Buffers are substances that help resist shifts in pH by
both donating H+ to a solution when bases are added, and accepting H+ when acids are added.
Water is a source of ______________ for chemical reactions in cells.
(a) hydrogen atoms
(b) oxygen atoms
-Good answer! Many reactions incorporate O and H from water into biological molecules. This happens when you digest starch and protein, for example.
Which statement is true of water’s tensile strength?
(a) It results from hydrogen bonding.
(b) It helps to pull water through plants.
(c) It involves both cohesion and adhesion.

-Right! Because of hydrogen bonding, water coheres to itself and adheres to cell walls. That makes it possible to pull water through plants without breaking the water column.

Water has surface tension because …
hydrogen bonds between surface water molecules resist being stretched.
-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.
-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.

-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 …
it takes energy to break hydrogen bonds.
-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.
-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.
The open spaces in water’s crystal structure make it possible for …
a) aquatic life to exist at the North Pole.
-Good thinking! The openings in the lattice make ice less dense than liquid water, so it floats. That causes water to freeze from the top down, so there’s always liquid in which organisms can live.
Why doesn’t oil mix with water?
Polar molecules attract one another.
-Correct! Water molecules cling to one another and won’t part to make room for uncharged (nonpolar) molecules. There’s no repulsion.
Which statement must be mentioned in explaining why amphipathic molecules line up at a water surface?
Polar groups attract one another.
Dissolving is best described as …
a mingling of molecules and/or ions.
-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.
Increased atmospheric CO 2 {
m CO_2} concentrations might have what effect on seawater?
Seawater will become more acidic, and carbonate concentrations will decrease.
The water molecule has a bent shape because …
oxygen has two unbonded pairs of valence electrons
-That’s right! Repulsion between electrons governs the angles between bonds. The two pairs of unbonded electrons repel the two pairs of bonding electrons, creating the bend in the molecule.
Which statement is true of water?
The H atoms in water have partial positive charges.
-Good work! H is less electronegative than O, so the shared electrons spend most of their time near O.
Each water molecule is joined to _____ other water molecules by ____ bonds.
four … hydrogen
The unequal sharing of electrons within a water molecule makes the water molecule _____.
polar
-The electrons spend more time with the oxygen of the water molecule than with the hydrogens of water. Thus, the oxygen has a net negative charge and the hydrogens have a net positive charge.
The tendency of an atom to pull electrons toward itself is referred to as its _____.
electronegativity
In this molecule, what type of bond is found between the oxygen and hydrogens?
polar covalent
-While oxygen shares electrons with hydrogen, the electrons spend more time with the oxygen than with the hydrogens.
An acid is a substance that _____.
increases the hydrogen ion concentration of an aqueous solution
Which of the following solutions would require the greatest amount of base to be added to bring the solution to neutral pH?
gastric juice at pH 2
Sodas typically contain sugar, flavorings, coloring agents, and carbon dioxide dissolved in water. The best term to describe this mixture would be _____.
an aqueous solution
-For solutions where water is the solvent, the term “aqueous solution” is used.
This animation illustrates the _____ of water.
The transfer of a hydrogen atom from one water molecule to another is referred to as dissociation.
Which of these is the correct equation for the dissociation of water?
H2O + H2O ;==; H3O+ + OH-
About _____ molecules in a glass of water are dissociated.
1 in 500,000,000
Because organisms are made primarily of water, they resist rapid temperature changes. This useful quality is based on water’s _____.
high specific heat
-Water has a high specific heat, meaning that a relatively large amount of heat must be added or removed in order to get the temperature of the water to change significantly.
Which of the following is a hydrophobic material?
wax
How does the pH of the solution on the right compare with that of the solution on the left?
The solution on the right is basic relative to the solution on the left.
Water molecules have a polarity, which allows them to be electrically attracted to other water molecules and other polar molecules by weak chemical bonds known as _____.
The polarity of a water molecule allows the oxygen of one water molecule to bond weakly to the hydrogen of another water molecule.
Select the statement that best describes a buffer.
A buffer resists change in pH by accepting hydrogen ions when acids are added to the solution and donating hydrogen ions when bases are added.
Which type of bond must be broken for water to vaporize?
hydrogen bonds
If the pH of a solution is increased from pH 5 to pH 7, it means that the
concentration of OH ? {
m OH^{-}} is 100 times greater than what it was at pH 5.
What is the main reason that the oceans are becoming more acidic?
absorption by the oceans of carbon dioxide generated by burning fossil fuels
How much carbon dioxide from the atmosphere is absorbed by the oceans on a daily basis?
20 million metric tons
What is the chemical reaction responsible for the oceans becoming more acidic?
carbon dioxide reacts with seawater, forming carbonic acid
How are oyster larvae affected by ocean acidification?
Oyster larvae cannot grow shells in acidified ocean waters
Because of ocean acidification, the price of which of the following foods is most likely to increase?
shellfish
Which of the following are consequences of ocean acidification? Select all that apply.
Corals and coral reefs are not able to form their hard skeletons.
Clams, mussels, lobsters, and shrimp are not able to form strong enough shells.
Water has a high specific heat relative to other substances due to the hydrogen bonding that occurs between water molecules. Water is very resistant to temperature increases because the hydrogen bonds between the water molecules must be disrupted to raise the temperature of the water. By the same token, water must lose a large amount of heat and form many additional hydrogen bonds for its temperature to decrease.
Water’s high specific heat has some important consequences for life on Earth. For example, coastal areas generally have milder climates than inland areas due to the oceans’ high specific heat. Oceans are able to absorb a large amount of heat during the daytime and summer, keeping air temperatures relatively cool. At night and during the winter, the oceans cool gradually, releasing heat and warming the air. Water’s high specific heat also stabilizes ocean temperatures, resulting in a favorable environment for marine life.
Most acid precipitation results from the combination of _____ with water in the atmosphere, forming strong acids that fall with rain or snow.
sulfur oxides and nitrogen oxides
If the pH of a solution is decreased from 9 to 8, it means that the
concentration of H + {
m H^{+}} has increased tenfold (10X) and the concentration of OH- has decreased to one-tenth (1/10) what they were at pH 9
The bonds that are broken when water vaporizes are
hydrogen bonds between water molecules.
Many of water’s emergent properties, such as its cohesion, its high specific heat, and its high heat of vaporization, result from the fact that water molecules _____.
are attracted to each other by partial negative and positive charges on the oxygen and hydrogen atoms, respectively
-The formation of hydrogen bonds between the positive and negative regions of different water molecules accounts for most of water’s unique properties
x

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