A1.1 Water

Almost every reaction that keeps you alive happens in water. It makes up roughly two-thirds of the mass of most cells and is the medium in which metabolism, transport and signalling take place — which is exactly why the search for life beyond Earth is really a search for liquid water. For the IB exam, the key idea is that nearly every useful property of water can be traced back to a single cause: it is a polar molecule that forms hydrogen bonds. Get that chain of reasoning right and most A1.1 questions answer themselves.

Polarity and hydrogen bonding: the root cause

A water molecule (H₂O) is bent, not linear. Oxygen is far more electronegative than hydrogen, so it pulls the shared electrons of each O–H covalent bond towards itself. This leaves the oxygen with a slight negative charge (δ−) and each hydrogen with a slight positive charge (δ+). The molecule is therefore polar — a dipole — even though it is electrically neutral overall.

Because opposite charges attract, the δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbouring molecule. This weak attraction is a hydrogen bond. A single hydrogen bond is only about a twentieth as strong as a covalent bond and they constantly form and break, but at any instant a huge number exist together. That collective strength is what gives water its emergent properties.

Common exam trap: hydrogen bonds form between water molecules (intermolecular). The bonds within a molecule, joining O to H, are covalent. Mixing these up is one of the most frequent Paper 1 errors.

Cohesion and adhesion

Cohesion is the attraction between like molecules. Hydrogen bonding makes water molecules stick to one another, producing two biologically important effects. First, surface tension: molecules at the surface are pulled inwards and sideways, creating a "skin" strong enough for small organisms such as pond skaters to walk on. Second, water can be pulled through narrow tubes as an unbroken column under tension — this is what allows the transpiration stream to draw water up the xylem of tall plants without the column snapping.

Adhesion is the attraction between water and other polar or charged surfaces, such as the cellulose walls of xylem vessels. Adhesion helps water creep up narrow spaces (capillary action) and works alongside cohesion to keep the transpiration stream intact.

Water as a solvent and transport medium

Because it is polar and can hydrogen bond, water is an excellent solvent for hydrophilic ("water-loving") substances — ions and polar molecules. Water molecules surround each ion or polar solute, forming a hydration shell that keeps it in solution. Sodium chloride, glucose and amino acids all dissolve this way. Hydrophobic ("water-fearing") substances such as fats and oils are non-polar, cannot form hydrogen bonds with water, and so do not dissolve.

This single distinction explains how substances are carried in blood plasma — a favourite A1.1 question:

• Glucose, amino acids and sodium chloride are hydrophilic, so they dissolve directly in the watery plasma.
• Oxygen is only slightly soluble in water, so the great majority is carried bound to haemoglobin inside red blood cells rather than dissolved in plasma.
• Fats (triglycerides) and cholesterol are hydrophobic, so they are packaged with proteins as lipoproteins (for example HDL and LDL) that can travel through the aqueous plasma.

Thermal properties — and why we compare water with methane

Breaking hydrogen bonds takes energy, which gives water unusual thermal behaviour:

• High specific heat capacity: a lot of energy is needed to raise water's temperature, so large bodies of water and the cytoplasm of cells resist sudden temperature swings — a stable environment for life.
• High latent heat of vaporisation: a lot of energy is absorbed when water evaporates, which is why evaporation is such an effective coolant — sweating in mammals and transpiration in plants both remove heat.
• High boiling and melting points: water stays liquid across the broad range of temperatures where most life operates.

The syllabus asks you to compare water with methane (CH₄). Methane has almost the same molecular mass as water (16 vs 18) but is non-polar, so it cannot hydrogen bond. The result: methane's boiling point is about −161 °C, compared with water's 100 °C, and its specific heat capacity and latent heat are far lower. Because the two molecules are a similar size, the comparison isolates one variable — it shows that water's life-supporting properties come specifically from hydrogen bonding, not just from molecular size.