Question 1 · 1.1 Structure of Water and Hydrogen Bonding
Four liquids were compared at one atmosphere of pressure. The data most strongly support which explanation of the boiling points?
- A Boiling point rises with molar mass, because heavier molecules move more slowly
- B Boiling point rises with the number of oxygen atoms, regardless of the groups they form
- C Boiling point rises with the count of O-H groups each molecule carries
- D Boiling point rises with the strength of the covalent bonds within each molecule
Show the answer
C — Boiling separates molecules from one another, so the boiling point measures how strongly neighboring molecules are held together. A hydrogen bond requires a hydrogen already attached to an electronegative atom, which is what an O-H group provides. P and Q settle the question: identical molar mass, identical oxygen content, and a 102 °C gap that follows the single structural difference of one O-H group. S, with two such groups, boils highest of all. The oxygen alone is insufficient, because with no attached hydrogen the molecule has no donor.
Question 2 · 1.2 Elements of Life
Carbon forms the skeleton of every major class of biological macromolecule, while silicon, which sits directly below carbon in the periodic table, is rare in living tissue. Which property of carbon best explains its central role?
- A It forms four covalent bonds at once, so chains, branches, and rings are all possible
- B It makes up more of a cell's mass than any other element, so cells are built mostly from it
- C It gains four electrons easily, forming a stable ion that other elements are attracted to
- D It bonds to hydrogen with a strong partial charge, so carbon chains dissolve freely in water
Show the answer
A — A carbon atom has four valence electrons and four vacancies, so it holds four partners at once and bonds to other carbons as readily as to anything else. That single fact generates the whole architecture of organic chemistry: straight chains, branched chains, rings, and rings fused to chains, each carrying different functional groups at different positions. Silicon also forms four bonds, but silicon-silicon bonds are weak and hydrolyze readily, so silicon cannot sustain the long stable skeletons that biological molecules require.
Question 3 · 1.3 Introduction to Macromolecules
A hydrocarbon chain is modified by replacing three of its hydrogen atoms with hydroxyl groups. Which change in the molecule's behavior is most likely to result?
- A It becomes less soluble in water, because the modification increases the molecule's mass
- B It becomes more soluble in lipid, because hydroxyl groups are nonpolar like the chain itself
- C It becomes more soluble in water, because the added groups can hydrogen bond with water
- D It becomes acidic, because hydroxyl groups readily release protons into the surrounding solution
Show the answer
C — A hydroxyl group is polar: its oxygen carries a partial negative charge and its hydrogen a partial positive one, so it can hydrogen bond with water. Adding three such groups to an otherwise nonpolar chain substantially increases solubility.