bio1406 Properties of Water

A solution has a pH of 10. Another solution has a pH of 7. The first solution is _____ times more basic than the second.

1,000 Explanation: The pH scale is log based, and an increase from 7 to 10 means solution one is 1000 times more basic than solution two.

The strongest bases would have a pH of ____.

14 Explanation: The strongest bases have a pH of 14. The pH scale ranges from a value of 0 to 14, with 7 being neutral. A value under 7 is acidic, while a value greater than 7 is basic.

In water, spheres of hydration form around

A and C Explanation: Spheres of hydration form around hydrophilic molecules. Polar molecules are hydrophilic because their partially charged atoms are attracted to the partial charges of oxygen and hydrogen. Ions are generally hydrophilic because they are charged and attracted to the partial charges of oxygen and hydrogen in water molecules.

Which of the following is not true regarding buffers?

Buffers are only necessary in very few biological processes. Explanation: This statement is not true. Constant pH is necessary in any number of biological processes; enzymes, for example, require a certain pH to function properly.

Carbon dioxide in the blood reacts with water to form carbonic acid. Which of the following is not true about this reaction?

Carbonic acid can release −OH. Explanation: Carbonic acid, being acidic, releases protons.

Which of the following is not true about the role of hemoglobin in the blood?

Hemoglobin is necessary in the blood, but it must be carefully controlled so that it does not make the blood too acidic. Explanation: This statement is incorrect. Hemoglobin does not make the blood acidic, it limits changes in pH.

A polar solvent would easily dissolve which of the following?

NaCl

Oxygen is much more electronegative than hydrogen. Which of the following statements about the OH group in methanol (CH3OH) is true?

The hydrogen atom has a partial positive charge. Explanation: Electronegativity is the electron-attracting power of a bonded atom. Because oxygen has a higher electronegativity than hydrogen, oxygen is able to attract the electrons in the hydrogen-oxygen bond. This attraction pulls the electrons in that bond away from hydrogen, resulting in a partially positive hydrogen and a partially negative oxygen.

A molecule that is able to mix polar and nonpolar molecules together because it has polar and nonpolar ends that can interact with both substances is called

a detergent. Explanation: Detergents are used to cause hydrophobic molecules (like grease) to interact with polar molecules (like water).

The partial negative charge on the oxygen of a methanol molecule (CH3OH) is attracted to the partial positive charge of another methanol molecule. This attraction is called

a hydrogen bond. Explanation: The attraction between a partially negative atom within a molecule and a partially positive hydrogen is called a hydrogen bond.

In the presence of a strong base, a water molecule acts like a (an) _________ and becomes a ___________.

acid, hydroxide ion Explanation: A strong base is able to take protons out of solution. Water will behave like an acid, giving up protons to the strong base, and forming hydroxide ions (OH − ).

When in the presence of a strong acid, a water molecule acts like a (an) ________ and becomes a __________.

base, hydronium ion Explanation: In the presence of a strong acid, water acts like a base and takes up protons forming hydronium ions (H3O+ ).

A chemical solution that is capable of donating or accepting hydrogen ions is a(an)

buffer. Explanation: A buffer is a solution that can accept or donate protons. It is composed of either a weak acid and its conjugate base, or a weak base and its conjugate acid. When an acid is added to a buffer, the conjugate base reacts with the acid to reduce the concentration of hydronium ion. When a base is added, the acid reacts similarly to offset the pH decrease resulting from the addition of base. In this way, the pH of buffer solutions is relatively stable to the addition of small quantities of acids or bases.

True or false? Fats do not mix with water because they are polar.

false

The intermolecular bonds in water are

hydrogen bonds. Explanation: Hydrogen bonds form between water molecules. Hydrogen bonds occur because of the polarity of the bond between hydrogen and oxygen atoms. The partially positive hydrogen atoms of one water molecule are attracted to the partially negative oxygen atoms of two other water molecules.

Can hydrogen bonds occur between water molecules?

yes because there are partially positive hydrogen atoms and partially negative oxygen atoms Explanation: The positive hydrogen atoms can form hydrogen bonds with the negative oxygen atoms.

An acid is a substance that

donates protons to a solution. Explanation: Acids are substances that donate protons (hydrogen cations) to solutions.

Hydrophobic interactions

hold nonpolar molecules together. Explanation: Nonpolar molecules are held together by hydrophobic interactions because the nonpolar molecules will aggregate to reduce their interaction with the water molecules.

Hydrogen bonding occurs between molecules in which hydrogen atoms are bound to small, highly electronegative atoms. Which of the following groups of atoms are most likely to result in hydrogen bonding when paired with hydrogen?

nitrogen, oxygen, fluorine Explanation: Nitrogen, oxygen, and fluorine are the three elements most likely to participate in hydrogen bonding. They are all highly electronegative (3.0, 3.5, 4.0, respectively) and small relative to the other elements.

Which of the following statements regarding hydrogen bonding is true?

Hydrogen bonding is possible when hydrogen is in a polar covalent bond. Explanation: A hydrogen bond forms when a partially positive hydrogen atom is attracted to a partially negative atom. Partially positive hydrogen atoms result when hydrogen is covalently bonded to a significantly more electronegative atom, such as oxygen or nitrogen.

Lecithin is a natural detergent because of what molecular property?

It has hydrophobic and hydrophilic regions. Explanation: Lecithin is a natural detergent because it has both hydrophobic and hydrophilic regions. The hydrophobic regions can surround hydrophobic molecules, and the hydrophilic regions interact with the surrounding water molecules.

A muddy stream flows from under a glacier and into a lake. The stream water is 4°C. The lake water has warmed in the summer sun to about 20°C on top. What will happen to the stream water as it enters the lake?

The muddy stream water will flow along the bottom of the lake. Explanation: The stream water is the densest water can be so it will flow below the rest of the water in the lake.

Which of the following is an effect of the polarity of the water molecule?

The polarity of water molecules requires it to absorb a lot of heat to change from liquid to gas. Explanation: The hydrogen bonding of water molecules increases the amount of energy required to increase the motion of the molecules relative to a substance without hydrogen bonding.

Water can move through the xylem (circulatory tissue) of plants, apparently in defiance of gravity. What property of water makes it able to do this?

Water molecules can both hydrogen bond with themselves and with other materials. Explanation: Cohesion (which is water molecules bonding to each other) and adhesion both allow water to move up through plant xylem.

A solution with a pH of 9 is more __________ than a solution with a pH of 6.

basic Explanation: A solution with a pH of 9 is more basic than a solution with a pH of 6 because it has a lower concentration of hydrogen ions. The pH is the negative base-10 logarithm of the hydrogen ion concentration. A solution with a pH of 9 has a hydrogen ion concentration of 1 × 10−9, while a solution with a pH of 6 has a hydrogen ion concentration of 1 × 10−6.

Which of the following pairings would result in a polar covalent bond?

carbon and oxygen Explanation: Carbon has an electronegativity of 2.5, and oxygen has an electronegativity of 3.5. When these two atoms bond covalently, this difference in electronegativity causes the bond to be polar.

The oxygen atom in an H2O molecule

carries a partial negative charge.

A hydrophobic compound would dissolve easily in

corn oil. Explanation: Corn oil is a hydrophobic substance, so it would dissolve another hydrophobic substance.

Unlike most liquids which steadily decrease in density as they increase in temperature, water exhibits unusual behavior because of the hydrogen bonds between water molecules. Place the following in order of increasing density: ice, water at 4°C, water at 20°C.

e correct answer is A: ice, water at 20°C, water at 4°C Explanation: Ice floats because it is not as dense as liquid water. When water is at 4°C, it is at its densest.

The bonds broken when water vaporizes to form a gas are

hydrogen bonds. Explanation: The input of heat required to transform water from a liquid to a gas is used to break hydrogen bonds, which hold the molecules together.

A base is a substance that

increases the pH of a solution. Explanation: Bases are proton acceptors, so they increase the pH of a solution.

If a solution has a pH of 9, then

it contains 1 × 10−9 moles of hydronium per liter. Explanation: pH is the negative inverse log of the molar concentration of hydronuim.

Hydrogen bonds can provide strength to large molecules because

large molecules may have multiple sites where hydrogen bonding can occur. Explanation: The strength that hydrogen bonding gives to large molecules is a result of the additive strength of the individual hydrogen bonds. Liquids with multiple hydrogen bonds generally have higher boiling points than liquids without such bonds because the molecules are very resistant to being broken apart.

Water at 4°C occupies ________ volume than it does at 0°C, and water is _______ dense at 4°C than it is at 0°C.

less, more Explanation: The mass of water at 4°C occupies less volume than it does at 0°C; thus, water is more dense at 4°C than it is at 0°C. This is why ice floats on water. Frozen water is less dense because the crystal structure of ice is more loosely spaced, thus it floats on water.

A solution contains 1 × 10−13 moles per liter of hydrogen ions. What is the pH of this solution?

pH = 13 Explanation: The pH is the negative inverse of the concentration of hydrogen ions.

Hydrophilic interactions

play a role in the solubility of a compound in water. Explanation: Hydrophilic interactions do play a role in the solubility of a substance in water. A hydrophilic substance will readily mix with water because the individual molecules are able to interact with water molecules. Hydrophobic substances, on the other hand, are not soluble in water because the individual molecules are not able to interact with water molecules.

Which of the following type(s) of compounds would you expect to be soluble in water?

polar compounds Explanation: Because water is a polar molecule, it does not readily dissolve nonpolar compounds. Nonpolar compounds tend to be hydrophobic and will aggregate when placed in water in order to minimize their interactions with the polar water molecules. Polar compounds will dissolve readily in water because the partial charges of the polar compound are attracted to the partial charges of the water molecule. Additionally, if there are highly electronegative atoms like oxygen, nitrogen, or fluorine in the polar compound, hydrogen bonds can form between the water molecule and the compound.

Which type of bond has unequally shared electrons?

polar covalent Explanation: Electrons are shared unequally in a polar covalent bond. In this type of bond, neither atom completely attracts the electrons. One of the atoms has a greater electronegativity, so the shared electrons are displaced towards this atom.

Label B in the diagram points to a

polar covalent bond. Explanation: The bond labeled B is a polar covalent bond. A polar covalent bond is formed from the sharing of electrons between two atoms that have differing electronegativities.

The intramolecular bonds in water are

polar covalent. Explanation: The bonds within a water molecule are polar covalent bonds. Oxygen shares two electrons with each hydrogen atom.

Some insects are able to walk on water. This is partially a result of the high __________ of water caused by the __________ between water molecules.

surface tension, hydrogen bonds Explanation: Water has the highest surface tension of all known liquids. This surface tension is a result of the hydrogen bonds formed between water molecules which cause the water molecules to be very strongly attracted to each other.

If a compound has a low specific heat, that means

that the amount of heat required to increase the temperature of one gram of the compound is low. Explanation: A low specific heat means that a small amount of energy is necessary to raise the temperature of one gram of a substance one degree Celsius.

The surface tension of water is caused by

the downward net cohesive force on the water molecules at the surface. Explanation: Because of the uneven pull of the water molecules on the surface of the water, there is a driving force pulling the surface molecules downward.

Ice is less dense than liquid water because

the open crystal structure of ice. Explanation: Ice floats because it is less dense than water. As water freezes, the hydrogen bonds become rigid, spacing water molecules so that they are less tightly packed—and less dense—than liquid water.

Which of the following chemical bonds is the weakest?

van der Waals Explanation: Van der Waals bonds (also known as van der Waals interactions) are weak interactions between nonpolar molecules that are very close together. They occur as a result in localized charge fluctuations within nonpolar molecules.

Long hydrocarbon chains such as oils are attracted to one another because of

van der Waals interactions. Explanation: Van der Waals interactions occur in nonpolar molecules that are closely packed together. Electrons within the molecule may shift, resulting in random charge fluctuations within the molecule. These positive and negative regions allow nonpolar molecules to attract one another.