properties of water

Overview: The Molecule That Supports All of Life
- Water is the biological medium here on Earth.
- All living organisms require water more than any other substance.

Three-quarters of the Earth’s surface is submerged in water.
The abundance of water is the main reason the Earth is habitable.
- Refer to Figure 3.1 for a visual representation of Earth's water distribution.

Water molecule composition:
- It consists of one oxygen atom and two hydrogen atoms.
- The orbitals of the water molecule are arranged in a tetrahedral shape.
- Two covalent bonds exist between the oxygen and the hydrogen atoms.
- The water molecule adopts a V-shape, with an angle of approximately 105 degrees between the hydrogen atoms.

Water molecules exhibit polarity due to a partial charge on both atoms.
Bonding electrons are shared unequally, resulting in higher electron density around the oxygen atom.
Water can ionize (form charged ions), facilitating hydrogen bonding between water molecules.
- This hydrogen bonding is a weak electromagnetic force occurring between the hydrogen and oxygen atoms.
- Hydrogen bonding contributes to many of water's important properties.

Definition: Cohesion is the bonding of a high percentage of water molecules to neighboring water molecules.
- It arises due to hydrogen bonding between like molecules (water attracting other water molecules).
Cohesion helps pull water upward through the microscopic vessels of plants, enabling capillarity.
- Refer to Figure 3.3 for a visual depiction of water conducting cells (100 µm).

Definition: Adhesion is the bonding between unlike molecules.
The adhesion of water (H2O) to vessel walls helps counteract the pull of gravity.

Definition: Surface tension is a measure of how hard it is to break the surface of a liquid.
Water molecules bond to molecules below them, causing a downward pull which draws water molecules closer together, forming a strong layer on the surface.
- Refer to Figure 3.4 for a visual representation.

Water plays a crucial role in moderating air temperature by absorbing heat from warmer air and releasing it to cooler air.

Definitions:
- Kinetic Energy: The energy of motion.
- Heat: A measure of the total amount of kinetic energy resulting from molecular motion.
- Temperature: A measure of the intensity of heat.

Definition: Specific heat of a substance is the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1ºC.
Water has a high specific heat which allows it to minimize temperature fluctuations within life-sustaining limits.
As water heats up, the kinetic energy increases and a significant amount of energy is used to break hydrogen bonds, making less energy available to increase temperature and thus creates a more stable environment.

Definition: Evaporation is the transformation of a substance from a liquid to a gas and requires energy.
Heat of vaporization: The quantity of heat a liquid must absorb for 1 gram of it to be converted from a liquid to a gas.
- For example, water at 100ºC (steam) has more heat than boiling water at the same temperature (540 calories).
Evaporative cooling occurs because a significant amount of energy is required to break hydrogen bonds.
- This allows water to cool surfaces; the energy absorbed by water molecules to vaporize results in a cooling effect on surrounding areas.
- Sweating cools the body by utilizing the heat energy from the body to convert sweat into gas through the process of evaporation.

Solid water (ice) is less dense than liquid water and thus floats in it.
Ice insulates the water and organisms below the ice layer.
The kinetic energy of water molecules is low, allowing them to form the maximum number of hydrogen bonds (4), resulting in a crystal lattice structure.

The hydrogen bonds in ice are more “ordered” than in liquid water, which contributes to its lower density (crystal lattice).
- Liquid water: Hydrogen bonds are constantly breaking and re-forming.
- Ice: Hydrogen bonds are stable and form a structured lattice.
- Refer to Figure 3.5 for visual representation.

Since ice floats in water, life can exist beneath the frozen surfaces of lakes and polar seas.

Water is recognized as a versatile solvent due to its polarity and ability to form aqueous solutions.
Water is often referred to as the universal solvent because many substances dissolve in it.

The different regions of the polar water molecule interact with ionic compounds (solutes) and facilitate their dissolution.
- For instance, the negative oxygen regions of polar water molecules are attracted to sodium cations (Na+), while the positive hydrogen regions of water molecules cling to chloride anions (Cl-).
- Refer to Figure 3.6 for a visual representation of this interaction.

Upon contact with water, ions and polar groups become surrounded by water molecules, leading to their separation or dissociation from each other.
Once dissociated, the molecules or ions can move about freely, enhancing their chemical reactivity.