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LAB: Why is WATER essential to all forms of life?
Learning Target: I can define and explain the unique properties of water that are essential to living organisms.
PART A. Structure of WATER- 3D Water Molecule Kits
Materials: On your table is a cup of water. Observe the water inside the beaker. With the spoon, stir the water,
then scoop the water out. Scoop the water back into the cup.
1. How would you know that these models represent water molecules (even if not on the cup or stated on
the lab sheet)?
2. How do the water molecules interact with each others? What about the models is making them do this?
3. Remove one water molecule from the water kit cup. What is the chemical formula of water?
4. Discuss with your partner what the H, O and 2 represent. Draw, label and color a model of the water molecule,
identifying which parts represent the hydrogen and which the oxygen. Water, as a real molecule, does not
have colors. It is impossible to see a single water molecule with the naked eye or even with the most
powerful light microscope. There are 3 x 10 of molecules in one drop of water
(3,000,000,000,000,000,000,000). A “water drop” made of 3 x 1021 of 3D Water Molecules would be
larger than the Earth!
5. Name and define the type of bond that holds the oxygen and hydrogen atoms together.
21
Water is a polar molecule. You can think about polar molecules in the same way that you can think about a
magnet’s poles. Polar molecules have a region with a slight positive charge and a region with a slight negative
charge.
6. On a water molecule each hydrogen atom has a partial positive charge and the oxygen atom has two
partial negative charges. Show this on your drawing by placing a + symbol on each hydrogen and two –
symbols on the oxygen.
7. What is it about real water molecules that would make them behave the way the models do?
When the oxygen atoms of 2 different water molecules come together, they repel. When the hydrogen atoms of
2 different water molecules come together, they repel. When an oxygen atom and a hydrogen atom from two
different water molecules come together, they attract. This attraction between molecules with partially charged
regions is called a hydrogen bond.
8. What are the similarities and differences between magnets and water
molecules?
STOP for Learning Check! (Raise for hand for teacher.)
Return all the water molecule models to the cup, replace lid, and turn in cup to kit box.
Complete FUN WITH WATER Activities.
Water fact: A hydrogen
bond is about 1/20 as
strong
as a covalent bond.
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PART B. Fun With Water- Observing Properties of Water
Activity #1 PILE IT ON.
Materials: 1 DRY penny, 1 eye dropper, water.
Procedure: Make sure the penny is dry. Begin by estimating the number of drops of water that can
be piled on the penny before it spills over. Gently place drops of water on the penny until the water
spills over.
9. Record the results (the total number of drops).
10. Identify the properties of water responsible for this observed phenomena.
Activity #2 Capillarity
Materials: 1 capillary tube (glass—VERY fragile) water
Procedure: Touch the tip of tube just to the surface of the water.
11. Record observations.
12. Identify the properties of water responsible for this observed phenomena.
Activity #3 Water as Glue?
Materials: 2 glass microscope slides, dropper, small beaker of water
Procedure: Place 1 drop of water onto one of the slides. Place the other slide on top. Try to pull apart the two slides.
13. Record observations.
14. Identify the properties of water responsible for this observed phenomena.
15. Write an example of a situation where you have seen water cause an item to stick to another or used water to stick
something (without glue).
Activity #4 Soap it up.
Materials: Wax paper, dropper, toothpick, soap, water
Procedure: Place one drop of water on the wax paper.
16. Draw the drop in your lab notebook (sideview).
17. Identify the properties of water responsible for this observed phenomena.
Dip the tip of toothpick in speck of soap. (Just touch the tip to the soapy spout of soap bottle. Touch the tip of toothpick
to water droplet.
18. Record your observations.
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Activity # 5 Can You Take the Heat?
Materials: 2 equal amounts of water and vegetable oil, 2 250-mL beakers, 2 thermometers, 2 hot plates, 2 ring stands:
each ring stand will have a clamp to hold the thermometer, Cold water bath for cooling the Erlenmeyer flasks or beakers
Procedure:
Set the cooled flasks containing their solutions on the ring stands or hot plate.
Take the initial temperature reading of each of the liquids.
Turn on the hot plate to a medium temperature.
19. Record the temperature of the liquid in each flask every 2 minutes until 4 minutes
20. After each liquid boils, record the temperature in the table on your lab sheet.
21. Based on your evidence, which substance has the highest specific heat? The lowest?
22. Think about and explain the relationship between high specific heat of a liquid and
hydrogen bonding.
23. Compare the boiling temperatures of water and of oil. What is the relationship
between hydrogen bonding and boiling temperature?
24. What happened to the temperature of the water and the oil after boiling? Explain why.
25. Identify the properties of water responsible for this observed phenomena.
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2.2 Properties of Water
KEY CONCEPT Water’s unique properties allow life to exist on Earth.
MAIN IDEAS
• Life depends on hydrogen bonds
in water.
Connect When you are thirsty, you need to drink something that is mostly
water. Why is the water you drink absolutely necessary? Your cells, and the cells
of every other living thing on Earth, are mostly water. Water gives cells structure
and transports materials within organisms. All of the processes necessary for life
take place in that watery environment. Water’s unique properties, which are
related to the structure of the water molecule, are important for living things.
Life depends on hydrogen bonds in water.
How do fish survive a cold winter if their pond freezes? Unlike most
substances, water expands when it freezes. Water is less dense as a solid
than as a liquid. In a pond, ice floats and covers the water’s surface. The ice
acts as an insulator that allows the water underneath to remain a liquid. Ice’s
low density is related to the structure of the water molecule.
Water and Hydrogen Bonds
Water is a polar molecule. You can
think about polar molecules in the
same way that you can think about a
magnet’s poles. That is, polar molecules
have a region with a slight positive
charge and a region with a slight
negative charge. Polar molecules, such
as the water molecule shown in
FIGURE 2.5, form when atoms in a molecule
have unequal pulls on the electrons
they share. In a molecule of water,
the oxygen nucleus, with its eight
protons, attracts the shared electrons
more strongly than do the hydrogen nuclei, with only one proton each. The
oxygen atom gains a small negative charge, and the hydrogen atoms gain small
positive charges. Other molecules, called nonpolar molecules, do not have these
charged regions. The atoms in nonpolar molecules share electrons more equally.
Opposite charges of polar molecules can interact
to form hydrogen bonds. A hydrogen bond is an
attraction between a slightly positive hydrogen
atom and a slightly negative atom, often oxygen
or nitrogen. Hydrogen bonding is shown
among water molecules in FIGURE 2.6, but these
bonds are also found in many other molecules.
For example, hydrogen bonds are part of the
structures of proteins and of DNA, which is the
genetic material for all organisms.
Properties Related to Hydrogen Bonds
Individual hydrogen bonds are about 20 times weaker than typical covalent
bonds, but they are relatively strong among water molecules. As a result, a
large amount of energy is needed to overcome the attractions among water
molecules. Without hydrogen bonds, water would boil at a much lower
temperature than it does because less energy would be needed to change
liquid water into water vapor. Water is a liquid at the temperatures that
support most life on Earth only because of hydrogen bonds in water.
Hydrogen bonds are responsible for three important properties of water.
• High specific heat Hydrogen bonds give water an abnormally high specific
heat. This means that water resists changes in temperature. Compared to
(ice)
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many other compounds, water must absorb more heat energy to increase
in temperature. This property is very important in cells. The processes that
produce usable chemical energy in cells release a great deal of heat. Water
absorbs the heat, which helps to regulate cell temperatures.
• Cohesion The attraction among molecules of a substance is cohesion.
Cohesion from hydrogen bonds makes water molecules stick to each other.
You can see this when water forms beads, such as on a recently washed car.
Cohesion also produces surface tension, which makes a kind of skin on
water. Surface tension keeps the spider in FIGURE 2.6 from sinking.
• Adhesion The attraction among molecules of different substances is called
adhesion. In other words, water molecules stick to other things. Adhesion
is responsible for the upward curve on the surface of the water in FIGURE 2.7
because water molecules are attracted to the glass of the test tube. Adhesion
helps plants transport water from their roots to their leaves because water
molecules stick to the sides of the vessels that carry water.