Structure of water and hydrogen bonding
Life on Earth is built in water and runs on water.
Your cells are about 70% water by mass, every chemical reaction in them happens in water, and water's odd behavior keeps your body temperature steady, lets trees lift water 100 meters to their top leaves, and lets an insect stand on a pond.
All of that comes from one feature of a small molecule: water is polar, so its molecules stick to each other by hydrogen bonds.
This section explains where the polarity comes from, what a hydrogen bond is, and how hydrogen bonding produces each property you need to know.
On the exam, this topic usually appears as a short recall prompt (a Part A Describe about hydrogen bonding, cohesion or cooling), as the biology behind a plant water-transport or temperature experiment, and in Section I questions that ask which property best explains an observation.
Used again in: every later section of this unit (hydrogen bonds hold DNA strands together and shape proteins; polarity decides which molecules mix with water); Unit 2 (water moving through membranes and up the xylem; water potential); Unit 8 (water in ecosystems).
You also need: the idea of an atom's electrons and of a covalent bond from your earlier science courses, reviewed below.
- The AP Biology Exam takes 3 hours and has two sections, each worth half of your score.
- Section I: 60 multiple-choice questions in 90 minutes, each with four options (A) to (D). Some stand alone; others come in sets of about four or five that share one graph, table or model.
- Section II: six free-response questions in 90 minutes. Questions 1 and 2 are long (9 points each, about 25 minutes each) and are built around an experiment and its data; questions 3 to 6 are short (4 points each, about 10 minutes each).
- You may use a four-function, scientific or graphing calculator in both sections, and the AP Biology Equations and Formulas sheet is available in both.
- Chemistry of Life most often supplies the Part A recall question that opens a free-response question (what a hydrolysis reaction does, the parts of a nucleotide, how a charged group changes a protein), and its ideas about polarity, charge and protein shape run through questions set in later units.
- Each point is one row of the scoring guidelines, tied to one prompt sentence. There are no half points and no levels.
- The row names a key idea. Any correct wording of that idea earns the point, and extra words cost nothing unless they contradict what you wrote.
- When a prompt asks for one example, only the first one you give is read, so give the number asked for and no more.
- A drawing on its own earns nothing unless the prompt asks you to draw. Answer in sentences.
- Predict and Justify are separate points. A justification earns only when it backs the correct prediction, so get the direction right first.
- A Calculate point is for the value. Show the working and the unit anyway, because the definition of Calculate asks for them and a slip is easier to spot.
- A graph you construct carries 3 points: the right type of graph, the data and error bars plotted accurately, and both axes labeled with units plus a legend.
- Describe asks for the relevant characteristics; Explain asks how (the mechanism) or why. Naming a structure without its characteristic (an Identify answer) does not earn a Describe point.
- Restating the question or the information you were given earns nothing.
No equation on the AP Biology Equations and Formulas sheet belongs to this unit.
Everything in Chemistry of Life is learned and recalled.
- The bond types (nonpolar covalent, polar covalent, ionic, hydrogen bond) and what each does in biological molecules.
- Hydrolysis adds water across a covalent bond and splits it; dehydration synthesis removes the equivalent of a water molecule and joins two monomers. Joining n monomers in one chain releases n − 1 water molecules.
- The three parts of a nucleotide, and the four parts attached to an amino acid's central carbon, with the three classes of R group.
- The four levels of protein structure and the bonds and interactions behind each.
- Saturated fatty acids have only single carbon-carbon bonds; unsaturated ones have at least one double bond and a kink.
- pH = −log[H⁺]; a change of 1 pH unit is a tenfold change in the hydrogen ion concentration.
- Living systems depend on the properties of water to sustain life. i. Water has polarity, because of the formation of polar covalent bonds between hydrogen and oxygen within water molecules. This polarity contributes to hydrogen bonding between and within biological molecules; ii. Water has a high specific heat capacity, which allows for the maintenance of homeostatic body temperature within living organisms; iii. Water has a high heat of vaporization, which allows for the evaporative cooling of the surrounding environment. In living organisms, this property allows for body temperature to be maintained.
- The hydrogen bonds between adjacent polar water molecules result in cohesion, adhesion, and surface tension.
The bonds you need for this whole unit
Every section of Chemistry of Life uses the same small set of bonds and attractions, and the exam expects you to name the right one.
The commonest lost point in this unit is calling a hydrogen bond a covalent bond, or the reverse, so learn the difference now.
A covalent bond forms when two atoms share a pair of electrons.
It is strong: breaking one takes a chemical reaction.
When the two atoms pull on the shared electrons equally (two carbon atoms, or carbon and hydrogen), the bond is nonpolar.
When one atom pulls harder, the electrons spend more time near it.
Oxygen and nitrogen pull electrons strongly (they are highly electronegative), so in an O–H or N–H bond the oxygen or nitrogen carries a small negative charge, written δ− ("delta minus"), and the hydrogen a small positive charge, δ+.
That is a polar covalent bond.
The other attractions are between molecules, or between distant parts of one large molecule, and are much weaker than covalent bonds.
Bond or interaction
What happens to the electrons or charges
Strength
Where you meet it in AP Biology
Nonpolar covalent bond
Two atoms share electrons equally (C–C, C–H)
Strong
The backbones of all macromolecules; fatty acid tails
Polar covalent bond
Two atoms share electrons unequally; the more electronegative atom (O or N) gets a partial negative charge (δ−) and its partner a partial positive charge (δ+)
Strong
O–H in water and in sugars; N–H in amino acids and bases
Ionic interaction
Attraction between fully charged groups of opposite sign
Moderate (weaker in water)
Charged R groups; DNA's negative phosphates and positive proteins
Hydrogen bond
Attraction between a δ+ hydrogen (bonded to O or N) and a δ− O or N on another molecule or another part of the same molecule
Weak one at a time; strong in large numbers
Between water molecules; DNA base pairs; protein helices and sheets
Hydrophobic interaction
Nonpolar molecules or regions cluster together because water molecules hydrogen bond to each other and exclude them
Weak, but adds up
Lipid bilayers; the nonpolar core of a folded protein
Two words in the table matter for the whole course.
Polar means having partial charges because electrons are shared unequally.
Hydrophilic ("water-loving") describes polar or charged molecules and regions, which attract water; hydrophobic ("water-fearing") describes nonpolar ones, which do not.
Why water is polar
A water molecule is one oxygen atom joined by two polar covalent bonds to two hydrogen atoms.
Oxygen pulls the shared electrons toward itself, so the oxygen end is δ− and each hydrogen is δ+.
The molecule is also bent (the two hydrogens sit at an angle of about 104.5°, not in a straight line), so the two δ+ hydrogens are on one side and the δ− oxygen on the other.
A straight molecule with the same bonds would have its charges cancel; the bent shape is why the whole molecule has a positive side and a negative side.
Hydrogen bonds between water molecules
Opposite partial charges attract.
The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbor.
That attraction is a hydrogen bond.
Each water molecule can form up to four of them (two through its hydrogens and two through its oxygen), so liquid water is a constantly shifting network of molecules held to each other.
The diagram below shows a small piece of that network.
Two things in the diagram are easy to mix up.
The solid lines are covalent bonds inside each molecule; they are not broken when water warms, evaporates or freezes.
The dashed lines are hydrogen bonds between molecules; each lasts only a fraction of a second before breaking and re-forming with another neighbor.
When water boils, it is the hydrogen bonds that break, never the O–H covalent bonds.
The statement you need to know also says that polarity contributes to hydrogen bonding "between and within biological molecules".
Hydrogen bonds are not only a water story.
They hold the two strands of DNA together (Nucleic Acids, later in this unit), hold the coils and sheets of proteins in shape (Proteins), and let sugars and other polar molecules dissolve, because water molecules hydrogen bond to their O–H groups.
Like dissolves like
Polar water surrounds other polar molecules and ions with hydrogen bonds and attractions, so they dissolve.
Nonpolar molecules (fats and oils, the tails of phospholipids) cannot form hydrogen bonds with water.
Water molecules keep hydrogen bonding to each other and push the nonpolar molecules together into clusters, which is why oil forms drops and a layer on water.
The rule to use in answers is like dissolves like: polar with polar, nonpolar with nonpolar.
Students often write that oil and water separate because "opposites attract" or "opposites repel".
Polar and nonpolar are not opposite charges, so neither phrase applies.
Oil separates because it has no partial charges for water to attract, and the water molecules attract each other instead.
That same exclusion makes the nonpolar tails of phospholipids cluster in a membrane (Lipids) and makes nonpolar R groups fold into the center of a protein (Proteins).
High specific heat capacity
Specific heat capacity is the amount of energy needed to raise the temperature of 1 gram of a substance by 1 °C. Water's is high: about 4.2 joules per gram per °C, around twice that of vegetable oil and about five times that of sand.
The reason is hydrogen bonding.
Temperature measures how fast molecules move.
When you heat water, much of the energy goes into breaking hydrogen bonds before the molecules can move faster.
So water absorbs a lot of energy for each degree its temperature rises, and it releases a lot of energy for each degree it cools.
For living things, this means the water in cells and blood soaks up the heat released by metabolism, or the heat of a hot day, with only a small rise in temperature.
A body that is mostly water changes temperature slowly, which allows for the maintenance of a homeostatic body temperature.
The same property keeps lakes and oceans at steady temperatures, so aquatic organisms live in a stable environment, and it keeps coastal climates milder than inland ones.
The graph below shows the same amount of energy going into equal masses of water and vegetable oil.
The oil's temperature rises by 25 °C in 10 minutes, the water's by only 12 °C. With identical heaters, both received the same energy, so each degree of temperature rise in water took about twice as much energy (25 ÷ 12 ≈ 2.1).
That ratio is a sensible size: it matches water's specific heat being roughly double that of the oil.
High heat of vaporization and evaporative cooling
Heat of vaporization is the energy needed to turn a given mass of liquid into gas.
For water it is high, because a molecule can only escape from the liquid once every hydrogen bond holding it has broken.
At any moment the molecules in a film of water have a range of speeds.
The fastest ones, with the most energy, are the ones that break free and evaporate.
They carry their energy away, so the molecules left behind have a lower average energy: the remaining liquid, and the surface it sits on, cool down.
That is evaporative cooling.
Because each gram of water that evaporates takes so much energy with it, a small amount of sweat removes a lot of heat.
In living organisms, this property allows for body temperature to be maintained.
You sweat during exercise; dogs pant, evaporating water from the mouth and airways; plants lose water vapor through stomata in transpiration, which cools leaves in full sun.
A leaf that cannot transpire can heat several degrees above a leaf that can.
Cohesion, adhesion and surface tension
The second statement you need to know lists three results of hydrogen bonds between adjacent water molecules.
- Cohesion is water molecules sticking to each other by hydrogen bonds. In the xylem of a plant, water forms an unbroken column from root to leaf. As water evaporates from the leaves, each molecule leaving pulls on the one behind it through hydrogen bonds, and the whole column is pulled upward.
- Adhesion is water sticking to other polar or charged surfaces, again by hydrogen bonds. Water adheres to the cellulose walls of xylem vessels, which helps it climb against gravity and stops the column from sliding back down. Adhesion is also why water creeps up a narrow glass tube or a paper towel (capillary action), and climbs higher in a narrower tube, where more of the water touches the wall.
- Surface tension is the resistance of a water surface to being stretched or broken. Molecules at the surface have no water molecules above them, so they are pulled inward and sideways by hydrogen bonds to their neighbors, and the surface behaves like a stretched film. Water striders and some spiders stand on it, and a drop of water holds a rounded shape.
The photograph below shows a water strider resting on a pond.
Look at where its legs touch the water: each one presses a small dimple into the surface, but the surface bends without breaking.
Cohesion and adhesion work together in a plant.
Cohesion alone would let the column snap; adhesion alone would only wet the walls.
The column holds because molecules hold each other (cohesion) and hold the vessel walls (adhesion).
Water and pH
A tiny fraction of water molecules split into a hydrogen ion (H⁺) and a hydroxide ion (OH⁻).
pH measures the hydrogen ion concentration: pH = −log[H⁺].
Because the scale is logarithmic, a change of 1 pH unit is a tenfold change in [H⁺]: pH 4 has ten times the hydrogen ion concentration of pH 5.
You need to recall this; the current reference sheet does not print it, though the sheets printed with the 2023 and 2024 exams did.
You will use pH most in Unit 3, where enzyme activity depends on it.
- Polar covalent bond: a covalent bond in which the electrons are shared unequally, so one atom carries a partial negative charge and the other a partial positive charge.
- Hydrogen bond: an attraction between a partially positive hydrogen atom, bonded to oxygen or nitrogen, and a partially negative oxygen or nitrogen atom elsewhere.
- Specific heat capacity: the energy needed to raise the temperature of 1 gram of a substance by 1 °C.
- Heat of vaporization: the energy needed to change a given mass of a liquid into a gas.
- Cohesion: the attraction of water molecules to each other by hydrogen bonds.
- Adhesion: the attraction of water molecules to other polar or charged substances.
- Surface tension: the resistance of a liquid surface to being stretched or broken, caused by cohesion among the surface molecules.
Coast redwood trees in northern California grow taller than 100 meters, and their leaves lose water vapor through stomata all day.
Describe how hydrogen bonds between water molecules allow water to move up the xylem of a tall tree.
Full-credit answer. Water molecules are attracted to each other by hydrogen bonds (cohesion), so the water in a xylem vessel forms a continuous column.
As water evaporates from the leaves, the molecules that leave pull on the molecules below them, and the whole column is drawn upward.
Scoring row. Part A (1 point): Hydrogen bonds hold water molecules together (cohesion), so water lost from the leaves pulls the continuous column of water up the xylem.
Weaker answer. "Cohesion."
This names the property and stops.
It is an Identify answer to a Describe prompt: it gives neither the characteristic (molecules held together by hydrogen bonds) nor its effect (the column is pulled up as a unit), so it earns nothing.
Explain how sweating lowers the temperature of the skin.
Full-credit answer. The water molecules in sweat with the most energy break their hydrogen bonds to other water molecules and evaporate.
Because water has a high heat of vaporization, each gram that evaporates removes a large amount of heat energy, so the water left on the skin and the skin beneath it cool down.
Scoring row. Part B (1 point): Evaporation of water requires/removes a large amount of energy (to break hydrogen bonds), so heat is carried away from the skin as the highest-energy molecules leave.
Weaker answer. "Sweating keeps the body at 37 °C so that enzymes keep working."
That is why sweating is useful (its purpose), not how it cools.
An "explain how" prompt needs the mechanism: high-energy molecules evaporate, carrying heat away from the skin.
Students placed 200 g of water and 200 g of vegetable oil in identical beakers on identical heaters, and recorded the temperature every 2 minutes (the graph earlier in this section).
Identify the temperature rise of each liquid after 10 minutes.
Explain the difference using the structure of water.
- Step 1: read the graph. Water rose from 20 °C to 32 °C, a rise of 12 °C. Oil rose from 20 °C to 45 °C, a rise of 25 °C.
- Step 2: interpret the size. 25 ÷ 12 ≈ 2.1, so the oil warmed about twice as much for the same energy input. That fits water's specific heat capacity being about twice that of the oil.
- Step 3: the biology. In water, much of the energy supplied goes into breaking hydrogen bonds between the polar molecules before the molecules can move faster, so the temperature rises less. Oil molecules are nonpolar and form no hydrogen bonds, so more of the same energy raises their speed, and their temperature.
Scoring rows. Identify (1 point): Water 12 °C; oil 25 °C (accept 24 to 26 °C).
Explain (1 point): Energy breaks hydrogen bonds between water molecules (high specific heat), so less goes into raising the temperature; the nonpolar oil has no hydrogen bonds to break.
Weaker answer. "Water is denser than oil, so it heats more slowly."
Density is not the cause, and the answer names no bond.
The point needs the hydrogen bonds.
Water (H₂O) and methane (CH₄) are molecules of similar size and mass.
Water boils at 100 °C, but methane boils at about −162 °C.
Which of the following best explains the difference?
- (A) Boiling water breaks the O–H covalent bonds, which are stronger than the C–H bonds broken when methane boils.
- (B) Water molecules are polar and are held to each other by hydrogen bonds, which must be broken before a molecule can escape as gas; methane molecules are nonpolar and form no hydrogen bonds.
- (C) Methane molecules are polar, so they repel each other and escape from the liquid easily.
- (D) Water has a lower specific heat capacity than methane, so it must be heated to a higher temperature.
Answer: (B). Only polar water molecules form hydrogen bonds with each other, so more energy, and a higher temperature, is needed to separate them.
- (A) is wrong: boiling breaks the attractions between molecules, not the covalent bonds inside them; steam is still H₂O.
- (C) is wrong: methane is nonpolar (carbon and hydrogen share electrons almost equally), and polar molecules attract each other.
- (D) is wrong: water's specific heat capacity is high, and specific heat does not set the boiling point.
Every water property on the exam traces back to one chain: polar covalent bonds make water polar, polarity lets water molecules form hydrogen bonds, and hydrogen bonds produce the property.
When asked to describe or explain a property, name the hydrogen bonds and then say what they do in that case (hold the column together, absorb energy, carry heat away as molecules evaporate).
Never say that a covalent bond inside the water molecule breaks.
- "Evaporation breaks the bonds in water molecules." Evaporation and boiling break hydrogen bonds between molecules; the O–H covalent bonds inside each molecule stay intact. The mistake is tempting because both are called "bonds". Say "hydrogen bonds between water molecules".
- "Hydrogen bonds join the hydrogen and oxygen in a water molecule." Those are polar covalent bonds. A hydrogen bond is the weaker attraction between a δ+ hydrogen and a δ− oxygen on a different molecule (or a different part of the same large molecule).
- "Oil and water separate because opposites attract (or repel)." Polar and nonpolar are not opposite charges. Oil separates because it cannot hydrogen bond with water, so the water molecules bond to each other and exclude it: like dissolves like.
- Mixing up cohesion and adhesion. Cohesion is water to water; adhesion is water to another surface. In a xylem answer you usually need both, and naming the wrong one loses the point.
- "High specific heat means water heats up quickly." It means the opposite: water needs a lot of energy for each degree, so it heats and cools slowly. The word "high" tempts students to think "high temperature".
Check your understanding
1. During a run on a dry, hot day, a person's skin stays near 34 °C while the air is at 38 °C, as long as the skin stays wet with sweat.
Which of the following best explains how sweat keeps the skin below the air temperature?
- (A) Sweat contains salts that absorb heat from the skin and store it.
- (B) As sweat evaporates, the highest-energy water molecules break their hydrogen bonds and leave, carrying heat away from the skin.
- (C) Evaporation breaks the covalent bonds inside water molecules, and breaking these bonds absorbs heat from the skin.
- (D) Water has a low heat of vaporization, so sweat evaporates quickly and cools the skin.
Total [1]
2. A student shakes vegetable oil and water together in a closed jar. Within a minute the oil forms a separate layer on top of the water.
Which of the following best explains why the oil separates from the water?
- (A) The oil molecules are polar and repel the polar water molecules, because like charges repel.
- (B) The oil molecules are nonpolar and cannot form hydrogen bonds with water, so the water molecules hydrogen bond to each other and exclude the oil.
- (C) Polar and nonpolar molecules are opposites, so the water attracts the oil only at the surface where they meet.
- (D) The oil molecules form covalent bonds with each other when shaken, making drops too large to dissolve.
Total [1]
3. Plants lose water vapor through stomata, pores that in bean plants (Phaseolus vulgaris) are found mostly on the lower surface of the leaf. Researchers placed bean plants in full sunlight with the air at 31 °C. They coated the upper surface of some leaves with petroleum jelly, which blocks water loss, and the lower surface of others, and left a third group uncoated. After 30 minutes they measured leaf temperature with an infrared thermometer (Table 1; 10 leaves per group).
Part A
Describe how hydrogen bonding gives water a high heat of vaporization.
Table 1. Temperature of Bean Leaves in Full Sunlight
Treatment
Leaf temperature (°C, mean ± 2SE)
Uncoated leaves
30.2 ± 0.5
Petroleum jelly on the upper surface
30.5 ± 0.6
Petroleum jelly on the lower surface
34.1 ± 0.7
Part B
i.
Identify the independent variable in the investigation.
ii.
Justify including the group of leaves with petroleum jelly on the upper surface.
Part C
Describe the effect of coating the lower surface of the leaf on leaf temperature.
Part D
The investigation was repeated in a chamber where the air was saturated with water vapor, with all other conditions the same.
i.
Predict how the temperature of uncoated leaves in the saturated air would compare with the temperature of uncoated leaves in Table 1.
ii.
Justify your prediction.
Total [6]
4. Water striders are insects that stand and run on the surface of ponds without breaking through. Their long legs are covered with fine water-repelling hairs. Detergents are molecules with a polar end and a nonpolar end that collect at a water surface and come between water molecules there.
Part A
Describe how hydrogen bonds produce surface tension at the surface of a pond.
Part B
Explain how surface tension allows a water strider to stand on the water.
Part C
Predict the effect on a water strider of adding detergent to the water in its tank.
Part D
Justify your prediction in Part C.
Total [4]
5. Explain how the high specific heat capacity of water helps a large fish in a lake keep a stable body temperature between day and night.
Total [1]
6. A student stood three clean glass tubes with internal diameters of 0.5 mm, 1.0 mm and 2.0 mm upright in a dish of water. Water rose up each tube.
Which of the following best predicts the result?
- (A) Water rises equally high in all three tubes, because cohesion is the same in each.
- (B) Water rises highest in the 2.0 mm tube, because it holds the most water.
- (C) Water rises highest in the 0.5 mm tube, because adhesion to the glass acts on a larger share of the water in a narrow tube.
- (D) Water does not rise in any tube, because glass is nonpolar.
Total [1]
Answers
1. Answer: (B). Evaporation removes the highest-energy molecules, and because breaking their hydrogen bonds takes a lot of energy (high heat of vaporization), each gram that evaporates carries a lot of heat away from the skin.
- (A) is wrong: salts do not store heat; the cooling comes from water evaporating.
- (C) is wrong: the covalent bonds inside water molecules do not break when water evaporates; the hydrogen bonds between molecules do.
- (D) is wrong: water's heat of vaporization is high, which is why evaporating it removes so much heat.
2. Answer: (B). Like dissolves like: nonpolar oil has no partial charges for water to attract, so water molecules hydrogen bond to each other and push the oil together.
- (A) is wrong: oil is nonpolar, and polar molecules attract each other, not repel.
- (C) is wrong: polar and nonpolar are not opposite charges, so "opposites attract" does not apply.
- (D) is wrong: shaking does not form covalent bonds between oil molecules.
3. This uses the variables and control rules from Science Practices.
Part A (1 point): Water molecules are held to each other by hydrogen bonds, so a large amount of energy is needed to break these bonds before a molecule can escape as vapor.
Part B i. (1 point): Which leaf surface was coated (the position of the petroleum jelly) / whether water loss from the stomata was blocked.
Part B ii. (1 point): It shows that coating a leaf with petroleum jelly does not by itself raise leaf temperature (by absorbing heat or insulating the leaf), so the higher temperature of leaves coated on the lower surface can be attributed to blocking water loss through the stomata.
Part C (1 point): Coating the lower surface increased leaf temperature, from about 30.2 °C to 34.1 °C (by about 3.9 °C) / leaves coated on the lower surface were warmer than uncoated leaves, and the ±2SE ranges do not overlap.
Part D i. (1 point): The temperature of uncoated leaves will be higher in the saturated air (closer to that of leaves coated on the lower surface).
Part D ii. (1 point): In saturated air, little water can evaporate from the leaves, so less heat is removed by evaporative cooling and the sunlight warms the leaves more.
4. Part A (1 point): Water molecules at the surface are pulled inward and sideways by hydrogen bonds to the molecules beside and below them (there are none above), so the surface resists being stretched or broken.
Part B (1 point): The strider's weight is spread over its long legs and is not enough to break the hydrogen bonds holding the surface together, so the surface bends without breaking and supports it (the water-repelling hairs stop water wetting the legs and pulling them through).
Part C (1 point): The water strider sinks into the water / can no longer stand on the surface.
Part D (1 point): Detergent molecules come between surface water molecules and reduce the hydrogen bonding among them, lowering surface tension so the surface breaks under the strider's weight.
5. (1 point): Water must absorb (or lose) a large amount of energy to change temperature by 1 °C because energy goes into breaking (or forming) hydrogen bonds, so the lake water and the water in the fish's body change temperature only slightly as heat is gained in the day and lost at night.
6. Answer: (C). Adhesion pulls water up along the glass wall, and in a narrow tube a larger share of the water column is in contact with the wall, so it is lifted higher; cohesion carries the rest of the column with it.
- (A) is wrong: the height depends on the tube's width, because the wall area per volume of water differs.
- (B) is wrong: the wide tube holds more water, which is heavier relative to the wall area pulling it up, so it rises least.
- (D) is wrong: glass is polar (its surface carries O–H groups), which is why water adheres to it.