Lesson 1.1 · Unit 1 · CED topic 1.1
Structure of water and hydrogen bonding
Life began in water, and most cells are still 70 to 95 percent water by mass. Water is not just filler. A molecule of only three atoms explains why a tree can lift water 100 meters without a pump, why sweating cools you, why a pond does not freeze solid, and why a water strider can stand on the surface. Every one of those facts traces back to the same two ideas: water molecules are polar, and polar molecules stick to each other with hydrogen bonds.
Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function. Notice the verb: explain asks for the how or why, not just a list of properties. You will practice that skill all through this lesson.
A polar molecule
A water molecule is one oxygen atom covalently bonded to two hydrogen atoms at an angle of about 104.5°. Oxygen attracts the shared electrons more strongly than hydrogen does, so the electrons spend more time near the oxygen. That makes each O–H bond a polar covalent bond: a partial negative charge (δ−) on the oxygen and a partial positive charge (δ+) on each hydrogen. Because the molecule is bent, the charges do not cancel. One side of the molecule is slightly negative and the other slightly positive, so the whole molecule is polar. (Carbon dioxide, by contrast, is a straight line with two polar bonds pointing opposite ways. The charges cancel and CO2 is nonpolar.)
A hydrogen bond is the attraction between the δ+ hydrogen of one polar molecule and a δ− atom (in water, the oxygen) of another. A water molecule can donate two hydrogen bonds and accept two, so it can be linked to as many as four neighbors. Hydrogen bonds also form between and within biological molecules: they hold the two strands of DNA together and help fold proteins, as you will see later in this unit.
| Covalent bond | Hydrogen bond | |
|---|---|---|
| What it is | Two atoms share electrons | Attraction between a δ+ hydrogen and a δ− oxygen or nitrogen |
| Where | Inside a molecule (the O–H bonds of water) | Between water molecules, or between and within biological molecules |
| Strength | Strong: about 460 kJ per mole for O–H | Weak: about 20 kJ per mole in water, roughly one-twentieth as strong |
| Lifetime | Stays intact under cell conditions | Constantly breaks and re-forms, each lasting only picoseconds |
One hydrogen bond is flimsy. But liquid water has an enormous number of them, and each molecule keeps swapping partners, so the liquid holds together while still flowing. Every property below is the combined effect of those many weak bonds.
Cohesion, adhesion, and surface tension
- Cohesion: attraction between water molecules, from hydrogen bonds between them.
- Adhesion: attraction between water and a different polar or charged surface, such as glass or the wall of a plant cell.
- Surface tension: the resistance of the water surface to being stretched or broken. A molecule at the surface has no water above it, so it is pulled sideways and inward and the surface acts like a stretched skin.
Surface tension is large in water, about 72 millinewtons per meter at 25 °C, higher than for most common liquids. It lets insects walk on ponds and lets a drop of water stand up as a dome instead of spreading flat. Anything that interferes with the hydrogen bonds at the surface lowers it. Soap and detergent molecules do exactly that.
Cohesion and adhesion work as a team in plants. Water evaporates from leaf cells (transpiration), which pulls on the water column below. Cohesion keeps the chain of water molecules unbroken, and adhesion to the walls of the xylem keeps the column from slipping back down. The tallest redwoods lift water more than 100 meters this way, with no pump, powered by the evaporation of water from the leaves.
A class counts how many drops of each liquid a penny can hold before the liquid spills over the edge. They use the same dropper, the same penny, and the same temperature for every trial. Distilled water is the control. Five trials each:
| Liquid | Trial 1 | 2 | 3 | 4 | 5 | Mean | 2 × SEM |
|---|---|---|---|---|---|---|---|
| Distilled water | 24 | 27 | 23 | 26 | 25 | 25.0 | ±1.4 |
| Salt solution | 26 | 30 | 25 | 27 | 27 | 27.0 | ±1.7 |
| Soapy water | 9 | 12 | 10 | 8 | 11 | 10.0 | ±1.4 |
Step 1, name the variables. The independent variable is the type of liquid. The dependent variable is the number of drops the penny holds. Distilled water is the control, the baseline the other liquids are compared with.
Step 2, describe the result. Soapy water held a mean of 10 drops compared with 25 for water, which is 60 percent fewer.
Step 3, explain it. Detergent molecules push in between water molecules at the surface and disrupt the hydrogen bonds that hold the surface together. With weaker cohesion, the surface tension drops, the drop can no longer stand up as a dome, and the penny holds fewer drops.
Step 4, evaluate the salt result. The means differ (27 against 25), but the intervals of mean ± 2 × SEM overlap: 23.6 to 26.4 for water and 25.3 to 28.7 for the salt solution. These data do not show a real difference. A sentence that claims salt “increases surface tension” would not be supported without more trials. The soap result is different: 8.6 to 11.4 does not come near 23.6 to 26.4.
Heat: specific heat capacity and heat of vaporization
Heat is energy transferred between objects, measured in joules. Temperature measures the average kinetic energy of the molecules. They are different quantities, and water is unusual in how much heat it can absorb for a small change in temperature.
The specific heat capacity \(c\) of a substance is the heat needed to raise 1 g of it by 1 °C. Water's value is \(4.18\) J/g·°C, among the highest of common substances. \[ q = mc\Delta T \] \(q\) is heat in joules, \(m\) is mass in grams, and \(\Delta T\) is the change in temperature in °C. Water's high value exists because heat added to water first goes into breaking hydrogen bonds, not just into making molecules move faster, so the temperature rises slowly. As water cools, hydrogen bonds re-form and release that energy back, so the temperature falls slowly. The result is thermal stability: cells, bodies, ponds, and oceans resist rapid temperature change.
100.0 g of water and 100.0 g of ethanol (\(c = 2.44\) J/g·°C) each absorb 2090 J. Find \(\Delta T\) for each and explain the difference.
\[ \Delta T = \frac{q}{mc} = \frac{2090}{(100.0)(4.18)} = 5.00\ ^\circ\mathrm{C}\quad\text{(water)} \] \[ \Delta T = \frac{2090}{(100.0)(2.44)} = 8.57\ ^\circ\mathrm{C}\quad\text{(ethanol)} \] Ethanol warms about 1.7 times as much. To explain rather than only describe, add the reason: each water molecule can form up to four hydrogen bonds while an ethanol molecule can form fewer, so more of the 2090 J is used to break hydrogen bonds in water and less is left to speed up its molecules.
The heat of vaporization is the heat needed to change 1 g of a liquid into a gas at constant temperature. For water it is about 2260 J/g at 100 °C and about 2400 J/g at the temperature of skin, much higher than most liquids, because a molecule can only escape into the air after nearly all of its hydrogen bonds are broken. Evaporative cooling follows: the fastest, highest-energy molecules are the ones that escape, and they carry their energy with them, so the liquid left behind is cooler. Sweating, panting, and transpiration from leaves all cool an organism this way.
A runner evaporates 250 g of sweat. Using 2400 J/g, how much heat leaves the body, and by how many degrees would that heat have raised the temperature of a 70 kg body (specific heat about 3.5 J/g·°C)?
\[ q = (250\ \mathrm{g})(2400\ \mathrm{J/g}) = 6.00 \times 10^{5}\ \mathrm{J} = 600\ \mathrm{kJ} \] \[ \Delta T = \frac{q}{mc} = \frac{6.00 \times 10^{5}}{(70\,000)(3.5)} = 2.4\ ^\circ\mathrm{C} \] Evaporating a quarter liter of sweat removes enough heat to offset a 2.4 °C rise. This is one way the body keeps its temperature steady (homeostasis). On a humid day the air already holds a lot of water vapor, so sweat evaporates more slowly, less heat leaves, and the same exercise feels hotter.
Two more properties you will meet again
The CED does not list these under topic 1.1, but the rest of the course relies on them. Ice floats. In ice, each molecule is held by four stable hydrogen bonds in an open lattice, which keeps the molecules slightly farther apart than in the liquid, so ice is about 8 percent less dense (0.92 g/cm3 against 1.00). A lake freezes from the top down, and the ice layer insulates the water beneath it. Water is an excellent solvent for ions and polar molecules. Their charges attract water molecules, which surround them and pull them apart. Substances that mix with water are hydrophilic; nonpolar substances such as oils do not mix and are hydrophobic. That single contrast is why cell membranes form (Unit 2) and why proteins fold the way they do (lesson 1.7).
- A three-part explanation earns the point. (1) Name the property. (2) Give the molecular cause: polarity leads to hydrogen bonds. (3) Give the biological consequence. “Water has a high specific heat, which keeps body temperature stable” names the property and the consequence but skips the cause.
- Get the cause in the right direction. Water forms hydrogen bonds because it is polar. It is not polar because it has hydrogen bonds.
- Do not mix up the two bonds. The bond between O and H inside a molecule is covalent. The hydrogen bond is between molecules.
- Cohesion is water to water; adhesion is water to something else.
- Heat is not temperature. Say “absorbs a lot of heat with a small temperature change.”
- Explain why a water strider can stand on the surface of a pond without sinking. Use the term hydrogen bond in your answer.
Show answer
Water molecules at the surface form hydrogen bonds with the molecules beside and below them but have no water molecules above. The pull is sideways and inward, so the surface resists stretching, an effect called surface tension (a result of cohesion). The strider's legs press the surface down, and the stretched film pushes back strongly enough to support the insect. Naming the property alone is a description; naming the hydrogen bonds is the explanation.
- Describe the roles of cohesion and adhesion in moving water up the xylem of a tall tree.
Show answer
Evaporation of water from the leaves pulls on the water column in the xylem. Cohesion, the hydrogen bonding between water molecules, keeps the column as one unbroken chain, so water at the bottom is pulled up when water at the top leaves. Adhesion, the attraction between water and the polar walls of the xylem, helps hold the column up and keeps it from slipping back. The energy that lifts the water comes from evaporation, not from the plant pumping.
- A 400 g sample of pond water (\(c = 4.18\) J/g·°C) and a 400 g sample of dry sand (\(c = 0.80\) J/g·°C) each absorb 5000 J. Calculate the temperature change of each and state which has the higher specific heat.
Show answer
Water: \(\Delta T = 5000 / (400 \times 4.18) = 2.99\) °C. Sand: \(\Delta T = 5000 / (400 \times 0.80) = 15.6\) °C. Water has the higher specific heat: it warms by only about one-fifth as much as sand for the same heat input, because much of the energy goes into breaking hydrogen bonds.
- Predict how the yearly temperature range (hottest month minus coldest month) of a coastal city compares with a city at the same latitude far inland, and justify your prediction.
Show answer
Prediction: the coastal city has the smaller range. Justification: the nearby ocean is water with a high specific heat capacity, so it absorbs a large amount of heat in summer with a small rise in temperature and releases heat slowly in winter. Winds from the water moderate the coast, while the inland land surface warms and cools much faster.
- A student heats 100 g of water at a constant rate and records its temperature. Describe the pattern and explain the data between minutes 8 and 16.
Time (min) 0 2 4 6 8 10 12 14 16 Temperature (°C) 20 40 60 80 100 100 100 100 100 Show answer
Description: the temperature rises steadily by 10 °C per minute (20 °C each 2 minutes) from 20 °C to 100 °C at minute 8, then stays at 100 °C from minute 8 to minute 16. Explanation: at 100 °C the added energy is no longer raising the molecules' kinetic energy. It is being used to break the hydrogen bonds between water molecules so they can escape as vapor. This large energy cost is water's high heat of vaporization, so the temperature stays constant until the liquid has boiled away.
- A student coats the inside of a glass capillary tube with a nonpolar wax and stands it in water beside an untreated tube. Predict how far the water rises in the waxed tube compared with the untreated tube, and justify your prediction.
Show answer
Prediction: the water rises much less in the waxed tube, and may not rise at all. Justification: capillary rise depends on adhesion, the attraction between the δ+ hydrogens of water and the δ− oxygens of the polar glass. Wax is nonpolar and has no partial charges, so water cannot adhere to the wall. Cohesion alone holds the water together but gives it nothing to climb. A prediction without a named mechanism is only a guess; the justification point requires the link to hydrogen bonding.