Water and Life: Chemical Properties and pH

Polar Molecule

A molecule with an uneven distribution of charge, such as water ($\text{H}_2\text{O}$), where oxygen carries a partial negative charge ($\delta-$) and hydrogen carries a partial positive charge ($\delta+$).

Hydrogen Bond

A weak chemical attraction formed when the partial positive charge ($\delta+$) of a hydrogen atom in one water molecule is attracted to the partial negative charge ($\delta-$) of an oxygen atom in another water molecule.

Cohesion

The binding together of like molecules, such as hydrogen bonds holding water molecules together, which contributes to high surface tension.

Adhesion

The attraction between different substances, such as water molecules forming hydrogen bonds with plant cell walls to help transport water against gravity.

Surface Tension

A measure of how difficult it is to stretch or break the surface of a liquid, which is exceptionally high in water due to hydrogen bonding.

Specific Heat

The amount of heat that must be absorbed or lost for $1\,\text{g}$ of a substance to change its temperature by $1\,^\circ\text{C}$. Water has a high specific heat due to its hydrogen bonds.

Evaporative Cooling

The process in which the remaining surface of a liquid cools as it evaporates, because the molecules with the highest kinetic energy leave as a gas.

Hydration Shell

The sphere of water molecules that surrounds each individual ion when an ionic compound is dissolved in an aqueous solution.

Acid

A substance that increases the concentration of hydrogen ions ($\text{H}^+$) in a solution, resulting in a $\text{pH}$ value less than $7$.

Base

A substance that reduces the concentration of hydrogen ions ($\text{H}^+$) in a solution, resulting in a $\text{pH}$ value greater than $7$.

Buffer

A substance that minimizes changes in $\text{pH}$ by accepting hydrogen ions ($\text{H}^+$) when they are in excess and donating them when they are depleted.