottom up, these organisms would be much more likely to die
Background Information:
Water covers about three fourths of the surface of the earth. It is one of
the simplest yet most important molecules in living systems. It makes up
from 50 to 95 percent of the weight of living organisms. The cytoplasm of a
cell is a water-based solution that contains a variety of ions, salts, and
molecules which make life 'happen.' Water is literally involved in every facet
of life.
The simplicity of the water molecule belies the complexity of its properties.
Based on its small size and light weight, one can predict how it should
behave, yet it remains liquid at a much higher temperatures than expected.
It also boils and freezes at much too high, or low, of a temperature for a
molecule of its size. Many of these unexpected properties of water are due
to the fact that water molecules are attracted to each other like small
magnets (cohesion). This attraction results in turn from the structure of the
water molecule and the characteristics of the atoms it contains.
Each molecule of water is made up of two atoms of hydrogen connected to
one atom of oxygen, as shown below. This is summarized in the familiar
formula, H2O.
Hydrogen Bonding in Water
Note: Hydrogen has a positive charge and oxygen has a negative charge.
These charges cause water molecules to stick together like magnets. This
“stickiness” is due to hydrogen bonding. Hydrogen bond are much weaker
than covalent bonds so they easily break and form again.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
Station One
Surface Tension and Adhesion
Part I. Problem: How many drops of water you can you place on the surface
of a penny before it overflows?
1. How many drops do you predict?
Number of Drops Predicted
person
#1
person
#2
person
#3
person
#4
Total
1-4
Average
2. Drop water from the dropper onto a penny, keeping careful count of each
drop. Draw a diagram below showing the shape of the water on the penny
after one drop, when the penny is about half full, and just before it
overflows.
Drawing of Drops
Questions:
1. How many drops were you able to place on the surface of the penny before
it overflowed? __________ drops
2. If the number of drops is very different from your prediction, explain
what accounts for the difference.
Students' predictions will typically underestimate this effect so they will likely be
surprised. The students will find that they can put a surprising number of water drops on
the head of a penny (typically 30 - 40) and they will observe how the water "piles up" on
the penny.
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3. Explain your results in terms of cohesion.
This exercise provides a demonstration of the 'stickiness' of water caused by cohesion.
The effect results from the formation of a weak hydrogen bond between the oxygen of one
water molecule (which carries a partial negative charge) and the hydrogen of another
water molecule (which carries a partial positive charge). The many weak bonds that are
formed are additive in strength, working in unison to bind water together. The water
molecule is polar because of its partial charges
Station I: Part II. Effects of Detergent
1. With your finger, spread one small drop of detergent on the surface of a
dry penny.
2. How many drops do you think this penny will hold after being smeared with
detergent, more, less, or the same as before? Why?
3. Using the same dropper as before, add drops of water to the penny
surface. Keep careful count of the number of drops, and draw the water on
the penny after one drop, about half full, and just before overflowing.
Drawing of Drops on a Penny with Detergent
4. How many drops were you able to place on the penny before it overflowed
this time? __________ drops
5. Did the detergent make a difference? Describe the effect of the
detergent.
Because of the amphipathic structure, one end of the detergent molecule forms hydrogen
bonds with water molecules, but the other end does not. When water hydrogen bonds with
detergent, it is not bonding to other water molecules. As a result, the overall forces
binding the liquid together are greatly diminished.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
6. What does the detergent do to have this effect on water?
The detergent is similar in structure to phospholipid molecules in our bodies which make
up cell membranes. Phospholipids are also amphipathic, having a polar phosphate
group at one end and two non-polar fatty acids at the other. The 'oil / water', 'polar /
non-polar' distinction is a very important factor in the structure and behavior of living
cells.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
Station I: Part III. Drop Shape on Glass and Wax Paper
1. What will the shape of a drop of water on (a) wax paper and (b) a glass
slide? Draw the shape you expect on each surface:
___________
wax paper
____________
glass
2. Why did you predict as you did? What assumptions are guiding your
thinking?
It is a good idea to ask students to discuss their predictions and the reasons for making
them. This prompts the students to identify and examine their underlying assumptions
about how water behaves.
3. Perform the experiment. Place several drops of water on each
surface and draw the results below.
___________
wax paper
____________
glass
4. Compare your predictions with your observations and explain.
On wax paper water binds to itself and not to the non-polar waxy surface, forming a little
ball due to the cohesiveness and the resultant surface tension of water molecules. Some
students will predict this effect (especially if they have polished cars or had similar
experiences), but many will not. The effect is explained by hydrogen bonding among
polar molecules and by the spontaneous separation of polar and non-polar molecules.
5. Explain the differences in drop behavior in terms of adhesion.
The drop shape on glass and wax paper demonstrates the interaction of water with polar
(glass) molecules and non-polar (wax paper) molecules. Water will bind via hydrogen
bonds to the glass slide and spread out in a flat puddle; the attraction between polar
molecules of different types like this is called adhesion. In contrast, the water is not
attracted to the wax paper.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
Station II
Cohesion of Water
Station II: Part I. Oil and Water
1. Put 8 mL of water into a 10 mL graduated cylinder.
2. What will happen if you add cooking oil (choose one of the answers
below)?
a. The oil will float to the top of the water.
b. The oil will sink to the bottom of the water.
c. The oil will dissolve in the water.
d. The oil will become mixed up with the water.
e. Other predictions (explain)
3. Gently add 2 mL of cooking oil by tilting the cylinder of water slightly
and letting the oil run slowly down the inside of the cylinder.
4. What happened?
Students should observe that added oil tends to float on the water. Sometimes a bubble of
oil or all the oil will be trapped below water due to the cohesiveness of water, which
forms a boundary that the oil has difficulty penetrating. However, if a glass stirring rod
is used to disrupt the interface between the oil droplet and the water, the oil will rise to
the top. This illustrates again the powerful effect of polarity, in which polar water and
non-polar oil molecules clearly separate from one another and avoid interaction or
intermixing. It also shows that oil is less dense than water, meaning that oil weighs less
per unit volume (e.g., per cc) than water does.
5. Which is less dense, oil or water?
Oil is less dense than water.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
6. What mechanism causes water molecules and oil molecules to separate
from one another? Use the terms polar and nonpolar molecules.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
Station II: Part II. Water, Oil and Dye
1. Predict what will happen if you add a few drops of a water-soluble dye
solution to your graduated cylinder containing water and oil. Will the
dye mix with water, oil or both?
Students should observe that when a water-soluble dye is mixed with water and oil, it will
dissolve into the water layer and not the oil layer.
2. Perform the experiment. Add a few drops of dye to each cylinder.
Use a glass stirring rod to penetrate the interface between each
layer, giving the dye access to both water and oil. How does the dye
behave in each cylinder? Does it diffuse into the oil? Into the
water?
As noted above, if the dye is water-soluble, it will go into the water layer.
3. Compare your predictions and results. Explain any differences.
Students should be encouraged to think and talk about the ways in which their
observations differed from their predictions. When there is a contradiction, students often
assume their mental model is correct and their observations were incorrect. Comparing
observations made by all the students in the class helps to build a consensus model of
what actually happened. If this consensus model of observations is different from
students' predictions, then the students need to think about the assumptions they were
making, if those assumptions need to be changed. And if so, how.
4. Stir the contents of the cylinder with a stirring rod and then let it
sit.
5. Will the contents remain mixed? Why do you think so?
One can stir vigorously and to mix oil and water pretty thoroughly, as with salad
dressing. When allowed to sit, however, such mixtures will separate once again, with oil
and water each seeking their own layers
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
6. Observe what happens, compare with your prediction, and explain why
it happens. Your explanation should involve polarity, polar and nonpolar molecules, solution, and hydrogen bonding.
Students should observe the oil and water separating, although the process may occur
slowly. Again, this is a case of polar molecules seeking a polar environment and avoiding
a non-polar environment. Any substance that can dissolve in water is polar.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
Station III
Water as the Universal Solvent
Procedure:
1. Without stirring add ½ teaspoon of salt to a beaker.
2. Without stirring add ½ teaspoon of sugar to a beaker.
Questions:
1. What property of water is being demonstrated?
2. Name two things you can do to make the sugar and salt dissolve
faster.
3. Name the solute and the solvent.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
Station IV
Acids and Bases
Procedure: Touch the end of the litmus paper to each of the different
substances.
Questions:
1. What color does the acid turn the litmus paper? Where is an acid on
the pH scale?
2. What color does the base turn the litmus paper? Where is a base on
the pH scale?
3. What property of water does this demonstrate?
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
Station V
Climbing Property of Water
I.
Background: Water moves to the tops of tall trees due to
capillary action combined with root pressure and evaporation
from the stomata in the leaves. Water will also climb up
paper, and will often carry other molecules with it. The
distance traveled by these other molecules will vary with
their mass and charge.
II.
Predict: How fast do you think water would climb a strip of
absorbent paper about a ½ inch wide? Answer: about one
inch per ______________ (unit of time)
III. Procedure:
1. Obtain a 50 mL graduated cylinder and tear off a strip fo
chromatography paper that is just long enough to hang
over the side of the cylinder (inside) and reach the
bottom.
2. Run the paper strip along the edge of a scissor blade to
take the curl out of it if necessary.
3. Place a single drop of ink from a black vis-à-vis (overhead
pen) on the paper, about 1 inch from the bottom and let it
dry completely.
4. Put 10 mL of water into the graduated cylinder and place
the strip of paper in the cylinder so that the bottom is
immersed in water and the drop of ink is just above the
surface of the water. Fold the paper over the top side.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
50 ml Graduated Cylinder with Chromatography Paper & Ink
5. Note the time starting below.
__________
6. Watch and note the time at 5 minute intervals. When
the water climbs to the top of the paper, remove the
paper from the water, and let it dry.
Time of Water Climbing
Time (min)
0
5
10
15
20
25
30
Distance (in)
7. How did the ink change? Glue the paper onto your lab
report and label each color on the strip.
With a water-soluble ink, such as that in a Vis-a-Vis pen, the dye in the ink is dissolved in
the water. That is, the dye breaks up into its component polar molecules, and each
molecule of dye is surrounded by and perhaps hydrogen-bonded with water molecules an effect often described as the cage effect. The surprise in this experiment is that black
Vis-a-Vis ink contains molecules of several different colors (this is not true of all black
inks). Students will observe bands of colors formed as the water rises up the paper.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html
8. How do you explain the results? Your explanation should
include capillary action, polar molecules and hydrogen
bonding.
Hydrogen bonding and polarity also account for the ability of water to climb up a piece
of paper hung vertically, telling us that, like glass, paper contains polar molecules. The
flow of water up paper or through a polar tube is called capillary action. When water
moves up paper or through a gel, the molecules that are dissolved in it are carried along
with the water. Each type of molecule moves at a different rate depending upon its mass
and its charge. Since the different types of molecules move at different rates and over
different distances, the molecules are sorted into bands of similar molecule types. In this
case, the different types of molecules are revealed by their different colors. This
fascinating effect is widely used in biological research for separating, purifying, and
identifying components in a substance. More often than not in research studies, the
molecules themselves are not colored and dyes must be added in order to distinguish the
bands.
Lesson modified from http://www.biologylessons.sdsu.edu/ta/classes/lab1/lab1.html