1.1 Structure of Water and Hydrogen Bonding

1. AAn attraction between the partially positive H of one molecule and the partially negative O of another Correct
   Line 2 is a hydrogen bond. The model shows it joining a δ+ H atom of one water molecule to the δ− O atom of a neighboring molecule; the two molecules attract because of these partial charges, without sharing electrons.
2. BA covalent bond in which the H atom of one molecule shares a pair of electrons with the O atom of another
   A student who thinks a hydrogen bond is a covalent bond picks this. The covalent bonds are within each molecule (line 1); between molecules there is only an attraction between partial charges, and no electrons are shared.
3. CAn ionic bond between an H⁺ ion in one molecule and an O²⁻ ion in a second, neighboring molecule
   A student who thinks water is made of ions picks this. The model shows partial charges (δ+ and δ−), not the full charges of ions: electrons are shared unequally within each molecule, not transferred.
4. DA strong bond, stronger than line 1, that holds the two water molecules in a fixed position
   A student who thinks hydrogen bonds are stronger than covalent bonds picks this. A hydrogen bond is much weaker than the covalent bond shown by line 1, and in liquid water hydrogen bonds constantly break and re-form.

CED 1.1.A.1.i · Read this in Fix

1. ACohesion, the attraction of water molecules to a substance that is not water
   A student who swaps the meanings of cohesion and adhesion picks this. Cohesion is the attraction of water molecules to one another, shown by the lines labeled 2; attraction to the glass is adhesion.
2. BSurface tension, the pull of water toward the walls of the container holding it
   A student who thinks surface tension is attraction to the container picks this. Surface tension is the resistance of water's own surface to being broken, caused by cohesion among surface molecules; attraction to the glass wall is adhesion.
3. CAdhesion, the attraction of water molecules to the molecules of a different polar substance Correct
   The attractions labeled 1 are hydrogen bonds between the glass's O–H groups and water molecules. Attraction between water and a different substance is adhesion; the attractions labeled 2, between water molecules, represent cohesion.
4. DCovalent bonding, in which water molecules share pairs of electrons with the glass
   A student who thinks hydrogen bonds are covalent bonds picks this. The dashed lines are attractions between partial charges on the glass's O–H groups and on water molecules; no electrons are shared between them.

CED 1.1.A.2 · Read this in Fix

1. AEach H atom transfers its electron completely to the O atom, so the molecule consists of ions of opposite charge.
   A student who thinks 'polar' means 'ionic' picks this. In water the electrons are shared, though unequally; they are not transferred, so the atoms carry partial charges rather than full ionic charges.
2. BHydrogen bonds inside the molecule join each H atom to the O atom, and these bonds give the atoms their charges.
   A student who thinks the O–H bonds inside a water molecule are hydrogen bonds picks this. The H atoms are joined to O by polar covalent bonds; hydrogen bonds form between molecules, as a result of the polarity, not as its cause.
3. CThe molecule picks up extra electrons from its surroundings, so as a whole it carries a negative charge.
   A student who thinks a polar molecule carries a net charge picks this. The partial negative charge on O is balanced by the partial positive charges on the two H atoms, so the molecule is electrically neutral overall.
4. DO attracts the shared electrons more strongly than H, so O is partially negative and each H partially positive. Correct
   Oxygen is more electronegative than hydrogen, so the electrons shared in each O–H bond spend more time near the O atom. The O end of the bent molecule carries a partial negative charge and the H atoms carry partial positive charges.

CED 1.1.A.1.i · Read this in Fix

1. AIt dissolves more readily, as each –CH₃ group has more H atoms that can hydrogen-bond with water.
   A student who thinks any H atom can form a hydrogen bond picks this. Only an H atom bonded to a strongly electronegative atom such as O or N is partially positive enough to hydrogen-bond; H atoms on carbon are not.
2. BIt dissolves less readily, as it forms fewer hydrogen bonds with the water molecules. Correct
   The polar –OH groups let X form hydrogen bonds with water, which is why it dissolves. The H atoms of a –CH₃ group are bonded to carbon and carry almost no partial charge, so the modified molecule forms fewer hydrogen bonds with water and dissolves less readily.
3. CIt dissolves just as readily as X, as water can dissolve almost any substance placed in it.
   A student who takes 'universal solvent' literally picks this. Water dissolves substances that can form hydrogen bonds or other attractions with it; replacing polar groups with nonpolar ones reduces these attractions.
4. DIt breaks apart into separate atoms, as water splits the covalent bonds of any substance it dissolves.
   A student who thinks dissolving is a chemical breakdown picks this. When a molecule dissolves, whole molecules separate from one another and are surrounded by water; the covalent bonds within each molecule stay intact.

CED 1.1.A.1.i · Read this in Fix

1. ABy day the water absorbs little of the Sun's energy; at night it has little energy left to lose.
   A student who equates a small temperature change with a small amount of heat picks this. Water absorbs a large amount of energy by day and releases a large amount at night; its temperature changes only a little because much of that energy goes into breaking or is released by forming hydrogen bonds.
2. BHydrogen bonds in the pond break at night and release energy, which keeps the water from cooling as the air does.
   A student who thinks breaking bonds releases energy picks this. Breaking hydrogen bonds absorbs energy; at night, as water cools, hydrogen bonds form, and it is their formation that releases energy.
3. CBy day, breaking hydrogen bonds takes up much of the energy the water gains; at night, re-forming them gives it back. Correct
   The air temperature varies by about 17 °C over the day, the pond's by only about 3 °C. Water has a high specific heat capacity: by day, much of the energy it absorbs breaks hydrogen bonds rather than speeding up the molecules; at night, hydrogen bonds form and release energy. Both effects keep the pond's temperature steady.
4. DEnergy is used to break the O–H bonds inside each water molecule, so the pond's temperature rises only slowly.
   A student who thinks the hydrogen bonds of water are the O–H bonds within each molecule picks this. Those covalent bonds are not broken at pond temperatures; the energy goes into hydrogen bonds between molecules.

CED 1.1.A.1.ii · Read this in Fix

1. AThe heat makes each base molecule swell in size, so the swollen bases then push the two strands of DNA away from each other.
   A student who thinks molecules expand when heated picks this. Heating makes molecules move faster; it does not make them larger. The strands separate because the hydrogen bonds between them are broken.
2. BHydrogen bonds are much weaker than covalent bonds, so heat breaks the bonds between the strands but not those within each. Correct
   Heating makes the molecules move faster. The weak hydrogen bonds between bases on opposite strands are broken at 95 °C, so the strands separate, but the much stronger covalent bonds that join the nucleotides within each strand are not.
3. CThe heat breaks the bonds between pairs of H atoms that link the bases of one strand to those of the other.
   A student who thinks a hydrogen bond joins two hydrogen atoms picks this. Each hydrogen bond between the strands joins a partially positive H atom to a partially negative atom (O or N) on the opposite strand.
4. DBreaking the bonds between the two strands releases energy, and this released energy forces the strands apart.
   A student who thinks breaking bonds releases energy picks this. Breaking hydrogen bonds requires energy, which here is supplied by heating; it does not release energy that pushes the strands apart.

CED 1.1.A.1.i · Read this in Fix

1. AA second thermometer with its bulb wrapped in wet cloth, placed in front of the same fan as A
   A student who confuses a replicate with a control picks this. A second wet thermometer repeats the treatment; it shows how much results vary but not what happens without evaporation.
2. BAn identical thermometer with its bulb wrapped in dry cloth, placed in another room with no fan
   A student who thinks a control may differ in several ways picks this. This control lacks both water and moving air, so a difference from thermometer A could be caused by either factor.
3. CAn identical thermometer with its bulb in wet cloth, placed in a room with no fan at all
   A student who builds a control by removing the most obvious addition, the fan, picks this. Without the fan, water still evaporates from the cloth, so this setup tests the effect of the fan, not the effect of evaporation.
4. DAn identical thermometer with its bulb wrapped in dry cloth, placed in front of the same fan Correct
   The hypothesis concerns evaporation of water, so the control should differ from thermometer A only in having no water to evaporate. A dry cloth in the same airflow keeps the cloth, the fan and the room the same.

CED 1.1.A.1.iii · Read this in Fix

1. ABoth wet-cloth thermometers cooled, and the one in moving air cooled faster. Correct
   Both wet-cloth thermometers fell below 24 °C. In the first 2 minutes the one in moving air fell by 3.4 °C and the one in still air by 1 °C, and the one in moving air kept the larger fall throughout. The dry-cloth thermometer stayed at 24 °C.
2. BBoth thermometers in moving air cooled, and the two of them cooled by similar amounts.
   A student who thinks moving air cools any object, wet or dry, picks this. The dry-cloth thermometer in front of the fan stayed at 24 °C throughout, while the wet-cloth thermometer in moving air fell to about 17 °C; only the wet-cloth thermometers cooled.
3. CBoth wet-cloth thermometers cooled, and the one in still air cooled faster than the other.
   A student who reads the higher line as the faster-changing one picks this. The still-air line is higher because that thermometer cooled less; its line is less steep, so it cooled more slowly.
4. DBoth wet-cloth thermometers cooled at a steady rate that stayed the same all the way through.
   A student who assumes changes happen at a constant rate picks this. The wet thermometer in moving air fell by 3.4 °C in the first 2 minutes but only 0.1 °C in the last 2 minutes; its rate of cooling fell as its temperature leveled off.

CED 1.1.A.1.iii · Read this in Fix

1. ASalt water held significantly more drops than plain water, as its mean is the higher of the two.
   A student who thinks any difference in means is significant picks this. Salt water's error bar (30 to 36) overlaps plain water's (28 to 34), so these data do not show a significant difference.
2. BDetergent water likely held fewer drops than plain water did, as the two error bars do not overlap. Correct
   Detergent water's error bar (10 to 14 drops) lies well below plain water's (28 to 34), so the difference in means is likely significant. Detergent weakens the cohesion at the water surface, so the surface breaks after fewer drops.
3. CSalt has no effect on the number of drops held, as its error bar overlaps that of plain water.
   A student who thinks overlapping error bars prove there is no difference picks this. The overlap means no significant difference was detected; a small effect of salt could still exist.
4. DCounting mistakes were probably fewest for detergent water, as its error bar is the shortest.
   A student who thinks error bars measure mistakes picks this. Error bars show the uncertainty in each mean, which comes from natural variation among the coins and from sample size, not from mistakes.

Working Error bars are ±2 SE, roughly 95% confidence intervals. Plain water 31 (28 to 34); salt water 33 (30 to 36); detergent water 12 (10 to 14). Plain and detergent: the intervals 28–34 and 10–14 do not overlap, so the difference is likely significant. Plain and salt: 28–34 and 30–36 overlap, so no significant difference is shown, and no difference is established either.

CED 1.1.A.2 · Read this in Fix

1. AEach depends on covalent bonds that keep forming and breaking between water molecules.
   A student who thinks hydrogen bonds are covalent bonds picks this. No electrons are shared between water molecules; the attractions between them are hydrogen bonds between partial charges.
2. BEach depends on ionic bonds between the H⁺ ions and O²⁻ ions that make up water.
   A student who thinks water is made of ions picks this. Water consists of neutral polar molecules with partial charges, held to one another by hydrogen bonds.
3. CEach depends on hydrogen bonds that form between neighboring polar water molecules. Correct
   A high specific heat (1), a high heat of vaporization (2) and surface tension (3) all arise because polar water molecules form hydrogen bonds with one another, and energy is needed to break these bonds or to stretch the surface.
4. DEach depends on the bonds that hold the H atoms to the O atom inside every water molecule.
   A student who thinks hydrogen bonds are the O–H bonds within a water molecule picks this. Those covalent bonds stay intact in all three observations; the properties come from hydrogen bonds between molecules.

CED 1.1.A.1 · Read this in Fix