Water as the medium for life and hydrogen bonding

Molecules (Unity and diversity) · Water · note 1 of 5

Spec A1.1.1, A1.1.2Download all Water notes (PDF)Download all notes (PDF)7 pages

Water as the medium for life and hydrogen bondingSpec A1.1.1, A1.1.2

In short

Water is the medium for life: the first cells originated in water and most processes of life still happen in aqueous solution. Oxygen attracts the shared electrons more strongly than hydrogen, so the O–H bonds are polar covalent, and hydrogen bonds form between the slightly positive hydrogen of one molecule and the slightly negative oxygen of another.

Water as the medium for life

The first cells originated in water, and water remains the medium in which most processes of life occur. Cytoplasm is mostly water, metabolic reactions take place in aqueous solution, and substances are carried around organisms dissolved in water (blood plasma, xylem sap, phloem sap).

Polar covalent bonds

A water molecule (H₂O) has two covalent bonds between oxygen and hydrogen. The electrons in each bond are shared unequally: the oxygen nucleus attracts them more strongly than the hydrogen nuclei. So the oxygen end carries a partial negative charge (δ−) and each hydrogen carries a partial positive charge (δ+). Because the molecule is bent rather than straight, the charges do not cancel, and the whole molecule is polar.

Hydrogen bonds between water molecules

A hydrogen bond is the attraction between the δ+ hydrogen of one water molecule and the δ− oxygen of a neighbouring water molecule. It is an intermolecular force, much weaker than a covalent bond, but there are vast numbers of them in liquid water. They constantly break and re-form, and together they explain cohesion, adhesion, the solvent properties and the high specific heat capacity of water.

Three bent water molecules, each oxygen labelled δ− and each hydrogen δ+, with solid polar covalent O–H bonds and dashed hydrogen bonds from the hydrogen of one molecule to the oxygen of the next. — Hydrogen bonds form between water molecules: the δ+ hydrogen of one is attracted to the δ− oxygen of the next. IB notation shows δ+ and δ− and dashed hydrogen bonds.

Exam tip:

When asked to draw hydrogen bonding, draw at least two water molecules, show δ+ on hydrogens and δ− on oxygen, and use a dashed line from H to O. In water, hydrogen bonds form between molecules, not within one.

Common mistake:

Polarity is caused by unequal sharing of electrons in the covalent bond, not by water having ions. Water molecules are uncharged overall.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why is water called the medium for life?

Water is called the medium for life because the first cells originated in water and most processes of life still occur in it. Cytoplasm is mostly water, most enzymes catalyse reactions in aqueous solution, and blood plasma, xylem and phloem carry substances dissolved in water around organisms.

How do hydrogen bonds form between water molecules?

Hydrogen bonds form because water molecules are polar. Oxygen attracts the shared electrons more strongly than hydrogen, so oxygen is slightly negative and hydrogen slightly positive. The δ+ hydrogen of one molecule is attracted to the δ− oxygen of a neighbouring molecule, and this weak intermolecular attraction is a hydrogen bond.

How does cohesion help water move up the xylem?

Cohesion holds water molecules together by hydrogen bonds, so the water in a xylem vessel behaves as a continuous column. When water evaporates from leaves, it pulls on this column, which is under tension, and the whole column moves upwards without breaking. Adhesion to the vessel walls also helps.

All 5 questions on Water