Chapter 3: Properties of Water

Question 1

How is water being a “polar molecule? significant?

Answer 1

It means that it forms bonds with other polar molecules(including itself).
-> polar molecules bond with each other because of their +ve/-ve end attracting -ve/+ve regions of other polar molecules(dipole-dipole interactions)

Question 2

What does “water molecules are cohesive(same)” mean?

Answer 2

Water molecules will H-bond with other adjacent water molecules(due to polarity) if given the chance( why surface tension exists)

Question 3

What does “water molecules are adhesive(different)” mean?

Answer 3

Water molecules form H-bonds with other non-water substances. (eg. transpiration in plants is cuz H2O bonds with xylem and moves up though stomata to evaporate/ H2O+protein)

Question 4

What are the 5 key properties of water from this chapter?

Answer 4

• water is the solvent of our cells
• water molecules are cohesive
• water moles are adhesive
• water can moderate temperature changes
• water is less dense as a solid than a liquid

Question 5

What does a solvent do? What makes an aqueous solution? Water makes up _ % of our cells(diso

Answer 5

• A solvent dissolves the solute(solutes can be liquids as well but are in a smaller proportion to the solvent)
• solute + H2O(solvent) = aqueous soln. ( also solutes that dissolve in water are polar, can’t generally dissolve non-polar molecules)
• 70 %(transports nutrients/waste products/rxns-> primary solvent/facilitator)

Question 5

A solution is also called a ___

Answer 5

homogenous mixture

Question 6

What substances are hydrophilic?

Answer 6

ionic and polar substances(hydrophilic-> dissolve in water)

Question 7

What substances are hydrophobic?

Answer 7

non-polar substances(water is polar) (Hydrophobic->does not dissolve in water)

Question 8

How does H2O dissolve ions?

Answer 8

partially -ve O attracts +ve cations, partially +ve H attracts -ve anions

Question 9

cations vs anions

Answer 9

cations-> +ve charge (cats are +ve, don’t confuse with cathode)
anions-> -ve charge (ants are -ve cuz they bite)

Question 10

Why can water moderate temperature changes?

Answer 10

Water has high specific heat capacity->can absorb a lot of heat energy before it changes temp->DUE TO H BONDS-> energy goes into breaking weak H bond network btwn water molecules and then KE increases(molecules can’t move otherwise)/temp. increases

Question 11

benefits from water’s high specific heat capacity

Answer 11

Moderates ocean temp/ cellular temp/coastal climates/evaporative cooling

Question 12

Why is water less dense as a solid than a liquid?
btw helps cuz ice floats on water and allows marine life to survive underneath

Answer 12

• Solid/frozen water(ice): molecules are frozen apart(H bonds are stable btwn them)–> larger volume for same mass (d dec. = m/v inc.) DENSITY DECREASES AS WATER FREEZES
• Liquid water: molecules are constantly moving due to high KE /H bonds breaking and reforming (H bonds are unstable)-> d inc. = m/ V dec.(molecules move closer together/not stuck apart)

Question 12

Why do water molecules dissociate and what happens when they do?

Answer 12

H2O molecules dissociate cuz of their polar nature(self-ionize); H2O-> H+ + OH-(hydroxide),
H+ can’t stay alone in aq soln(OH- can), so it bonds with the water molecules around it-> H+(hydrogen) + H2O-> H3O+(hydronium)->used interchangeably in bio

Question 13

Acids are substances that ____ the H conc. of a soln

Answer 13

increase the H conc. of a soln/give H

Question 14

Bases are substances that ___ the H conc. of a soln

Answer 14

decrease the H conc of a solution/ take H

Question 15

What does pH measure? pH=?

Answer 15

• H conc of a soln.
• pH= -log(H+) -> eg. pH 7= -log (10^-7) where each log unit in ten fold difference in H conc.
• pH scale: <7- acidic, =7-neutral, >7- basic(blood/water)

Question 16

What are buffers and what purpose do they serve?

Answer 16

• Buffers minimize(neutralize) the pH change of a soln. on addition of acid/base
• Formed by weak acid+ its conjugate base(eg. CH3COOH-> CH3COO- + H+)
• Weak acid does not completely ionize->Take H+ when pH decreases(acid is added), give H+ when pH increases(base is added)

Question 17

What would happen to the pH of water with the addition of an acid or a base?

Answer 17

It would become more acidic/basic (pH change) as H2O is not a buffer (a buffer would show minimal/no change in pH)

Question 18

Why are buffers important to cells?

Answer 18

maintain a stable pH level within the cell by taking excess H+or OH- which would otherwise drastically alter the cell’s chemical environment, disrupting important biological processes like enzyme function and protein structure

Question 19

In a neutral solution the concentration of _____

Answer 19

conc. of H+ ions = conc. of OH- ions

Question 20

A water molecule can bond with to up to _____ other water molecules by ____ bonds

Answer 20

4, hydrogen(within molecules-polar covalent, between separate molecules- H bond?)

Question 21

Many of water’s emergent properties, such as its cohesion, high specific heat, and high heat of vaporization, result from the fact that water molecules __________.

Answer 21

are attracted to each other by partial negative and positive charges on the oxygen and hydrogen atoms, respectively

Question 22

Do buffers hold pH constant?

Answer 22

no, they just limit variation by taking/donating H+

Question 23

Buffers work best when ...

Answer 23

about half of the buffer molecules are dissociated(not most either way; needs plenty of both associate and dissociated molecules)

Question 24

buffer example to keep in mind while thinking of all buffer questions

Answer 24

CH3COO(weak acid)<----> CH3COO-(its conjugate base) + H+ [REVERSIBLE RXN]

Question 25

why do all living cells need pH buffers?

Answer 25

Amino acid side chains have many carboxyl and amino groups; that can act as acids or bases based on the surrounding environment (also very acidic pH causes denaturation of proteins)

Question 26

How to make buffer?

Answer 26

Put weak acid/weak base in half-ionized water (when half of the molecules are ionized, there are plenty of molecules that can either absorb H+ or give off H+ when the pH is disturbed from outside)

Question 27

What are amphipathic molecules?

Answer 27

molecules that have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions(eg. parts of cell membrane interact with aq. stuff and lipid bilayers)

Question 28

Can water form intramolecular hydrogen bonds?

Answer 28

No, water only forms polar covalent bonds within itself( H/O/H->polar covalent bonds are found btwn them)

Question 29

dissolving is best described as

Answer 29

a mingling of molecules and ions

Question 30

do solutes always dissolve into ions in water?

Answer 30

no, ionic compounds dissociate into ions. Molecular compounds dissociate into molecules (eg. CO2/CH4)

Question 31

Why does water have tensile strength(resistance to breaking under tension)?

Answer 31

water surface tension->due to H bonds, due to adhesion(with other substances)/cohesion, water's tensile strength helps to pull water through plants

Question 32

What factors allow us to cool down while sweating?

Answer 32

- Weak H bonds - molecules collide with various angles and speeds

Question 33

Whathelps to explain why ice is less dense than liquid water?

Answer 33

- Water molecules make hydrogen bonds at definite angles - Cold molecules move less than warm molecules

Question 34

The open spaces in water's crystal structure make it possible for ...

Answer 34

aquatic life to exist at the North Pole

Question 35

Why doesn't oil mix with water?

Answer 35

Polar molecules attract one another (water molecules cling to one another and won't part to make room for uncharged (nonpolar) molecules. There's no repulsion. )

Question 36

ionization of water

Answer 36

- dissociation of water is reversible -dissociation of water produces equal numbers of OH- and H+(not masses) - water ionizes to form hydroxide and hydronium ions

Question 37

Right before a predicted overnight freeze, farmers spray water on crops to protect the plants. Use the properties of water to explain how this method works. Be sure to identify why hydrogen bonds are responsible for this phenomenon

Answer 37

Due to intermolecular hydrogen bonds, water has high specific heat. When water is cooled, many of these bonds are formed, which releases a significant amount of heat. This can provide some protection against freezing of the plants' leaves, thus protecting the cells from damage.

Question 38

Global warming is causing atmospheric CO2 levels to increase, which is expected to cause ocean acidification.

Answer 38

CO2 + H2O -> H2CO3(carbonic acid) -> HCO3-(bicarbonate ions in the ocean increase) + H+ also H+ + CO3 2- (carbonate) -> HCO3-(carbonate ion conc. in oceans decrease, turning acidic, calcification and coral reef growth decreases)(also CO2 dissolving doesn't increase CO3 2-)