Chemistry of Life: Bonding and Properties of Water

Home , Chloride, Cohesion (chemistry), Miscibility

Instructor Answer Key

Observations

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

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.

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.

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

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

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

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

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.

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 Cl- in 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.