Water & hydrogen bonding

A lopsided molecule

A water molecule is one oxygen atom bonded to two hydrogen atoms: H₂O. The atoms are held together by covalent bonds, which means they share electrons. But they don't share them equally.

Oxygen is much more electronegative than hydrogen: it pulls shared electrons toward itself more strongly. So the electrons spend more of their time near the oxygen. That gives the oxygen end a slight negative charge (written δ⁻) and each hydrogen a slight positive charge (δ⁺).

Water is also bent, like a wide letter V, rather than straight. Because of that shape, the charges don't cancel out: one side of the molecule is slightly negative and the other slightly positive. A molecule with an uneven spread of charge like this is called polar.

Hydrogen bonds: how water molecules stick together

Opposite charges attract. The slightly positive hydrogen of one water molecule is drawn toward the slightly negative oxygen of a neighbouring molecule. This attraction is called a hydrogen bond.

A single hydrogen bond is weak and short-lived: in liquid water, hydrogen bonds break and re-form constantly. But each water molecule can hydrogen-bond to as many as four neighbours, and even a single drop holds an astronomical number of molecules. Added together, all those small attractions are strong enough to give water its unusual properties.

What hydrogen bonds do

Cohesion: water sticks to water. At the surface of a pond, water molecules are pulled sideways and inward by their neighbours, forming a tight film. This surface tension is what lets a water strider stand on water, and why water beads up into drops.

Adhesion: water sticks to other things. Water also hydrogen-bonds to other polar surfaces, like glass or the walls of plant cells. Adhesion and cohesion together let water creep up a narrow tube (capillary action). In plants, water evaporating from the leaves pulls on an unbroken column of water that reaches all the way down to the roots. The column holds together because each molecule is hydrogen-bonded to the next.

High specific heat: water resists changing temperature. It takes 1 calorie to warm 1 gram of water by 1 °C, about five times as much as for sand. That's because when you heat water, much of the energy goes into breaking hydrogen bonds before the molecules can move faster. So oceans and lakes warm and cool slowly, coastal climates stay mild, and your body (which is mostly water) resists sudden temperature swings.

High heat of vaporisation: evaporating takes a lot of energy. To escape as vapour, a water molecule has to break free of all its hydrogen bonds. Evaporating one gram of water takes about 586 calories. That's why sweating cools you down: as sweat evaporates, it carries a lot of heat away from your skin.

Ice floats. In most substances the solid is denser than the liquid. Water is the exception. As water freezes, hydrogen bonds lock the molecules into an open, orderly lattice that holds them farther apart than in the liquid. So ice is less dense and floats. A lake freezes from the top down, and the ice layer insulates the water beneath it, which lets fish survive the winter.

Water as a solvent

Because water is polar, it's very good at dissolving substances that are charged or polar. Drop table salt (NaCl) into water and it falls apart into Na⁺ and Cl⁻ ions. Water molecules crowd around each ion: their slightly negative oxygens face the Na⁺, and their slightly positive hydrogens face the Cl⁻. Each ion ends up wrapped in a hydration shell, which keeps it from rejoining the crystal.

Polar molecules dissolve in a similar way. Sugar is covered in polar –OH groups that hydrogen-bond with water. Substances that mix easily with water are called hydrophilic ("water-loving").

Oils and fats are different. They're made mostly of carbon and hydrogen, which share electrons almost equally, so they're nonpolar and have no charges for water to grab onto. These substances are hydrophobic ("water-fearing").

Why oil and water separate

It's tempting to think water pushes oil away. It doesn't: there's no repelling force. What actually happens is that water molecules prefer to hydrogen-bond with each other. Next to an oil molecule they can't, so they arrange themselves into a more rigid, ordered cage around it. Forcing water into that order is unfavourable.

When oil droplets merge, less oil surface is touching water, so fewer water molecules are stuck in cages. The water is freer to move, and the whole system becomes more stable. This is called the hydrophobic effect.

The hydrophobic effect builds much of the cell. It's why phospholipids arrange themselves into a membrane with their oily tails tucked inside, and why a protein folds with its hydrophobic amino acids buried in its core.

Key terms

Polar

Having an uneven spread of charge, with a slightly positive end and a slightly negative end.

Electronegativity

How strongly an atom pulls shared electrons toward itself.

Hydrogen bond

A weak attraction between a slightly positive hydrogen on one molecule and a slightly negative atom (such as O) on another.

Cohesion

Water molecules sticking to each other.

Adhesion

Water sticking to other polar or charged surfaces.

Specific heat

The energy needed to raise 1 g of a substance by 1 °C.

Heat of vaporisation

The energy needed to turn 1 g of liquid into gas.

Hydration shell

The layer of water molecules surrounding a dissolved ion or polar molecule.

Hydrophilic / hydrophobic

Mixes easily with water / doesn't mix with water.

Check yourself

Try answering in your head before you open each answer.

• 1.A water strider stands on a pond. Which property of water makes this possible, and what causes it?Show answerHide

  Surface tension, which comes from cohesion. Hydrogen bonds between water molecules at the surface pull them into a film strong enough to hold the insect's small weight.

• 2.Imagine ice were denser than liquid water. What would happen to a lake in a cold winter?Show answerHide

  Ice would sink as it formed, so the lake would freeze from the bottom up and could freeze solid, with no floating ice layer to insulate the water beneath. Many organisms living there would die.

• 3.Sweat only cools you if it evaporates, not if it just drips off. Why?Show answerHide

  Evaporation needs energy to break each molecule's hydrogen bonds, and that energy is taken from your skin as heat. Water's high heat of vaporisation means a lot of heat leaves with every gram. Sweat that drips off never absorbs that energy.