Unit 1: Structure of Water and Hydrogen Bonding

Matter

Anything that takes up space and has mass.

Element

A substance that cannot be broken down into another substance by chemical reactions.

Electronegativity

A measure of an atom’s ability to attract shared electrons toward itself.

Atomic number

The number of protons in the nucleus of an atom.

Atomic mass

The weighted average of the masses of an element’s naturally occurring isotopes.

Octet rule

Atoms tend to gain, lose, or share electrons to complete their valence shells and become more stable.

Chemical bond

An attraction between atoms resulting from the sharing or transfer of valence electrons.

Compound

A substance consisting of two or more different elements combined in a fixed ratio.

Covalent bond

A bond formed when two atoms share one or more pairs of valence electrons.

Nonpolar covalent bond

A covalent bond in which electrons are shared approximately equally between atoms, as in oxygen gas (O₂).

Polar covalent bond

A covalent bond in which electrons are shared unequally because one atom is more electronegative, as in water.

Ionic bond

The electrostatic attraction between oppositely charged ions formed after electrons are transferred between atoms.

Adhesion

The attraction between water and another polar or charged substance.

Cohesion

The attraction between molecules of the same substance, such as water molecules attracting other water molecules.

Capillary action

The movement of water through a narrow space due to adhesion to the surface, cohesion between water molecules, and surface tension.

Solvent

The dissolving agent in a solution.

Solution

A homogeneous mixture of two or more substances.

Solute

The substance dissolved in a solvent.

pH

A measure of how acidic or basic a solution is based on its hydrogen-ion concentration.

Acid

A substance that increases the hydrogen-ion concentration of a solution.

Base

A substance that reduces the hydrogen-ion concentration of a solution by accepting H⁺ or releasing OH⁻.

Buffer

A substance that resists changes in pH by accepting excess H⁺ or donating H⁺ when its concentration decreases.

Which elements make up nearly all living matter?

Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (CHNOPS).

What is the difference between a solute, solvent, and solution?

A solute is the substance being dissolved, a solvent is the dissolving agent, and a solution is the homogeneous mixture formed by the solute and solvent.

Describe covalent bonds and give two examples.

Covalent bonds form when atoms share electrons. Examples include the O—H bonds in water (H₂O) and the O=O bond in oxygen gas (O₂).

Describe ionic bonds and give two examples.

Ionic bonds are electrostatic attractions between oppositely charged ions. Examples include sodium chloride (NaCl) and lithium fluoride (LiF).

How are hydrogen bonds different from covalent and ionic bonds?

A hydrogen bond is a relatively weak attraction between a partially positive hydrogen atom in one polar molecule and a partially negative electronegative atom in another molecule; it does not involve the direct sharing or transfer of electrons.

What bonds are found within and between water molecules?

Polar covalent O—H bonds occur within each water molecule, while hydrogen bonds form between the δ⁺ hydrogen of one molecule and the δ⁻ oxygen of another.

How does electronegativity affect interactions between water molecules?

Oxygen is more electronegative than hydrogen and attracts their shared electrons more strongly, producing δ⁻ oxygen and δ⁺ hydrogen regions. These partial charges allow hydrogen bonds to form between water molecules.

What would happen if oxygen and hydrogen had the same electronegativity?

The O—H bonds would be nonpolar, water would lack partial charges and normal hydrogen bonding, and it would lose many properties essential for life, including strong cohesion, high specific heat, and its ability to dissolve ions and polar substances.

Water’s polarity

Unequal sharing of electrons gives oxygen a δ⁻ charge and each hydrogen a δ⁺ charge. Example: polar water molecules orient toward a charged plastic rod, causing a water stream to bend.

Water’s cohesion

Hydrogen bonds cause water molecules to attract one another. Example: cohesion maintains a continuous column of water inside plant xylem.

Water’s adhesion

Water is attracted to other polar or charged surfaces. Example: water adheres to the walls of plant xylem.

Water’s capillary action

Adhesion, cohesion, and surface tension allow water to move upward through narrow spaces. Example: water and dissolved minerals rise through plant xylem.

Water’s temperature control

Water’s high specific heat allows it to absorb or release considerable heat with only a small temperature change. Example: large bodies of water absorb heat during the day and release it at night, moderating nearby temperatures.

Water’s density as ice

Hydrogen bonds form an open crystalline structure when water freezes, causing ice to expand, become less dense than liquid water, and float. Example: floating ice insulates the liquid water below so aquatic organisms can survive winter.

Water as a solvent

Water’s polarity allows it to surround and dissolve ions and other polar substances. Example: water forms hydrogen bonds with sugar molecules and dissolves them.

Water, pH, and buffering

Water can dissociate into H⁺ and OH⁻, while buffers minimize changes in hydrogen-ion concentration. Example: the carbonic acid–bicarbonate system maintains human blood near pH 7.4.

How does capillary action benefit organisms?

Capillary action helps transport water and dissolved minerals upward through the narrow xylem vessels of plants.

How does water’s temperature control benefit organisms?

Water’s high specific heat stabilizes organisms’ internal temperatures and moderates environmental temperatures near large bodies of water.