Which of the following is a hydrophobic material?
(B) table salt
REASON: nonpolar and nonionic
We can be sure that a mole of table sugar and a mole of vitamin C are equal in their...
(C) # of atoms
(D) # of molecules
REASON: 1 mole of a substance has the same # of molecules as 1 mole of another substance
Measurements show that the pH of a particular lake is 4.0. What is the hydrogen ion concentration of the lake?
(A) 4.0 M
(B) 10^-10 M
(D) 10^4 M
REASON: pH of the lake is 4... use the formula 10^-pH to find the hydrogen ion concentration, which is 10^-4
What is the hydroxide ion concentration of the lake described in question 3?
(A) 10^-10 M
(B) 10^-4 M
(C) 10^-7 M
(D) 10.0 M
REASON: [H+][OH-] = 10^-14
A slice of pizza has 500 kcal. If we could burn the pizza and use all the heat to warm a 50-L container of cold water, what would be the approximate increase in the temperature of the water? (Note: A liter of cold water weighs about 1 kg.)
(A) 50 degrees C
(B) 5 degrees C
(C) 100 degrees C
(D) 10 degrees C
REASON: 500 (kg * degrees C) divided by 50 kg (since 1L cold water = 1 kg)
Many mammals control their body temperature by sweating. Which property of water is most directly responsible for the ability of sweat to lower body temperature?
(A) The absorption of heat by the breaking of hydrogen bonds.
(B) The release of heat by the formation of hydrogen bonds.
(C) Water's ability to dissolve molecules in the air.
(D) Water's high surface tension.
(E) Water's change in density when it condenses.
REASON: absorption of heat by breaking bonds cools down surroundings => sweating
Sweating has a cooling effect because of water's high __________.
(A) buffering capacity
(B) heat of vaporization
(D) surface tension
(E) specific heat
REASON: water's high heat of vaporization results in evaporative cooling => surface of water cools down
this occurs b/c "hottest" (fast) molecules have already escaped into vapor, making the remaining molecules cool
Water is a polar molecule. This means that __________.
(A) the atoms in water have equal electronegativities
(B) water molecules are linear, like a pole
(C) the opposite ends of the molecule have opposite electrical charges
(D) water is one of the many hydrophobic molecules
(E) all of the listed responses are correct.
REASON: definition of a polar molecule (uneven sharing of electrons; more electronegative leads to strongly pulling more shared electrons towards itself)
Water moves up a plant because of hydrogen bonds by a process called __________.
(A) covalent interactions
(D) surface tension
(E) cohesion and adhesion
REASON: water moves up a plant because of h-bonds due to cohesion (between H2O molecules in cell walls) AND adhesion (between H2O molecules and cell walls to go against gravity)
Which statement must be mentioned in explaining why amphipathic molecules line up at a water surface?
Nonpolar groups attract one another.
Polar groups repel water.
Polar groups attract one another.
Nonpolar groups repel water.
All of the above.
REASON: Good thinking! Polar water molecules pull together, holding the polar parts of the amphipathic molecules and squeezing out the nonpolar parts. That leaves the nonpolar parts waving in the air.
Dissolving is best described as...
molecules breaking into ions.
a change from a solid to a liquid.
a separation of molecules into neutral atoms.
a mingling of molecules and/or ions.
breaking covalent bonds.
REASON: Good! When sugar dissolves in water, the sugar molecules remain intact but mingle with water molecules. When salt (NaCl) dissolves, its Na+ and Cl- ions separately mingle with water.
Water is a source of ______________ for chemical reactions in cells.
(a) hydrogen atoms
(b) oxygen atoms
Both (a) and (b)
(a), (b), and (c).
REASON: Good answer! Many reactions incorporate O and H from water into biological molecules. This happens when you digest starch and protein, for example.
Water has surface tension because ...
cohesion forces are weaker at the surface.
there is positive pressure inside the water mass.
molecules at the surface make more hydrogen bonds.
water tends to evaporate from the surface.
hydrogen bonds between surface water molecules resist being stretched.
REASON: Good choice! The hydrogen bonds between surface water molecules are normally slightly stretched. Like a stretched sheet of rubber, the surface tends to contract and resists being penetrated.
Which of the following helps most to explain why water has a high specific heat?
(a) A water molecule can make 4 hydrogen bonds.
(b) The water molecule has exceptionally strong covalent bonds.
(c) Water temperature is exceptionally sensitive to heat.
Both (a) and (b).
Both (b) and (c).
REASON: Good job! When you heat water, much of the heat is used to break hydrogen bonds. Only the remaining heat can increase molecular motion, raising the temperature.
Which factor is important in making it possible to cool yourself by sweating? Think carefully!
(a) Molecules collide with varied angles and speeds.
(b) Hydrogen bonds are relatively weak.
(c) Water has more energy at the body surface.
Both (a) and (b).
(a), (b), and (c).
REASON: Good choice! Random collisions allow some molecules to accumulate more energy than other molecules. The weakness of hydrogen bonds lets those molecules escape, leaving the cooler molecules behind.
Though you add heat, the temperature of boiling water remains constant because ...
water has a constant boiling temperature.
it takes energy to break covalent bonds.
it takes energy to break hydrogen bonds.
it takes energy to circulate water.
None of the above. The temperature rises during boiling.
REASON: Correct! At boiling, all the added heat is used to break hydrogen bonds. Free of the water mass, the departing steam carries away all the added energy, with none left over to raise the temperature.
Which statement helps to explain why ice is less dense than liquid water?
(a) Water molecules make hydrogen bonds at definite angles.
(b) Cold molecules move less than warm molecules.
(c) Hydrogen bonds lengthen in the cold.
All of the above.
Both (a) and (b).
REASON: Correct! The ice lattice has open spaces because of the angles at which hydrogen bonds form. Heat energy can break water molecules free of the lattice so they move into the openings.
Why doesn't oil mix with water?
(a) Nonpolar molecules repel water molecules.
(b) Polar molecules repel nonpolar molecules.
(c) Polar molecules attract one another.
(d) Nonpolar molecules attract one another.
Both (a) and (d).
REASON: Correct! Water molecules cling to one another and won't part to make room for uncharged (nonpolar) molecules. There's no repulsion.
Polar attractions are ...
(a) forces between atoms with partial charges.
(b) weaker than covalent bonds.
(c) important because they are numerous.
(a) and (b) are true. (c) is false.
(a), (b), and (c) are correct.
REASON: Polar attractions occur between atoms with partial charges and are weaker than covalent bonds. Still, they're important because so many of them occur in living cells.
are covalent bonds.
can form between H and N.
occur within a water molecule.
share electrons between the two bonded atoms.
All of the above.
REASON: They can also form between H and other highly electronegative atoms, such as O, if the atoms have strong partial charges.
Which of these is the correct equation for the dissociation of water?
H+<==> H2O + H2O+
H2O + H2O <==> H3O+ + OH-
H2O + OH-<==> H2O+
H2O <==> H+ + OH-
H2O + H2O+<==> H2O + OH-
REASON: it makes H3O+ and OH-, not H+ and OH-
About _____ molecules in a glass of water are dissociated.
1 in 5,000,000
1 in 1,000,000
1 in 500,000,000
1 in 2
1 in 1,000,000,000
Hydrophobic molecules __________.
(A) are nonionic molecules that are attracted to the nonpolar portion of the water molecule
(B) always have a partial charge at one end of the molecule
(C) are polar molecules that are attracted to the nonpolar portion of the water molecule
(D) are uncharged, nonionic substances that seem to repel water
(E) are ionic molecules that are attracted to the polar portion of the water molecule
REASON: Hydrophobic substances do not have an affinity for water. Substances that are nonionic and nonpolar (or otherwise cannot form hydrogen bonds) actually seem to repel water; these substances are said to be hydrophobic.
The partial charges on a water molecule occur because of __________.
(A) the unequal sharing of electrons between the hydrogen and the oxygen atoms of a water molecule
(B) the high electronegativity of hydrogen
(C) covalent bonding
(D) widespread ionization
(E) the achievement of a stable configuration by one atom of a bond but not by the other partner
REASON: Oxygen is one of the most electronegative of all the elements, attracting shared electrons much more strongly than hydrogen does. In a covalent bond between oxygen and hydrogen, the electrons spend more time near the oxygen nucleus than they do near the hydrogen nucleus. Because electrons have a negative charge and are pulled toward oxygen in a water molecule, the oxygen atom has a partial negative charge.
All of the following are hydrogen bonding properties except __________.
(A) nonpolar covalent bonding
(B) surface tension
(C) partial charges on the oxygen and hydrogen
REASON: Surface tension, adhesion, partial changes on the oxygen and hydrogen, and cohesion are hydrogen bonding properties, but nonpolar covalent bonding is not. Water's polar nature is responsible for these properties. The extraordinary qualities of water are emergent properties resulting from hydrogen bonding.
The absorption of human-generated CO2 by the oceans __________.
(A) reduces the carbonate ion concentration in the oceans and threatens calcifying organisms in marine ecosystems
(B) increases the oceans' acidity and lowers pH
(C) increases the hydrogen ion concentration in the oceans but decreases the carbonate ion concentration and threatens the livability of the oceans for calcifying organisms
(D) All of the responses are correct.
(E) increases the oceanic concentration of carbonic acid
REASON: The absorption of human-generated CO2 by the oceans increases the oceanic concentration of carbonic acid because it reacts with water to form carbonic acid, it increases the oceans' acidity and lowers the pH of seawater, reduces the carbonate ion concentration in the oceans and threatens calcifying organisms in marine ecosystems because the absorption of CO2 by the oceans leads to a decrease in the carbonate ion concentration, which is required for calcification, the production of calcium carbonate (CaCO3) by many marine organisms, and increases the hydrogen ion concentration in the oceans but decreases the carbonate ion concentration and threatens the livability of the oceans for calcifying organisms.
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