NAS2 Part 3 Review

Why do scientists search for water when evaluating whether another planet might support life?

Water is essential to life as we know it and supports the cellular chemistry and metabolism required by organisms.

Approximately what percentage of the human body is water?

About 60–70 percent.

Where does most cellular chemistry and metabolism occur?

In the watery contents of the cell’s cytoplasm.

How are water’s molecular structure and its life-supporting properties connected?

Water’s polarity allows hydrogen bonding, which produces its solvent ability, thermal stability, cohesion, adhesion, and unusual solid-state behavior.

Name the major properties of water emphasized in Section 2.2.

Polarity and hydrogen bonding; high heat capacity; high heat of vaporization; solvent ability; cohesion and adhesion; and ionization related to pH.

What type of bonds join hydrogen and oxygen within a water molecule?

Polar covalent bonds.

Why is the oxygen end of a water molecule partially negative?

Oxygen is more electronegative than hydrogen and pulls the shared electrons closer to its nucleus.

Why is each hydrogen end of a water molecule partially positive?

The shared electrons spend more time near oxygen, leaving less electron density around hydrogen.

True or false: A water molecule has an overall negative charge.

False. Water has no net charge, but it has partially positive and partially negative regions.

What is a hydrogen bond in the context of water?

A weak attraction between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.

In a diagram of several water molecules, where should a hydrogen bond be drawn?

Between the oxygen of one water molecule and a hydrogen of a neighboring water molecule, not within a single water molecule.

Are the O–H bonds inside one water molecule hydrogen bonds?

No. The O–H bonds within water are polar covalent bonds; hydrogen bonds occur between molecules.

What key idea does Figure 2.13 illustrate?

Different electronegativities create partial charges in water, allowing hydrogen bonds to form between neighboring water molecules.

What does hydrophilic mean?

Describes an ion or polar substance that interacts readily with or dissolves in water.

What does hydrophobic mean?

Describes an uncharged nonpolar substance that does not interact well with water.

Why do vinegar and oil separate in salad dressing?

Vinegar is water-based and polar, whereas oil is nonpolar and hydrophobic.

Contrast hydrophilic and hydrophobic substances.

Hydrophilic substances are ionic or polar and interact with water; hydrophobic substances are nonpolar and avoid interaction with water.

What continually happens to hydrogen bonds in liquid water?

They continually form and break as water molecules move past one another.

What causes hydrogen bonds in liquid water to break more frequently as temperature rises?

Increased kinetic energy of the moving water molecules.

Sequence the molecular events as liquid water boils.

Heat increases molecular kinetic energy; hydrogen bonds break; water molecules escape the liquid as water vapor.

What terms refer to gaseous water?

Steam or water vapor.

What happens to water molecules and hydrogen bonds when water freezes?

Molecular motion decreases and hydrogen bonds stabilize the molecules in an open crystalline lattice.

Why is ice less dense than liquid water?

Hydrogen bonds hold frozen water molecules farther apart in an open lattice than they are in liquid water.

How does water differ from most substances when it solidifies?

Water expands and becomes less dense, whereas most substances pack more tightly and become denser as solids.

What does Figure 2.14 show about the structure and behavior of ice?

Ice has a spacious lattice structure created by hydrogen bonding, making it less dense and able to float.

Why does ice form on the surface of a lake rather than sink?

Solid water is less dense than liquid water, so it floats.

How does floating ice help aquatic organisms survive winter?

The surface ice forms an insulating barrier that slows further heat loss and prevents the entire body of water from freezing solid.

Why can freezing irreversibly damage living cells?

Expanding ice crystals can rupture delicate cell membranes and disrupt cellular structures.

What substance does the section mention as a temporary water replacement that can help cells survive freezing?

Glycerol.

True or false: Ice floats because frozen water molecules are packed more tightly than liquid water molecules.

False. Ice floats because its hydrogen-bonded lattice holds molecules farther apart, making ice less dense.

What is specific heat capacity?

The amount of heat one gram of a substance must absorb or lose to change its temperature by 1°C.

How much heat is required to change the temperature of one gram of water by 1°C?

One calorie.

What is a calorie as defined in this section?

The amount of heat needed to raise one gram of water by 1°C.

Why does water have a high heat capacity?

Much of the added heat is used to disrupt hydrogen bonds before molecular motion and temperature rise substantially.

About how does water’s specific heat capacity compare with that of sand?

It is about five times greater.

Why does land usually heat and cool faster than the sea?

Sand and land have lower heat capacity than water, so their temperatures change more rapidly.

How does water help warm-blooded animals maintain a relatively even body temperature?

It absorbs, transports, and redistributes heat from warmer regions to cooler regions.

Does a high heat capacity mean water contains no heat?

No. It means a large amount of heat is required to change water’s temperature.

True or false: Water heats quickly because hydrogen bonds transfer heat efficiently.

False. Water heats slowly because energy must be used to disrupt hydrogen bonds.

What is heat of vaporization?

The energy required to change one gram of a liquid into a gas.

Approximately how much heat energy is required to vaporize one gram of water?

586 calories.

Why does water have a high heat of vaporization?

Hydrogen bonds must be broken before water molecules can separate and enter the gas phase.

What is water’s boiling point at standard conditions in Celsius and Fahrenheit?

100°C or 212°F.

Why does water require more heat to boil than ethanol?

Hydrogen bonding among water molecules is stronger and more extensive than hydrogen bonding among ethanol molecules.

What is evaporation?

The escape of liquid molecules from a surface into the gas phase, even below the boiling point.

How does evaporation differ from boiling?

Evaporation occurs at the surface and can happen below the boiling point; boiling occurs throughout the liquid at its boiling point.

Why does evaporation cool the remaining liquid or nearby surface?

The highest-energy molecules escape and the phase change absorbs substantial energy from the surroundings.

How does sweating help maintain body-temperature homeostasis?

Evaporation of water in sweat absorbs heat from the skin, cooling the body.

Approximately what percentage of sweat is water according to the section?

About 90 percent.

Why can water be described as a heat sink or heat reservoir?

It can absorb large quantities of heat with relatively small temperature changes and requires substantial energy to vaporize.

What is a solvent?

A substance capable of dissolving another substance.

Why is water an excellent solvent for ions and polar molecules?

Its partial charges attract charged or partially charged regions of solutes and surround them with water molecules.

What is a sphere of hydration, or hydration shell?

An organized layer of water molecules surrounding an ion or polar particle and keeping it dispersed in solution.

What is dissociation?

The separation of atoms or groups from a molecule to form ions.

What happens when sodium chloride is added to water?

Water’s polar regions attract Na+ and Cl−, disrupt the ionic lattice, the ions dissociate, and hydration shells form around them.

How do water molecules orient around a sodium ion (Na+)?

Their partially negative oxygen atoms point inward toward Na+.

How do water molecules orient around a chloride ion (Cl−)?

Their partially positive hydrogen atoms point inward toward Cl−.

What does Figure 2.15 illustrate?

NaCl dissociating in water and water molecules forming hydration shells around Na+ and Cl− with opposite partial charges facing each ion.

Does water dissolve NaCl by turning sodium and chloride atoms into neutral particles?

No. NaCl separates into charged Na+ and Cl− ions that remain stabilized by hydration shells.

Why do hydration shells keep dissolved ions from readily recombining?

Water molecules surround and separate the ions, reducing direct attraction between oppositely charged ions.

True or false: Water dissolves nonpolar oils especially well because water is polar.

False. Water dissolves ionic and polar substances well; nonpolar oils are hydrophobic.

What is cohesion?

Attraction between water molecules caused largely by hydrogen bonding.

Why can water form a dome slightly above the rim of a completely full glass?

Cohesive forces hold surface water molecules together.

What is surface tension?

The ability of a liquid surface to resist being pulled apart or ruptured under tension.

How does cohesion produce surface tension?

Hydrogen-bonded water molecules at the surface pull toward one another, creating a resistant surface film.

Why does water form droplets on a dry surface rather than immediately flattening completely?

Cohesion and surface tension keep the water molecules together.

What forces are balanced in Figure 2.16, allowing a needle to remain at the water’s surface?

The needle’s weight pulls downward while surface tension pulls upward around the indentation.

Does a needle supported by surface tension float because it is less dense than water?

Not necessarily. It can be denser than water but remain supported if placed gently without breaking the surface.

What is adhesion?

Attraction between water molecules and molecules of another material.

Distinguish cohesion from adhesion.

Cohesion is water-to-water attraction; adhesion is attraction between water and another material.

What is capillary action?

The movement of water along or upward through a narrow tube because adhesion to the tube’s walls can exceed cohesion among water molecules.

Why is water higher along the sides of a narrow glass tube than in the center?

Water adheres strongly to the charged glass surface and climbs the walls.

What relationship does Figure 2.17 demonstrate?

When adhesion to a glass tube exceeds water’s cohesion, water rises in the tube by capillary action.

How do cohesion and adhesion contribute to water transport from plant roots to leaves?

Adhesion helps water cling to conducting walls, cohesion maintains a continuous water column, and evaporation from leaves pulls connected molecules upward.

Besides water, what important materials are carried upward in plants because of these properties?

Dissolved minerals required by the plant.

What water property allows the water strider in Figure 2.18 to remain on the surface?

Surface tension arising from cohesive forces among water molecules.

True or false: Capillary action is caused only by cohesion.

False. It depends strongly on adhesion between water and the tube surface, together with cohesion within the water column.

What does the pH of a solution indicate?

Its acidity or basicity, based on hydrogen ion concentration.

Write the simplified reversible ionization equation for water.

H2O ⇌ H+ + OH−.

What is litmus paper, or pH paper?

Filter paper treated with a water-soluble dye that changes color according to pH.

What ions form when a small proportion of water molecules ionize?

Equal numbers of hydrogen ions (H+) and hydroxide ions (OH−).

In liquid water, does a hydrogen ion normally remain as a free, naked proton?

No. It quickly associates with a water molecule to form hydronium, H3O+, although scientists conventionally write H+.

What is the hydrogen ion concentration of pure water?

1 × 10^−7 moles of H+ per liter.

What is a mole?

An amount of substance containing 6.02 × 10^23 particles.

What is the pH of a solution with [H+] = 1 × 10^−7 M?

7.0.

How is pH calculated from hydrogen ion concentration?

pH = −log10[H+].

Why does a high hydrogen ion concentration correspond to a low pH?

Because pH is the negative base-10 logarithm of [H+].

Why does a low hydrogen ion concentration correspond to a high pH?

Because decreasing [H+] makes its negative logarithm larger.

What is an acid?

A substance that increases H+ concentration in solution, commonly by donating H+.

What is a base?

A substance that supplies OH− or accepts/binds H+, thereby lowering H+ concentration and raising pH.

How can hydroxide ions from a base raise pH?

OH− combines with free H+ to form water, decreasing hydrogen ion concentration.

What distinguishes a strong acid from a weak acid?

A strong acid donates H+ readily and dissociates extensively; a weak acid dissociates only partially.

Why is hydrochloric acid considered a strong acid?

It dissociates essentially completely into hydrogen and chloride ions.

Give two examples of weakly acidic liquids from the section.

Tomato juice and vinegar.

What distinguishes a strong base from a weak base?

A strong base readily releases OH− or accepts H+, whereas a weak base does so less extensively.

Give an example of a strong base named in the section.

Sodium hydroxide (NaOH).

What weakly basic solution in the section has a pH near 8?

Seawater.

What range does the pH scale cover?

0 to 14.

Which pH values are acidic, neutral, and alkaline?

Below 7 is acidic, 7 is neutral, and above 7 is alkaline.

What does Figure 2.19 communicate?

The pH scale is inversely related to H+ concentration and places common substances from highly acidic to highly basic.