Download Chemistry of Life: Bonding and Properties of Water

Chemistry of Life: Bonding and Properties of Water
Instructor Answer Key
Observations
Activity 1: Solubility
Expected results provided.
At room temperature, sodium chloride and sucrose are highly soluble in water
but only slightly soluble in isopropanol. Benzoic acid is soluble in isopropanol but
not in water.
1. What can you conclude about the polar nature of benzoic acid in Activity 1?
Most of the benzoic acid molecule (specifically the benzene ring) is nonpolar
and therefore hydrophobic. This hydrophobicity prevents it from dissolving in
water at room temperature.
2. What type of mixture is benzoic acid and water? What type of mixture is
sucrose and isopropanol? Explain your answers.
Both are suspensions, because the components of each mixture
spontaneously separate from one another.
Activity 2: Adhesion and Cohesion
Expected results provided.
The surface tension of the water is high due to hydrogen bonding. The bits of
pepper are small enough to be suspended on the surface of the water.
However, the surface tension of isopropanol is much lower than that of water,
allowing the pepper to sink to the bottom of the well.
When the wood splint is gently pushed into the pepper and water mixture, the
pepper prevents the water from contacting the splint. The high surface tension
of the water allows the pepper to rest on the surface of the water. The pepper
has no attraction to the water molecules and is, therefore, hydrophobic. It forms
a water-resistant shield around the wood splint, keeping it dry while initially
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submerged and creating a small “pocket” in the water. The adhesion of pepper
to water is stronger than that of pepper to the splint. Therefore, when the splint is
lifted out of the well, it is clean.
When the splint is inserted all the way into the water in the well, it penetrates the
pepper layer and the wood comes in contact with the water. Upon lifting the
splint out of the water, the wood pulls up both pepper and water. Adhesion
occurs between water and wood, as well as between water and pepper.
When soap is added to the splint, it disrupts the hydrogen bonds of the water,
reducing the surface tension. When the soap contacts the water, it does not
immediately dissolve. Instead, the soap quickly spreads out over the surface of
the water, pushing the pepper to the edges of the well.
1. What type of mixture is pepper and water? Explain your answer.
It is a suspension, because the pepper and water spontaneously separate
from one another.
Activity 3: Density and Miscibility
Expected results provided.
The water and isopropanol will form two distinct layers. Some of the pepper will
sink to the bottom of the well, but most will be found at the surface of the water
layer.
1. Explain the layers that formed from the two liquids in Activity 3.
Water is denser than isopropanol. When added slowly, the difference in
density allows the two liquids to remain separate as two distinct layers. When
pepper is added to the well, some of it sinks to the bottom. However, most of
the pepper moves through the isopropanol layer and remains suspended at
the surface of the water layer.
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Activity 4: Heat Capacity
Expected results provided.
As long as the paper comprising the cup is in contact with water, the water
absorbs the heat and prevents the paper from burning. When the flame is
directed at the part of the cup that does not touch water, the paper catches
on fire. The paper will burn down to the water level.
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Chemistry of Life: Bonding and Properties of Water
1. 580104 01 REMEMBERING
Which of these bonds is formed due to the opposite charges of cations and
anions being attracted to each other?
A. covalent
B. hydrogen
C. oxygen
*D. ionic
E. nonpolar
General Feedback:
In ionic bonds, the opposite charges of cations and anions are attracted
to each other.
2. 580104 02 REMEMBERING
Which of these refers to the relative ability of an atom to attract electrons?
A. solubility
B. saturation
*C. electronegativity
D. adhesion
E. cohesion
General Feedback:
Electronegativity refers to the relative ability of an atom to attract
electrons.
3. 580104 03 REMEMBERING
Which type of bond will form when two nonmetal atoms share one pair of
electrons?
*A. covalent
B. ionic
C. colloid
D. cationic
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E. anionic
General Feedback:
Covalent bonds form when two nonmetal atoms share electrons.
4. 580104 04 REMEMBERING
Which of these is the part of a solution that dissolves in the solvent?
A. polar substance
B. nonpolar substance
*C. solute
D. aqueous solution
E. immiscible substance
General Feedback:
The part of a solution that is dissolved in the solvent is called the solute.
5. 580104 05 UNDERSTANDING
What property of the vegetable oil in salad dressing requires you to shake the
dressing to mix up the oil, water, and vinegar before pouring it onto a salad?
A. surface tension
B. cohesion
C. adhesion
D. hydrophilic
*E. hydrophobic
General Feedback:
Vegetable oil separates from the water and vinegar to form its own layer
in salad dressing. This is because it is hydrophobic, or "water fearing."
6. 580104 06 REMEMBERING
What property allows a large body of water, such as a lake, to maintain
relatively constant temperature throughout a 24-hour period, even though the
ambient temperature may drastically rise and fall throughout the daytime and
nighttime hours?
A. saturation
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B. low surface tension
C. hydrophilic
D. adhesion
*E. specific heat capacity
General Feedback:
Specific heat capacity is the amount of energy that is requried to raise the
temperature of 1 gram of a substance by 1 degree Celsius. Water's specific heat
is very high, so it requires a relatively large amount of energy to change the
temperature of the water compared to other substances.
7. 580104 07 UNDERSTANDING
Which of these terms describes why a dog has to shake so hard in order to dry its
fur?
*A. adhesion
B. cohesion
C. surface tension
D. polarity
E. hydrophilic
General Feedback:
Adhesion is the attraction of water molecules with other different
molecules. When water clings to a dog's fur while the dog shakes extremely
hard, this illustrates the attractive nature of adhesion.
8. 580104 08 REMEMBERING
What does it mean for a substance to be immiscible?
A. It is completely soluble with another substance.
*B. It is insoluble with another substance.
C. It is an adhesive substance.
D. It is a cohesive substance.
E. Hydrogen bonds form between the substance and water molecules.
General Feedback:
A substance is immiscible if it is insoluble with another substance.
9. 580104 09 REMEMBERING
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When fine clay is mixed in water and left to stand, the clay will settle to the
bottom of the container. Which term best describes this mixture?
*A. suspension
B. colloid
C. aqueous solution
D. saturated solution
E. emulsion
General Feedback:
Suspensions are heterogeneous mixtures that spontaneously separate into
their individual components.
10. 580104 10 REMEMBERING
Which property of water allows a water strider to run across the surface of
standing water?
A. polarity
B. adhesion
C. low solubility
*D. high surface tension
E. nonpolarity
General Feedback:
Water is resistant to objects passing through the surface and thus is said to
have a high surface tension.
11. 580104 11 REMEMBERING
Smoke and gelatin are examples of what type of mixture?
A. suspension
*B. colloid
C. Solutions
D. Cations
E. Anions
12. 580104 12 UNDERSTANDING
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Which of these statements is NOT true of water?
A. Water has the property of cohesion.
B. Water has the property of adhesion.
C. Adjacent water molecules form hydrogen bonds with one another.
D. Water is the solvent in aqueous solutions.
*E. Water has a low specific heat capacity.
General Feedback:
Water requires a lot of energy to raise the temperature of 1 g of it by 1
degree Celsius. This means water has a high specific heat capacity, not a low
one.
13. 580104 13 APPLYING
Explain the "like dissolves like" rule and give an example.
Correct Answer:
The "like dissolves like" rule generally follows the idea that polar substances (and
ionic compounds) are more soluble in polar substances, and nonpolar
substances are more polar in nonpolar substances. An example of this is sodium
chloride (an ionic compound) dissolving in water (a polar substance).
14. 580104 14 ANALYZING
Explain how rain falling from the sky can be an example of both adhesion and
cohesion.
Correct Answer:
Rain falling from the sky exemplifies cohesion because adjacent water
molecules are attracted to one another via hydrogen bonding due to the
opposite charges at each pole of a water molecule. Rain can also exemplify
adhesion since particulate matter in the air can trigger condensation.
15. 580104 15 EVALUATING
Water can dissolve many ionic and polar covalent compounds. Describe at
least two ways in which this property of water has significant biological and
environmental implications.
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Correct Answer:
Possible answers include:
1) Water can carry nutrients to cells, and waste away from cells.
2) Water can dissolve harmful chemicals. For example, a river that flows near an
industrial facility may carry pollutants for great distances.
16. 580104 16 UNDERSTANDING
Is sodium chloride (NaCl) an ionic compound or covalent compound? What
happens to the atoms in NaCl when the compound is dissolved in water?
Correct Answer:
NaCl is an ionic compound. It dissociates into the cation Na+ and the anion Clin aqueous solution.
17. 580104 17 APPLICATION
Is sucrose (C12H22O11) an ionic or a covalent compound? What happens to the
sucrose molecules when this solute is dissolved in water?
Correct Answer:
Sucrose is a covalent compound. When dissolved in water, each intact sucrose
molecule simply separates from its neighboring sucrose molecules.
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