Water - Basic 10 Flashcards
(10 cards)
What makes water a polar molecule?
a) Symmetrical charge distribution
b) Linear geometry
c) Difference in electronegativity between H and O
d) Nonpolar covalent bonds
Answer: c) Difference in electronegativity between H and O
Explanation: Water is polar because of the high electronegativity of oxygen compared to hydrogen and its bent geometry, creating a dipole moment.
What types of molecules dissolve well in water?
a) Nonpolar molecules
b) Polar and charged molecules
c) Hydrophobic molecules
d) Long hydrocarbons
Answer: b) Polar and charged molecules
Explanation: Water is a polar solvent, so it interacts well with other polar or charged molecules. These solutes dissolve easily because they can form hydrogen bonds or ion-dipole interactions with water molecules, stabilizing them in solution. Nonpolar or hydrophobic molecules cannot do this and are poorly soluble.
What is the hydrophobic effect?
a) Attraction between nonpolar molecules
b) Exclusion of nonpolar molecules by water
c) Ionization of water molecules
d) Breaking hydrogen bonds between solutes
Answer: b) Exclusion of nonpolar molecules by water
Explanation: The hydrophobic effect occurs because water molecules form ordered cages (clathrates) around nonpolar solutes. To minimize this ordering (which lowers entropy), water excludes nonpolar molecules, pushing them together. This increases the entropy of the system, which drives processes like protein folding and micelle formation.
How many hydrogen bonds can one water molecule form?
a) 1
b) 2
c) 3
d) 4
Answer: d) 4
Explanation: Each water molecule can form up to 4 hydrogen bonds—two through lone pairs on oxygen and two through hydrogen atoms.
Why is ice less dense than liquid water?
a) Less hydrogen bonding
b) No dipole moment
c) Tetrahedral H-bond network
d) Increased entropy
Answer: c) Tetrahedral H-bond network
Explanation: In ice, water molecules form a crystalline lattice stabilized by maximum hydrogen bonding. This creates a tetrahedral arrangement that spaces molecules farther apart than in liquid water, making ice less dense and able to float.
Why does water have a high dielectric constant?
a) Ability to shield charges
b) Weak hydrogen bonds
c) Low polarity
d) Small molecular size
Answer: a) Ability to shield charges
Explanation: Water has a high dielectric constant because its polarity allows it to surround and separate charged particles. This reduces the electrostatic attraction between ions, making it easier for salts and polar molecules to dissolve. The high dielectric constant reflects water’s capacity to shield charges, increasing solubility and weakening ionic interactions.
Which of the following would NOT form hydrogen bonds with water?
a) –OH group
b) –CH₃ group
c) –NH₂ group
d) –COOH group
Answer: b) –CH₃ group
Explanation: Methyl groups (–CH₃) are nonpolar and cannot act as hydrogen bond donors or acceptors. They lack electronegative atoms (like N or O) with lone pairs that could participate in hydrogen bonding. In contrast, –OH, –NH₂, and
–COOH groups are polar and contain electronegative atoms capable of forming hydrogen bonds with water.
What drives the formation of micelles in aqueous solution?
a) Hydrophobic effect
b) Electrostatic attraction
c) Ion-dipole interactions
d) Van der Waals forces
Answer: a) Hydrophobic effect
Explanation: The hydrophobic effect drives micelle formation. Amphipathic molecules like detergents have hydrophobic (nonpolar) tails and hydrophilic (polar) heads. In water, the nonpolar tails aggregate to avoid contact with water, minimizing disruption to the hydrogen bonding network. This clustering reduces the ordering of water molecules, increasing entropy. The polar heads remain exposed to water, stabilizing the micelle.
What property of water allows it to be a good solvent for salts?
a) Low polarity
b) High surface tension
c) High dielectric constant
d) Low boiling point
Answer: c) High dielectric constant
Explanation: Water’s high dielectric constant allows it to shield charged ions from one another. This weakens electrostatic interactions (per Coulomb’s Law), making it easier for salts to dissolve. Combined with water’s polarity, this property makes it an excellent solvent for ionic and polar compounds.
Which thermodynamic principle explains the low solubility of nonpolar molecules in water?
a) High enthalpy of mixing
b) Low entropy of water near nonpolar solutes
c) Strong ionic bonding
d) High pKa values
Answer: b) Low entropy of water near nonpolar solutes
Explanation:
Water molecules form highly ordered cages (clathrates) around nonpolar solutes. This reduces the entropy (ΔS), making the process thermodynamically unfavorable. Since a decrease in entropy increases ΔG (ΔG = ΔH − TΔS), nonpolar molecules are poorly soluble in water due to the unfavorable entropy change.
