AP Biology · Chemistry of Life · free lesson

Structure of Water and Hydrogen Bonding

The idea, then worked examples solved step by step, then practice questions whose numbers change every time you try them.

The idea

Water: a small molecule with a big effect

Life evolved in water, and most of the chemistry inside a cell happens in the watery cytoplasm. Roughly 60 to 70 percent of the human body is water. The properties that make water so useful all trace back to two facts: its molecules are polar, and polar molecules attract each other.

Polarity: unequal sharing

A water molecule, H2O, has two covalent bonds: oxygen shares electrons with each of two hydrogen atoms. The sharing is not equal. Oxygen is more electronegative than hydrogen, which means a shared electron spends more of its time near the oxygen nucleus than near a hydrogen nucleus. A covalent bond with unequal sharing is a polar covalent bond. The result is a slightly negative charge on the oxygen and a slightly positive charge on each hydrogen. Chemists write these partial charges as δ− and δ+.

Notice what polarity does not mean. A water molecule has no net charge: the partial charges add up to zero, so it is not an ion. And the bonds inside the molecule are covalent, not ionic, because electrons are shared, not handed over.

Hydrogen bonds: the attraction between molecules

Opposite charges attract. The slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of a neighboring water molecule. That attraction is a hydrogen bond. More generally, a hydrogen bond forms between a slightly positive hydrogen of a polar bond and a slightly negative atom, usually oxygen or nitrogen, in another molecule or in another part of the same molecule.

In the picture, each solid line is a polar covalent bond inside a molecule, and each dashed line is a hydrogen bond between two molecules. One hydrogen bond is weak, far weaker than a covalent bond. In liquid water they are constantly formed and broken as the molecules slide past one another, but there are so many of them that together they strongly shape how water behaves.

Hydrogen bonds also form between and within biological molecules, and not just between water molecules. Hydrogen bonds hold the two strands of DNA together, and they help hold a folded protein in its three-dimensional shape. A hydrogen bond can form within a single large molecule when one part of it is slightly positive and another part, folded nearby, is slightly negative.

High specific heat capacity: a thermal buffer

Specific heat is the amount of heat that one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water that amount is one calorie. Water has a high specific heat capacity, and the cause is hydrogen bonding: some of the heat added to water goes into breaking and rearranging hydrogen bonds, so less is left to speed up the molecules and raise the temperature. Water therefore takes a long time to heat and a long time to cool. Water's specific heat capacity is about five times that of sand, which is why land cools faster than the sea.

The graph shows 100 g of water and 100 g of sand, both starting at 20 °C, as the same amount of heat is added. The sand climbs about five times as fast. For a living thing this matters: because it is mostly water, an animal's temperature changes slowly, and blood carries heat from warm places in the body to cooler ones, a little like the coolant in a car. This is how high specific heat capacity helps maintain a steady (homeostatic) body temperature.

High heat of vaporization: cooling by evaporation

The heat of vaporization is the amount of energy needed to change one gram of a liquid into a gas. For water it is high: about 586 calories per gram near room temperature, and about 540 at the boiling point, 100 °C. To escape as vapor, a water molecule must break its hydrogen bonds with its neighbors, and that takes a lot of energy. Even below the boiling point, some molecules at the surface gain enough energy to escape; this is evaporation.

The energy that the escaping molecules carry away comes from the surroundings, so evaporation cools the surface it leaves. This is evaporative cooling. When sweat, which is mostly water, evaporates from your skin, it removes heat from your body, and that is how body temperature is maintained on a hot day. The same idea applies to the surface of a leaf or of a pond.

Cohesion, adhesion and surface tension

Hydrogen bonds between adjacent water molecules give three related properties.

Cohesion is the attraction of water molecules to each other. It is why water forms droplets and why a glass can be filled slightly above its rim.
Adhesion is the attraction between water molecules and other molecules, such as the charged wall of a thin glass tube. When adhesion to the wall is stronger than cohesion, water climbs the wall: capillary action.
Surface tension is the ability of the surface of a liquid to resist being ruptured under stress. It comes from cohesion: the molecules at the surface are pulled toward each other and the water below. A water strider can stand on a pond because of surface tension.

In plants, cohesion and adhesion work together. As water evaporates from a leaf, molecules still joined by hydrogen bonds to the molecules below are pulled along, and adhesion to the walls of the tubes keeps the column from breaking away. This pull helps move water and dissolved minerals from the roots to the leaves.

Warning.

Three traps. First, a hydrogen bond is not a bond inside a water molecule; the bonds inside are polar covalent bonds, and hydrogen bonds are the attractions between molecules. Second, hydrogen bonds are weak one at a time; water's special properties come from huge numbers of them acting together. Third, high specific heat capacity and high heat of vaporization are different properties: the first is about changing temperature, the second about changing from liquid to gas.

Adapted from Biology by OpenStax (Rice University), licensed under CC BY 4.0. Access for free at https://openstax.org/details/books/biology

Worked examples

Each example is the kind of question the AP exam asks on this topic, worked step by step.

Example.

1. Sea and sand. On a sunny day, 100 g of seawater and 100 g of dry sand, both at 20 °C, each absorb 1000 calories of heat. The specific heat of water is 1 cal per gram per °C, and sand's is about one-fifth of that. Estimate the final temperature of each, and explain the result.

The heat absorbed equals mass times specific heat times the change in temperature, so the temperature change is the heat divided by the mass times the specific heat.

\begin{aligned} dw &= \frac{q}{mcw} \\ &= \frac{1000}{100 \times 1} \\ &= 10 \\ ds &= \frac{q}{mcs} \\ &= \frac{1000}{100 \times \frac{1}{5}} \\ &= 50 \\ tw &= t0 + dw \\ &= 20 + 10 \\ &= 30 \\ ts &= t0 + ds \\ &= 20 + 50 \\ &= 70 \end{aligned}

Water warms by $10$ °C, to $30$ °C. Sand warms by $50$ °C, to $70$ °C. The same heat raises the sand's temperature five times as much, because water's high specific heat capacity means it must absorb or lose a lot of heat to change its temperature. The reason is hydrogen bonding: part of the added heat goes into rearranging hydrogen bonds rather than speeding up the molecules.

Result.

Water $30$ °C, sand $70$ °C. This is why the sea warms and cools more slowly than the land beside it, and why an animal that is mostly water changes temperature slowly.

Example.

2. Heat lost by sweating. During a race a runner evaporates 120 g of sweat, which we treat as water. The heat of vaporization of water at skin temperature is about 586 cal per gram. How much heat leaves the runner's body, and why does evaporation, and not just warming, remove so much?

\begin{aligned} q &= mhv \\ &= 120 \times 586 \\ &= 70320 \end{aligned}

The heat removed is $120$ g times $586$ cal per gram, which is $70320$ cal.

The amount is large because a water molecule enters the gas phase after its hydrogen bonds to neighboring molecules are broken, and breaking them takes energy. That energy is taken from the skin, so the skin cools. This is evaporative cooling, and it is how body temperature is maintained when the air is hot.

Result.

About $70320$ cal of heat leaves the body as the sweat evaporates.

Example.

3. Which bond is which? In a diagram of two water molecules, a dashed line joins a hydrogen atom of one molecule to the oxygen atom of the other, and a solid line joins each hydrogen to its own oxygen. Name each kind of bond and say which is stronger.

The solid line inside a molecule is a polar covalent bond: oxygen is more electronegative than hydrogen, so the shared electrons lie closer to oxygen, giving it a slight negative charge and each hydrogen a slight positive charge.
The dashed line between molecules is a hydrogen bond: the slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of the other.
The covalent bond is the stronger one. A hydrogen bond is weak, but water has enormous numbers of them, and that is what gives water cohesion, surface tension and a high heat capacity.

Result.

Solid line: polar covalent bond (stronger). Dashed line: hydrogen bond (weaker, between molecules).

Example.

4. Water up a tall tree. A student puts one end of a thin glass tube in water and sees the water climb a little way up the inside. In a tall tree, water rises from the roots to the leaves. Which properties of water explain both?

In the glass tube, water molecules are attracted to the charged wall. Attraction between water and another substance is adhesion, and when it is stronger than the attraction among the water molecules, the water climbs the wall (capillary action).
In the tree, water evaporates from the leaves, and each molecule that leaves pulls on the one below it, because the molecules are held together by hydrogen bonds. Attraction of water molecules to each other is cohesion.
Adhesion to the walls of the tubes in the plant and cohesion among the water molecules together carry water and dissolved minerals from the roots to the leaves.

Result.

Adhesion (water to the wall) and cohesion (water to water), both resulting from hydrogen bonding between polar water molecules.

Practice

Answer AP-style questions on structure of water and hydrogen bonding. Each answer is checked, with the full solution, and every try brings new numbers. Practice needs a free account: it keeps your progress.

Practise structure of water and hydrogen bonding with new numbers