178 ❯ STEP 4. Review the Knowledge You Need to Score High
being the correct answer. The higher charge on
calcium (+2 versus +1) leads to a higher lattice
energy, which results in a higher melting point.- D—This answer describes a solid consisting of
discrete polar molecules. Answer A refers to a
nonpolar molecule. Answer B refers to a metal-
lic solid. Answer C might apply; however, even
though phosphoric acid is an acid that forms
ions in aqueous solution, there is no water pres-
ent for ionization. - A—Compound A can form hydrogen bonds.
Compound B has dipole–dipole interac-
tions. Compounds C and D are both nonpo-
lar; therefore, their strongest intermolecular
force is London dispersion force. For relatively
small molecules such as these, the stronger the
intermolecular force, the higher the viscosity.
Hydrogen bonding in the strongest of the three
intermolecular forces present in these molecules.- C—The carbonyl, C=O, and –OH groups can
participate in hydrogen bonds, as illustrated by
the dotted lines in the following diagram:
CH 3 CCCH 3OOOHOH
The hydrogen bonds hold the two molecules
together as a dimer.- B—The solid begins to melt at B and finishes
melting at C. Once the solid begins to melt,
both solid and liquid are present.
❯ Free-Response Question
You have 15 minutes to answer the following question. You may use a calculator and the
tables in the back of the book.Question
Write a brief explanation concerning each of the following observations.
(a) An aqueous solution in a glass buret has a meniscus that is concave, whereas in a plastic
buret, the meniscus is convex.
(b) The surface tension of liquid CBr 4 is greater than the surface tension of liquid CCl 4.
(c) At the same temperature, liquid HF has a higher viscosity than liquid HCl.
(d) When heated, glass does not melt; it becomes soft.❯ Answer and Explanation
(a) The concave surface results from capillary action, and the convex surface results from
surface tension. Surface tension is the result of strong intermolecular forces between the
solution and the glass; these forces are greater than those within the solution causing a
convex surface. The convex surface results from a stronger interaction within the solu-
tion than between the solution and the plastic.
Give yourself 1 point for this answer.
(b) The greater surface tension results from stronger intermolecular forces in CBr 4. Both
CCl 4 and CBr 4 are nonpolar molecules; therefore, the intermolecular forces are London
dispersion forces. For simple molecules, such as these, the one with more electrons,
CBr 4 , will have the stronger London dispersion forces and hence the greater surface
tension.
Give yourself 1 point for this answer.
(c) Viscosity, the resistance to flow, is related to the strength of attraction between the
molecules. Both HF and HCl are polar molecules with dipole–dipole forces; however,