Biochemistry : Essential Properties of Water

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Homeostasis And The Biological Environment

Which substance do you expect to be most soluble in water?

Possible Answers:

Nitrogen

Methane

Carbon trioxide

Carbon dioxide

Ammonia

Correct answer:

Ammonia

Explanation:

Ammonia, NH3, will be the most soluble in water simply because polar substances dissolve in polar solvents. This follows the principle that "like dissolves like." Ammonia and water are both polar due to the presence of lone pairs of electrons combined with a lack of geometrical symmetry (water is bent and ammonia is trigonal pyramidal).

Carbon dioxide and nitrogen are linear, negating any potential polarity. Methane is tetrahedral, and lacks any polarized bonds. Carbon trioxide is tigonal planar, negating polar interactions due to symmetry.

Example Question #2 : Homeostasis And The Biological Environment

What is true about the solubility of electrolytes in water?

Possible Answers:

They readily dissolve in water

They are usually only sparingly soluble in water

No reactions can occur between aqueous electrolytes

They are all insoluble in water

They often form super-saturated aqueous solutions

Correct answer:

They readily dissolve in water

Explanation:

Electrolytes readily dissolve and ionize in water. The term "electrolyte" refers to a molecule that will produce ions in solution, and can be synonymous with "salt" in certain contexts, as well as acidic and basic compounds.

Example Question #1 : Essential Properties Of Water

What property of water makes it a good solvent for ionic salts?

Possible Answers:

Molecular symmetry

Small molecule size

Low dielectric constant

High polarity

van der Waals forces

Correct answer:

High polarity

Explanation:

When dealing with solutions, it is helpful to remember that solubility depends on polarity and that "like dissolves like." Polar solutes (ionic salts) dissolve well in polar solvents (water); the same goes for nonpolar solutes and solvents.

Example Question #2 : Essential Properties Of Water

Which of the following statements about water are incorrect?

Possible Answers:

Water has a tetrahedral structure when considering lone pairs and a bent molecular structure when not considering lone pairs

Water has a high heat of vaporization and high surface tension

Hydrophobic interactions are driven by the increase in enthalpy that water molecules achieve by excluding a non-polar solute

In water, linear hydrogen bonds are stronger than bent hydrogen bonds

Liquid water is 15% less H-bonded than ice
Correct answer:

Hydrophobic interactions are driven by the increase in enthalpy that water molecules achieve by excluding a non-polar solute

Explanation:

Hydrophobic interactions are driven by the increase in entropy (not enthalpy) that water molecules achieve by excluding a nonpolar solute. In the hydrophobic effect, water initially forms cage-like structures called clathrates around the nonpolar solute, which is entropically unfavorable. However, entropy is regained when the water molecules exclude the solute and interact with each other in a disordered manner.

Example Question #5 : Essential Properties Of Water

The primary reason that water is liquid at room temperature is because of __________.

Possible Answers:

Cohesion

Surface tension

Adhesion

High density

Correct answer:

Cohesion

Explanation:

Cohesion is defined as attraction between the same type of molecule. Water attracts other water molecules because it is polar and has partial charges. This attraction means that water has strong intermolecular forces even at room temperature and more thermal energy is required to vaporize it. Adhesion involves the attraction of a substance with a container or surface.

Example Question #5 : Homeostasis And The Biological Environment

Which of the following is false about water?

Possible Answers:

Its electrons are symmetrically positioned around the molecule. 

Hydrogen bonds join together water molecules.

Water's total charge is neutral.

Water is polar, with an electropositive and electronegative region.

The hydrogen atoms have a slight net positive charge.

Correct answer:

Its electrons are symmetrically positioned around the molecule. 

Explanation:

The electrons around water are not symmetrically positioned; rather, they are distributed toward the oxygen atom, which is highly electronegative. Hence, water's two hydrogen atoms have a net positive charge. Nonetheless, the total charge on water is neutral, with electropositive and electronegative regions, rendering it polar. This polarity allows water molecules to hydrogen-bond with each other.

Example Question #3 : Essential Properties Of Water

Which of the following polymers is not formed by condensation?

Possible Answers:

Polystyrene

Nylon

Nucleic acids

Polysaccharides

Proteins

Correct answer:

Polystyrene

Explanation:

The important point here is that biological polymers are basically all condensation polymers. In the case of proteins, polysaccharides (carbohydrates), and nucleic acids (like DNA and RNA), synthesis occurs via a loss of a water molecule. Non-biological molecules can also be formed via condensation, such as nylon, which also often has water as its by-product. Polystyrene is an addition polymer; upon formation, monomers do not lose any molecules. Bonds are only rearranged. Although it may seem counterintuitive, the fact is that in nature, polymers do not typically form this way.

Example Question #4 : Essential Properties Of Water

Which accurately describes the effects of solute concentrations on cells?

Possible Answers:

Aquaporins prevent water flow via osmosis.

Cells tend to swell in both hypotonic and hypertonic solutions.

Cells tend to swell in hypotonic solutions, and shrink in hypertonic solutions.

Cells tend to shrink in hypotonic solutions, and swell in hypertonic solutions.

Cells tend to shrink in both hypotonic and hypertonic solutions.

Correct answer:

Cells tend to swell in hypotonic solutions, and shrink in hypertonic solutions.

Explanation:

Hypotonic solutions have a low solute concentration, and hypertonic solutions have high solute concentration. By the rules of osmosis, water moves in and out of cells along a concentration gradient, because membranes are only slightly permeable to water. Hence in a hypotonic solution, water will enter cells, causing them to swell and possibly break, while in a hypertonic solution, cells will lose water and shrink. Aquaporins are water channels that encourage osmosis, rather than hinder it.

Example Question #5 : Essential Properties Of Water

How many hydrogen bonds can a molecule of water participate in?

Possible Answers:

4

1

5

2

3

Correct answer:

4

Explanation:

A molecule of water is polar with the two hydrogen atoms being partially posistive and the oxygen atom being partially negative. The two hydrogens can therefore each act as a donor which accounts for two net hyrdogen bonds made by water. The oxygen contains a partial negative charge and two lone electrons pairs can each act as acceptors and from two more hydrogen bonds, making a total of 4.

Example Question #10 : Essential Properties Of Water

A dialysis bag containing pure water is placed in a beaker containing a 1 molar glucose in water. The pore size of the dialysis bag is only large enough to allow water to pass through it. What will be the net flow of water?

Possible Answers:

There will be no movement of water

Water will flow out of the dialysis bag into the glucose solution

Glucose will flow from the glucose solution into the dialysis bag

Water will flow both directions at equal rates

Water will flow from the glucose solution into the dialysis bag

Correct answer:

Water will flow out of the dialysis bag into the glucose solution

Explanation:

Osmosis states that a solvent will move from a region of high concentration to a region of relatively lower concentration. In this case the solvent is water. The concentration of water inside the dialysis bag is higher than the concentration of water in the beaker because this water also contains 1 molar glucose. Therefore the water in the dialysis bag will flow out into the beaker to bring the concentration of water in both spaces toward equilibrium

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