AP Biology

## Learning Outcomes

- Explain why the oxygen-hydrogen bonds in water are polar covalent bonds.
- Describe how electronegativity differences create partial charges in a water molecule.
- Distinguish between covalent bonds within water molecules and hydrogen bonds between water molecules.
- Link water’s polarity to hydrogen bonding, cohesion, adhesion, surface tension, high specific heat, and solvent behavior.
- Apply the structure-property-function chain of water to biological examples such as xylem transport and evaporative cooling.
- Avoid common AP Biology mistakes about ionic bonds, nonpolar covalent bonds, and hydrogen bonds in water.

## AP Biology Syllabus

In AP Biology Unit 1, Chemistry of Life, water is treated as a key molecule whose structure explains many features of living systems. Unit 1 is commonly assessed in multiple-choice and free-response questions, and water often appears in questions requiring a structure → property → biological function explanation. For this subtopic, you should be able to connect unequal electron sharing in water to polarity, hydrogen bonding, and life-related properties.

- Water consists of two hydrogen atoms covalently bonded to one oxygen atom.
- Oxygen is more electronegative than hydrogen, so the O-H bonds are polar covalent.
- Unequal electron sharing gives oxygen a partial negative charge and hydrogen atoms partial positive charges.
- Water is a polar molecule, allowing water molecules to form hydrogen bonds with one another.
- Hydrogen bonding explains cohesion, adhesion, surface tension, high specific heat, high heat of vaporization, and expansion on freezing.
- Water’s polarity allows it to dissolve many ionic and polar substances.
- AP questions often ask you to identify which bond is within a water molecule and which attraction occurs between water molecules.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why are the O-H bonds in a water molecule described as polar covalent rather than nonpolar covalent?
2. Which atom in water has a partial negative charge, and which atoms have partial positive charges?
3. What is the difference between a polar covalent bond and a hydrogen bond in water?
4. How does water’s polarity help explain its ability to dissolve salts and other charged substances?
5. Why does water’s hydrogen bonding contribute to high surface tension and evaporative cooling?

## Introduction

Water is the most abundant compound in living organisms, and many biological processes occur in watery environments. Cells contain water, blood plasma is mostly water, plant xylem transports water, and aquatic habitats depend on water’s unusual physical properties. These properties are not random. They arise from the structure of the water molecule, especially the polar covalent bonds between oxygen and hydrogen.

A water molecule is small, but its charge distribution makes it chemically powerful. Oxygen attracts shared electrons more strongly than hydrogen does, so the electrons in each O-H bond spend more time near oxygen. This unequal sharing creates a molecule with partial charges, allowing water molecules to attract one another through hydrogen bonds.

> **Key Term: covalent bond**
> A chemical bond in which atoms share one or more pairs of electrons.

### Unequal electron sharing creates polar covalent O-H bonds

A water molecule has the formula $$H_2O$$. It contains one oxygen atom bonded to two hydrogen atoms. The bonds holding hydrogen atoms to oxygen within a single water molecule are covalent bonds because electrons are shared between atoms.

However, the sharing is not equal. Oxygen has a greater ability to attract electrons than hydrogen. This property is called electronegativity. Because oxygen is more electronegative, the shared electrons in the O-H bonds spend more time near the oxygen atom than near the hydrogen atoms.

> **Key Term: electronegativity**
> The ability of an atom to attract electrons in a chemical bond.

This unequal sharing produces polar covalent bonds. In a polar covalent bond, electrons are shared, but one atom pulls the electrons closer than the other atom does. In water, oxygen pulls the shared electrons closer, so oxygen becomes partially negative, written as $$\delta^-$$. The hydrogen atoms become partially positive, written as $$\delta^+$$.

> **Key Term: polar covalent bond**
> A covalent bond in which electrons are shared unequally because the bonded atoms differ in electronegativity.

This is different from a nonpolar covalent bond. In a nonpolar covalent bond, electrons are shared more equally because the bonded atoms have similar electronegativities. For example, a bond between two identical atoms would be nonpolar because neither atom attracts the shared electrons more strongly.

It is also different from an ionic bond. In an ionic bond, electrons are transferred from one atom to another, forming ions with full positive or negative charges. Water’s O-H bonds are not ionic because the electrons are shared rather than transferred.

> **Test Tip:**
> If an AP question asks what type of bond joins hydrogen to oxygen within a water molecule, the answer is polar covalent bond, not hydrogen bond and not ionic bond.

The word “partial” matters. The oxygen atom in water does not have a full negative charge, and the hydrogen atoms do not have full positive charges. They have partial charges caused by unequal electron sharing. This partial charge distribution is enough to strongly affect how water molecules interact.

> **Key Term: partial charge**
> A slight positive or negative charge on part of a molecule caused by unequal sharing of electrons.

### From polar bonds to a polar water molecule

A molecule is polar if it has an uneven distribution of charge. Water is polar because the oxygen side is partially negative and the hydrogen side is partially positive. The molecule also has a bent shape, so the partial charges do not cancel out.

> **Key Term: polar molecule**
> A molecule with an uneven distribution of charge, giving it partially positive and partially negative regions.

The bent shape of water means the two hydrogen atoms are on one side of the molecule, with oxygen at the other end of the charge distribution. Since oxygen carries a partial negative charge and the hydrogens carry partial positive charges, water has distinct charged regions.

This polarity explains why water interacts strongly with other water molecules and with other polar or charged substances. The partially negative oxygen of one water molecule can attract the partially positive hydrogen of a neighboring water molecule. This attraction is called a hydrogen bond.

> **Key Term: hydrogen bond**
> A weak attraction between a partially positive hydrogen atom bonded to an electronegative atom and a partially negative electronegative atom nearby.

Hydrogen bonds in water form between different water molecules. They do not replace the covalent bonds inside each water molecule. Within a water molecule, oxygen and hydrogen are joined by polar covalent bonds. Between water molecules, partial charges attract, forming hydrogen bonds.

This distinction is heavily tested because the terms sound similar. A question may describe “the attraction between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another water molecule.” That is a hydrogen bond. A question may describe “the bond between oxygen and hydrogen within one water molecule.” That is a polar covalent bond.

> **Exam Warning:**
> Do not say that hydrogen bonds hold the hydrogen and oxygen atoms together within a water molecule. Hydrogen bonds occur between water molecules; polar covalent bonds occur within a water molecule.

Hydrogen bonds are individually weak compared with covalent bonds, but many hydrogen bonds together have major effects. Liquid water contains constantly forming and breaking hydrogen bonds. This network of attractions gives water many properties that support life.

### Hydrogen bonding between water molecules

The polarity of water leads directly to hydrogen bonding. The partially positive hydrogen atoms of one water molecule are attracted to the partially negative oxygen atoms of nearby water molecules. This attraction gives water a “sticky” quality.

Water molecules stick to other water molecules through cohesion.

> **Key Term: cohesion**
> Attraction between molecules of the same substance; in water, cohesion results largely from hydrogen bonding between water molecules.

Cohesion helps explain why water forms droplets and why raindrops hold together. In plants, cohesion helps maintain a continuous column of water in xylem. As water evaporates from leaves during transpiration, it pulls on nearby water molecules, which pull on others below them. This helps water move upward from roots to leaves.

Water molecules can also stick to other polar or charged surfaces through adhesion.

> **Key Term: adhesion**
> Attraction between molecules of different substances; for water, adhesion occurs when water is attracted to polar or charged surfaces.

Adhesion is seen when water sticks to glass or to plant cell walls. Together, cohesion and adhesion help produce capillary action, the movement of water through narrow tubes or spaces. In tall plants, capillary action contributes to water movement through xylem vessels.

> **Key Term: capillary action**
> The movement of water through narrow spaces due to cohesion between water molecules and adhesion to surrounding surfaces.

Hydrogen bonding also explains water’s high surface tension. At the surface of water, molecules are pulled inward by cohesive forces with neighboring water molecules. This makes the surface resist being broken. Small, lightweight insects can stand on water because the surface tension supports them.

> **Key Term: surface tension**
> The resistance of a liquid surface to being broken, caused by cohesive forces among molecules at the surface.

A common AP-style question may describe a student placing drops of water on a penny. Water can pile up above the penny’s surface because cohesion creates surface tension. If a surfactant is added, it disrupts hydrogen bonding between water molecules. Fewer hydrogen bonds reduce cohesion and surface tension, so fewer drops can remain on the penny before spilling.

> **Test Tip:**
> When explaining surface tension, name the molecular cause: hydrogen bonds between water molecules create cohesion, and cohesion causes high surface tension.

### Biological effects of water’s polarity

Water’s polar covalent bonds matter because they lead to partial charges, hydrogen bonding, and several properties essential for life. AP Biology questions often reward answers that connect each level clearly.

A useful chain is:

Oxygen is more electronegative than hydrogen → O-H bonds are polar covalent → water has partial charges and is polar → water molecules form hydrogen bonds → water has life-supporting properties.

One major property is high specific heat. Specific heat is the energy required to raise the temperature of a substance. Water has a high specific heat because energy must be used to disrupt hydrogen bonds before water molecules can move faster and increase in temperature. This means water resists rapid temperature change.

> **Key Term: specific heat**
> The amount of energy required to raise the temperature of a substance by a given amount.

High specific heat helps organisms and habitats maintain stable temperatures. Oceans and lakes absorb heat during warm periods and release heat during cooler periods, moderating nearby climates. Since organisms contain large amounts of water, water also helps cells resist sudden temperature shifts that could affect enzyme activity and homeostasis.

Water also has a high heat of vaporization. Evaporation requires energy because hydrogen bonds must be broken before water molecules can leave the liquid phase as gas. When sweat evaporates from skin, it absorbs heat from the body, producing a cooling effect. Plants can also cool leaves through water evaporation during transpiration.

> **Key Term: heat of vaporization**
> The amount of energy required to change a liquid into a gas.

A strong free-response answer on sweating should not simply state “water cools the body.” It should explain that hydrogen bonds between water molecules require energy to break, and that energy is absorbed as heat from the body during evaporation.

> **Exam Warning:**
> For high specific heat or evaporative cooling, do not focus on breaking the covalent O-H bonds within water molecules. The key energy cost is disrupting hydrogen bonds between water molecules.

Water’s polarity also makes it an effective solvent for many biological substances. A solvent is a substance that dissolves other substances. Because water has partial charges, it can interact with ions and polar molecules. For example, when an ionic salt dissolves in water, water molecules surround the separated ions. The partially negative oxygen ends of water molecules orient toward positive ions, while the partially positive hydrogen ends orient toward negative ions. These surrounding water molecules form hydration shells.

> **Key Term: solvent**
> A substance that dissolves solutes to form a solution.
>
> **Key Term: hydration shell**
> A layer of water molecules surrounding an ion or polar molecule in solution.

This solvent ability matters in cells. Cytosol is water-based and contains dissolved ions, sugars, amino acids, proteins, and other polar molecules. Blood plasma transports dissolved substances around the body. Plant sap carries minerals and sugars in aqueous solution.

However, water does not dissolve all substances equally well. Polar and charged substances are hydrophilic, meaning they interact well with water. Nonpolar substances are hydrophobic, meaning they do not mix well with water. Oils and many lipid tails are hydrophobic because they lack strong partial charges and cannot form favorable interactions with water.

> **Key Term: hydrophilic**
> Water-attracting; describes substances that are polar or charged and interact favorably with water.
>
> **Key Term: hydrophobic**
> Water-repelling; describes substances that are nonpolar and do not interact favorably with water.

This distinction helps explain biological membranes. Phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails. In water, the heads face outward toward the aqueous surroundings, while the tails turn inward away from water. This organization helps form the phospholipid bilayer of cell membranes.

Finally, hydrogen bonding affects freezing. Ice is less dense than liquid water because hydrogen bonds arrange water molecules into a more open structure when frozen. As a result, ice floats. Floating ice insulates water below, allowing aquatic organisms to survive in lakes and oceans during cold periods.

## Key Point Checklist

This article has covered the following key knowledge points:

- Water contains polar covalent bonds between oxygen and hydrogen.
- A covalent bond involves sharing electrons, not transferring electrons.
- Oxygen is more electronegative than hydrogen, so shared electrons spend more time near oxygen.
- Oxygen in water has a partial negative charge, while the hydrogen atoms have partial positive charges.
- Water is a polar molecule because its charge distribution is uneven.
- Hydrogen bonds form between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.
- Polar covalent bonds occur within a water molecule; hydrogen bonds occur between water molecules.
- Hydrogen bonding causes cohesion, which helps water molecules stick to each other.
- Adhesion allows water to stick to other polar or charged surfaces.
- Cohesion and adhesion contribute to capillary action in plant xylem.
- Water’s hydrogen bonds explain high surface tension, high specific heat, and evaporative cooling.
- Water’s polarity allows it to dissolve many ionic and polar substances but not most nonpolar substances.

## Key Terms and Concepts

- covalent bond
- electronegativity
- polar covalent bond
- partial charge
- polar molecule
- hydrogen bond
- cohesion
- adhesion
- capillary action
- surface tension
- specific heat
- heat of vaporization
- solvent
- hydration shell
- hydrophilic
- hydrophobic


Structure of water and hydrogen bonding - Polar covalent bonds in water