Name _______________________________________________________Period ____________Day______
Lab: Properties of Water
Water is a simple, small molecule, and without it, life on Earth could not exist! It is composed of two hydrogen
atoms and one oxygen atom. Each of the hydrogen atoms shares its single electron with oxygen in a covalent
bond.
Water has unique properties because it is a polar molecule. A polar molecule is one in which the elements
share electrons, but do not share them equally. One or more of the elements pulls on the electrons more
forcefully than the other elements. This causes a partial positive charge in one area of the molecule and a partial
negative charge in another area of the molecule. In the case of water molecules, the oxygen pulls more on the
shared electrons than the hydrogen does, resulting in a polar molecule.
Cohesion occurs when like molecules are attracted to each other and form bonds. Each water molecule can
establish hydrogen bonds with four other water molecules. Weak hydrogen bonds between the molecules of
water must be separated in order for water to change form. Separating these hydrogen bonds requires energy.
These bonds, which are constantly forming, breaking, and reforming, only last for a fraction of a second. At any
time, a large portion of the molecules present are bonded to other water molecules. Because of this, changing
the temperature of water requires adding or removing more heat than any other naturally occurring material on
Earth requires.
At the surface of a group of water molecules, the outer molecules share bonds with fewer molecules than the
inner molecules. As a result, the bonds between the outer layer of molecules are stronger and do not break as
easily. The resulting outer layer of strongly bonded molecules forms a film called surface tension. The amount
of surface tension a liquid has is a measure of the strength of the bonds in its outer layer.
The polarity of a water molecule also enables it to form bonds with many other kinds of molecules. The
positive hydrogen side of the molecule is attracted to negatively charged molecules or negative parts of other
molecules. The negative oxygen side of the molecule is attracted to positively charged molecules or positive
parts of other molecules.
Density is defined as the amount of mass in a unit of volume. Frozen water, or ice, is less dense than water in its
liquid form. As a result, ice floats. If ice sank, lakes and ponds could completely freeze. This would be
devastating to the fish and other life.
In this Virtual Lab, you will assemble water molecules, position water molecules to form hydrogen bonds,
identify some properties of water, and observe the relationships between water molecules at different
temperatures.
Objectives:
·Describe how the polarity of water molecules gives water unique properties.
· Define the terms adhesion and cohesion.
· Describe how cohesion relates to surface tension.
· Describe the density of water at different temperatures.
Procedure:
1. Drag the oxygen and hydrogen atoms to the Work Area. Arrange them to form a water molecule. Click
Check. If the molecule is correct, you will see covalent bonds appear between the oxygen atom and each
hydrogen atom. Notice the positive and negative charges of the molecule. (see picture p. 40 Biology text)
2. Drag another water molecule to the one you just made. Position it so it will form a hydrogen bond with the
first water molecule. Click Rotate if you need to turn the molecule. (see p. 41, note what is close to what)
3. Click Check to see if the water molecules form a hydrogen bond. (if too close or too far, they will not bond)
4. Repeat steps 2 and 3 until you have placed six water molecules.
5. Click the Reference Tool to read about some properties of water. Drag each label to the appropriate place in
the arrangement of water molecules. Then click Check.
6. To see the arrangement of water molecules at different temperatures, click one of the Temperature buttons: 20°C, 30°C, or 110°C.
7. To see the molecules in motion, click Motion. To see the motion of the molecules at different temperatures,
repeat step 6 and click Motion again.
Complete the following questions on YOUR OWN PAPER!
1.
2.
3.
4.
5.
How are adhesion and cohesion similar? How are they different?
How does this property make hydrogen bonds between water molecules possible?
How does this property make hydrogen bonds between water molecules possible?
Describe surface tension in a group of bonded water molecules.
Describe the arrangement of water molecules in solid, liquid and gas form. How does the movement of
the molecules differ in each form?
6. The density of a substance refers to how close together the molecules in that substance are packed.
Compare the density of water in its three phases.
7. Water is often called the 'universal solvent' because it can dissolve more substances than any other
liquid. Explain how the polarity of the water molecule makes this possible.
Conclusion: Write a conclusion in which you summarize the special properties of water and explain how they
make it possible for life to exist on Earth. This should be a couple of paragraphs. Use your book for
assistance in finding the importance of each property for life. This section is worth 35 points of the 50 point
lab score.