Chapter 3: The Chemistry of Water - Comprehensive Study Notes

Water is the primary substance that makes life possible on Earth, serving as the biological medium for all known organisms.
It is unique as the only common substance in the natural environment that exists in all three physical states of matter: solid, liquid, and gas.
The suitability of Earth for life is directly linked to the emergent properties of water, which arise from its specific molecular structure.
The structure of the water molecule is fundamental to its ability to interact with other molecules, facilitating the complex chemistry of living systems.
Climate change poses threats to environments dependent on water in its different states; for example, the black guillemots are an avian species threatened by the shifting climate.

The structural basis of water's properties lies in its polar covalent bonds. Within a water molecule, the electrons shared in these bonds spend significantly more time near the oxygen atom than the hydrogen atoms due to oxygen's higher electronegativity.
Water is defined as a polar molecule because its overall charge is unevenly distributed across the structure.
The oxygen atom carries a partial negative charge, represented as $\delta-$ , while the hydrogen atoms carry partial positive charges, represented as $\delta+$ .
This polarity enables water molecules to form hydrogen bonds with one another. A hydrogen bond is an attraction between the $\delta+$ region of one water molecule and the $\delta-$ region of another.

Water exhibits four major emergent properties that contribute to Earth's habitability: cohesive behavior, the ability to moderate temperature, expansion upon freezing, and versatility as a solvent.
Cohesion refers to the phenomenon where hydrogen bonds collectively hold water molecules together. This is crucial for plants, as it helps transport water against the pull of gravity.
Adhesion is the attraction between different substances. In plants, adhesion occurs between water molecules and the cell walls of water-conducting cells, further assisting in the upward movement of water.
The transport process in plants involves evaporation pulling water upward from the leaves, while cohesion and adhesion maintain a continuous column of water within the water-conducting cells (which are approximately $300\,\mu m$ in size).
Surface tension is a specific measure of how difficult it is to stretch or break the surface of a liquid. Water possesses an unusually high surface tension caused by the hydrogen bonding between molecules at the air-water interface and the molecules submerged below.

Water moderates air temperature by absorbing heat from air that is warmer and releasing that stored heat into air that is cooler.
A key feature of water is its ability to absorb or release large quantities of heat with only a minimal change in its own temperature.
Kinetic energy is defined as the energy of motion. The random motion of atoms or molecules constitutes thermal energy.
Temperature is a measure that represents the average kinetic energy of the molecules in a body of matter.
Heat is defined as the thermal energy being transferred from one body of matter to another.
Units of energy in biological systems include:
- Calorie (cal): The amount of heat needed to raise the temperature of $1\,g$ of water by $1^{\circ}C$ . This is also the amount released when $1\,g$ of water cools by $1^{\circ}C$ .
- Kilocalorie (kcal): 1,000 calories. This represents the "Calories" found on food packaging.
- Joule (J): Another unit of energy, where $1\,J = 0.239\,cal$ and $1\,cal = 4.184\,J$ .

Specific heat is the amount of heat required for $1\,g$ of a substance to change its temperature by $1^{\circ}C$ . The specific heat of water is defined as $1\,cal/(g \cdot ^\circ C)$ .
Water's high specific heat means it resists temperature changes. This property is directly traced to hydrogen bonding:
- Heat must be absorbed to break hydrogen bonds.
- Heat is released when hydrogen bonds are formed.
This high specific heat minimizes temperature fluctuations in the environment to within limits that permit the survival of life.
Regional temperature moderation is evident in coastal areas. For instance, in Southern California, the Pacific Ocean (at $68^{\circ}F$ ) keeps coastal cities like Santa Barbara ( $73^{\circ}F$ ) and San Diego ( $72^{\circ}F$ ) cooler than inland areas like San Bernardino ( $100^{\circ}F$ ) or Palm Springs ( $106^{\circ}F$ ).
Evaporation (or vaporization) is the transition of a substance from a liquid to a gas.
Heat of vaporization is the specific amount of heat a liquid must absorb for $1\,g$ to be converted into gas.
Evaporative cooling occurs as a liquid evaporates; the surface of the remaining liquid stays cool because the "hottest" molecules (those with the most kinetic energy) are the ones that leave as gas. This process stabilizes temperatures in organisms and bodies of water.

Unlike most substances, water expands when it freezes. Ice floats on liquid water because it is less dense.
In liquid water, hydrogen bonds are transient, constantly breaking and re-forming. In ice, hydrogen bonds become more "ordered" and stable, creating a crystalline lattice that keeps the molecules further apart than in the liquid state.
Water reaches its maximum density at $4^{\circ}C$ .
The floating of ice is essential for life; if ice sank, all bodies of water would eventually freeze solid from the bottom up, making life impossible.
Current climate trends show a rapid disappearance of glaciers and Arctic sea ice. Between September 1979 and September 2014, the extent of Arctic sea ice has drastically diminished.
Species affected by ice loss include:
- Harmed: Pacific walrus, Polar bears, and Black guillemots.
- Benefiting (due to open water): Phytoplankton, Bowhead whales, and Capelin.

A solution is a liquid consisting of a completely homogeneous mixture of substances.
The solvent is the agent that does the dissolving, while the solute is the substance being dissolved. An aqueous solution is any solution where water acts as the solvent.
Water is a versatile solvent because of its polarity. When an ionic compound (like sodium chloride, $NaCl$ ) is placed in water, each individual ion is surrounded by a sphere of water molecules called a hydration shell.
Water can dissolve nonionic polar molecules as well as large molecules like proteins, provided they possess ionic and polar regions on their surface.
Substances are categorized by their relationship with water:
- Hydrophilic: Substances that have an affinity for water.
- Hydrophobic: Substances that do not have an affinity for water. Nonpolar molecules, such as oils, are hydrophobic. Hydrophobic molecules related to oils are critical components of cell membranes.