chapter 3 : Water questions
The chemical formula for sucrose (sugar) is C6H12O6. In some parts of the sucrose molecule, oxygen is covalently bonded to hydrogen. This makes the sucrose molecule
A polar molecule
What does it mean to say that something is a "polar molecule"?
A polar molecule has a net dipole as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically. Water (H2O) is an example of a polar molecule since it has a slight positive charge on one side and a slight negative charge on the other.
Which of the following effects can occur because of the high surface tension of water?
A raft spider can walk across the surface of a small pond.
The partial negative charge at one end of a water molecule is attracted to the partial positive charge of another water molecule. What is this attraction called?
a hydrogen bond
Compounds that ionize when dissolved in water are called ________.
electrolytes
If the cytoplasm of a cell is at pH 7, and the mitochondrial matrix is at pH 8, then the concentration of H+ ions _____.
is 10 times higher in the cytoplasm than in the mitochondrial matrix
A molecule or ion is said to be hydrated when it ________.
is surrounded by water molecules
Hydrophobic substances such as vegetable oil are _____.
nonpolar substances that repel water molecules
Scientists predict that acidification of the ocean will lower the concentration of dissolved carbonate ions (CO32−), which are required for coral reef calcification. To test this hypothesis, what would be the independent variable?
ocean pH
The ________ pressure is the pressure required to prevent the flow of solvent through a solvent-permeable membrane that separates two solutions of different solute concentration.
osmotic
In a single molecule of water, two hydrogen atoms are bonded to a single oxygen atom by _____.
polar covalent bonds
Water molecules can form hydrogen bonds with _____.
compounds that have polar covalent bonds
You put a drop of water and a drop of alcohol on a paper towel and saw that the alcohol evaporated faster than the water. Alcohol molecules are not as polar as water molecules. Use the difference in polarity between water and alcohol molecules to explain why alcohol evaporates faster than water.
A more polar molecule will have stronger attractive forces between its molecules than a liquid with weaker polarity. It will require more energy to break the lattice of the liquid with the greater polarity. Therefore alcohol will require less energy to break its lattice and evaporate faster.
Which of the following can be attributed to water's high specific heat?
A lake heats up more slowly than the air around it.
Compared to an acidic solution at pH 5, a basic solution at pH 8 has
1.000 times less hydrogen ions.
If the seawater carbonate ion concentration is 250 µmol/kg, what is the approximate rate of calcification according to this graph?
10 mmol CaCO3 per m2 per day
Define a buffer and provide an example
A buffer solution is one which resists changes in pH when small quantities of an acid or a base are added to it. An acidic buffer solution is simply one which has a pH less than 7. Acidic buffer solutions are commonly made from a weak acid and one of its salts usually sodium salt. An example is carbonic acid + Na carbonate, the buffer which modulates the pH of blood. A basic buffer solution is simply one which has a pH greater than 7. Basic buffer solutions are commonly made from a weak acid and one of its salts usually chloride salt. An example is ammonium hydroxide + ammonium chloride.
Based on this graph, what is the relationship between carbonate ion concentration and calcification rate?
As the concentration of carbonate ions increased, the rate of calcification increased.
What is surface tension and why does water have a strong surface tension?
In a sample of water, there are two types of molecules. Those that are on the outside, exterior, and those that are on the inside, interior. The interior molecules are attracted to all the molecules around them, while the exterior molecules are attracted to only the other surface molecules and to those below the surface. This makes it so that the energy state of the molecules on the interior is much lower than that of the molecules on the exterior. Because of this, the molecules try to maintain a minimum surface area, thus allowing more molecules to have a lower energy state. This is what creates what is referred to as surface tension.
The amount of gas that can dissolve in water
Increases as the temperature of the water decreases
Consider the following reaction at equilibrium: co2+H2o <---->H2co3 What would be the effect of adding additional H2CO3?
It would drive the equilibrium dynamics to the left.
Which of the following takes place as an ice cube cools a drink?
Kinetic energy in the liquid water decreases.
If liquid A is more polar than liquid B, you might expect
Liquid A to evaporate more slowly than liquid B
Why does sugar not dissolve in mineral oil?
Mineral oil is a lipid and is nonpolar. Sugar is a polar molecule. Polar molecules will not dissolve in nonpolar liquids..
When certain substances dissolve, the solution gets warmer. This type of dissolving process is exothermic. In exothermic dissolving
More energy is released when water molecules bond to the solute than is used to pull the solute apart.
When certain substances dissolve, the solution gets colder. This type of dissolving process is endothermic. In endothermic dissolving
More energy is used to pull the solute apart than is released when water molecules bond to the solute.
Water has an unusually high specific heat. What does this mean?
More heat is required to raise the temperature of water.
You have two beakers. One contains pure water, the other contains pure methanol (wood alcohol). The covalent bonds of methanol molecules are nonpolar, so there are no hydrogen bonds among methanol molecules. You pour crystals of table salt (NaCl) into each beaker. Predict what will happen.
NaCl crystals will dissolve readily in water but will not dissolve in methanol.
Water is not a good dissolver of oil mainly because
Oil molecules are non-polar
Briefly explain, on the molecular level, how water dissolves salt.
On addition to water the Na+ section of NaCl is attracted to the oxygen side of the water molecules, while the Cl- side is attracted to the hydrogens' side of the water molecule. his causes the sodium chloride to split in water, and the NaCl dissolves into separate Na+ and Cl- atoms. A hydration shell is formed around them which prevents Na+ and Cl- to form ionic bonds.
Sodium chloride is made up of sodium ions and chloride ions which bond together in a salt crystal because
One is positive and one is negative so they attract
In living systems molecules involved in hydrogen bonding almost always contain either oxygen or nitrogen or both. How do you explain this phenomenon?
Oxygen and nitrogen are elements with very high attractions for their electrons.
Sucrose dissolves well in water because
Polar water molecules attract the opposite polar areas of the sucrose molecules
Increased atmospheric CO2 concentrations might have what effect on seawater?
Seawater will become more acidic, and carbonate concentrations will decrease.
The water molecule is
Slightly negative near the oxygen atom and slightly positive near the hydrogen atoms
Water has many exceptional and useful properties. Which is the rarest property among compounds?
Solid water is less dense than liquid water.
Water cannot dissolve all substances that are made from ionic bonds. This is probably because
Some ionic bonds are too strong for the attractions of water molecules to pull them apart
The abundance of water in the cells and tissues helps to minimize temperature fluctuations. This is due to what property of water?
Specific heat.
Why does ice float in liquid water?
Stable hydrogen bonds keep water molecules of ice farther apart than water molecules of liquid water.
This figure suggests that increased atmospheric concentrations of CO2 will slow the growth of coral reefs. Do the results of the previous experiment support that hypothesis?
Yes; more CO2 causes a decrease in the amount of CO32− in seawater, leading to slower reef growth.
A paper clip can stay on the surface of a sample of water because of water's strong surface tension. Water's surface tension is mostly a result of
The attraction of water molecules
Carbon dioxide gas dissolves pretty well in water because
The bond between the carbon and oxygen in the carbon dioxide is polar
Why does increasing the temperature cause more solute to dissolve in a solvent?
The movement of the molecules (kinetic energy) in a solvent increases with increasing temperature. The force of the collisions with a solute are greater enabling the solute to dissolve faster.
The cities of Portland, Oregon, and Minneapolis, Minnesota, are at about the same latitude, but Minneapolis has much hotter summers and much colder winters than Portland. Why?
The ocean near Portland moderates the temperature.
Water is able to dissolve sodium chloride because
The polar areas of water molecules attract the oppositely charged ions of sodium chloride
The surface of water bends but doesn't break under the weight of a paper clip or water strider. What is it about water molecules and the way they interact that gives water this strong surface tension?
The strong polarity of water will produce large numbers of hydrogen bonds forming a strong lattice.
You put drops of water and alcohol on the surface of two pennies. The water held together and beaded up more than the alcohol. Also, more drops of water than alcohol stayed on the penny. If water molecules are more polar than alcohol molecules, explain why this happened.
The strong polarity of water will produce large numbers of hydrogen bonds forming a strong lattice. This will keep the water molecules together forming a bead.
When sucrose dissolves in water
The water molecules cause the sucrose molecules to separate from one another
Which statement is true about hydrogen bonds between water molecules?
They involve the unequal sharing of a proton between water molecules.
Which of the following statements is true about buffer solutions?
They maintain a relatively constant pH when either acids or bases are added to them.
Which is true about the solubility of electrolytes in water?
They readily dissolve and ionize in water.
Alcohol molecules are not as polar as water molecules. If you mixed sucrose in alcohol you might expect the sucrose
To dissolve in alcohol not as well as in water
The loss of water from a plant by transpiration cools the leaf. Movement of water in transpiration requires both adhesion to the conducting walls and wood fibers of the plant and cohesion of the molecules to each other. A scientist wanted to increase the rate of transpiration of a crop species to extend its range into warmer climates. The scientist substituted a nonpolar solution with an atomic mass similar to that of water for hydrating the plants. What do you expect the scientist's data will indicate from this experiment?
Transpiration rates will fall to zero as nonpolar compounds do not have the properties necessary for adhesion and cohesion.
Why does water dissolve salt more effectively than isopropyl alcohol does?
Water is more polar than isopropyl alcohol.
What are the four emergent properties of water that are important for life?
cohesion, moderation of temperature, expansion upon freezing, solvent properties
Poorly soluble molecules such as lipids and nucleoside bases can be made more soluble in cells by attaching ________ to them.
carbohydrates
Which of the following acts as a pH buffer in blood?
carbonic acid and bicarbonate ion
How would acidification of seawater affect marine organisms? Acidification of seawater would _____.
decrease dissolved carbonate concentrations and hinder growth of corals and shell-building animals
Which of the following is a property of liquid water? Liquid water _____.
has a heat of vaporization that is higher than that for most other substances
Water shows high cohesion and surface tension and can absorb large amounts of heat because of large numbers of which of the following bonds between water molecules?
hydrogen bonds
Which type of bond must be broken for water to vaporize?
hydrogen bonds
The atoms in a water molecule are held together by covalent bonds. This means that the bonded atoms?
share elections
To act as an effective coolant in a car's radiator, a substance has to have the capacity to absorb a great deal of heat. You have a reference book with tables listing the physical properties of many liquids. In choosing a coolant for your car, which table would you check first?
specific heat
The partial negative charge in a molecule of water occurs because _____.
the electrons shared between the oxygen and hydrogen atoms spend more time around the oxygen atom nucleus than around the hydrogen atom nucleus
Solutes diffuse more slowly in cytoplasm than in water because of
the presence of many crowded molecules in the cytoplasm.
In a glass of old-fashioned lemonade, which is the solvent?
water
Electrolytes dissolve readily in water because
water molecules can cluster about cations and anions.