Chemistry Foundations: Water and the Elements of Life

Polar molecule

A molecule with an uneven distribution of electrical charge (partial positive and partial negative regions) but no net charge overall; water is polar.

Bent (V-shaped) geometry

The shape of a water molecule; because it is not linear, oxygen’s pull on shared electrons does not cancel out, contributing to water’s polarity.

Covalent bond

A strong chemical bond formed when atoms share electron pairs; oxygen forms two covalent bonds with hydrogen in H₂O.

Electronegativity

An atom’s ability to attract shared electrons in a covalent bond; oxygen is more electronegative than hydrogen.

Partial charges (δ− and δ+)

Slight charges created by unequal electron sharing in polar covalent bonds; in water, oxygen is δ− and hydrogens are δ+.

Hydrogen bond

A weak attraction between a δ+ hydrogen (bonded to an electronegative atom like O or N) and a nearby electronegative atom with a partial negative charge.

Cohesion

Attraction between molecules of the same substance; in water, cohesion results from hydrogen bonding between water molecules.

Adhesion

Attraction between molecules of different substances; water adheres to other polar surfaces (e.g., cellulose in plant cell walls).

Capillary action

The movement of water up narrow spaces (like tubes) due to the combined effects of cohesion and adhesion.

Surface tension

A “skin-like” effect at water’s surface caused by cohesive hydrogen bonding among surface molecules, making the surface resistant to breaking.

Specific heat

The amount of energy required to raise a substance’s temperature; water’s high specific heat helps buffer temperature changes because heat is used to disrupt hydrogen bonds first.

Heat of vaporization

The energy required to convert a liquid to a gas; water’s relatively high value is due to the energy needed to break hydrogen bonds.

Evaporative cooling

Cooling that occurs when high-energy molecules evaporate from a liquid surface, lowering the average kinetic energy (temperature) of the remaining liquid (e.g., sweating).

Density anomaly of ice

Water’s unusual property that solid ice is less dense than liquid water because hydrogen bonds form an ordered lattice that spaces molecules farther apart, causing ice to float.

Solution

A homogeneous mixture in which one substance is dissolved in another (e.g., saltwater).

Solvent

The dissolving agent in a solution; in biology, water is the most common solvent.

Solute

The substance dissolved in a solution (e.g., NaCl in saltwater).

Hydration shell

A layer of water molecules surrounding an ion in solution; water’s partial charges orient to stabilize separated cations and anions.

Hydrophobic interaction

The clustering of nonpolar molecules in water because it allows water to maintain hydrogen bonding with itself; important in membrane formation and protein folding.

Element

A pure substance made of only one kind of atom; living organisms are primarily built from a limited set of elements.

Atom

The smallest unit of an element that retains that element’s properties; composed of protons and neutrons in the nucleus with electrons around it.

Isotope

Atoms of the same element with different numbers of neutrons; isotopes can be stable or radioactive and are used as tracers in biology/medicine.

Valence electrons

Electrons in the outermost energy level of an atom; they largely determine bonding behavior and reactivity.

Ionic bond

An electrostatic attraction between oppositely charged ions formed after electron transfer; in water, many ionic compounds dissociate into ions.

CHNOPS

Mnemonic for major biologically important elements: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur—common in macromolecules like proteins, nucleic acids, carbohydrates, and lipids.