The Chemistry of Water (H₂O) Explained Simply

Water (H₂O) is a molecule of one oxygen atom bonded to two hydrogen atoms in a bent, V-shaped arrangement. Oxygen pulls the shared electrons harder than hydrogen does, so the oxygen side of the molecule carries a slight negative charge and the hydrogen side a slight positive one. That built-in polarity lets water molecules cling to each other through hydrogen bonds, and it explains almost everything unusual about water: a high boiling point, a huge appetite for dissolving salts and sugars, ice that floats, and a neutral pH of 7 at 25 °C. Here's how it all fits together, one step at a time.

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Water chemistry at a glance

These are the reference values worth knowing. The rest of this guide explains where each one comes from:

Values are rounded. Dissolved minerals and gases shift them slightly in real-world water.

Inside a single water molecule

Start with the atoms. Oxygen has six electrons in its outer shell and wants two more to fill it; each hydrogen has one and wants one more. So they share. Each hydrogen pairs its electron with one from oxygen, forming two covalent O–H bonds. That leaves oxygen with two further electron pairs that aren't shared with anyone — the lone pairs.

The four electron pairs around oxygen (two bonding, two lone) repel each other and spread out in a roughly tetrahedral arrangement. Lone pairs take up more room than bonding pairs, so they squeeze the two O–H bonds closer together. The result is a bent molecule with an H–O–H angle of about 104.5° — not the 109.5° of a perfect tetrahedron and nowhere near a straight line.

Now the charge. On the Pauling scale, oxygen's electronegativity (its pull on shared electrons) is 3.44 and hydrogen's is 2.20. The shared electrons therefore spend more time near oxygen, giving it a partial negative charge (written δ⁻) and each hydrogen a partial positive charge (δ⁺). Because the molecule is bent, those charges don't cancel: one side is negative and the other positive. Compare carbon dioxide, which also has polar bonds but is perfectly straight, so its charges cancel and the molecule is nonpolar. Water's shape is what makes it polar.

Hydrogen bonding: why water is unusual

The slightly positive hydrogen on one molecule is attracted to a lone pair on the oxygen of a neighbor. That attraction is a hydrogen bond. Each molecule can donate up to two and accept up to two, so it can link to as many as four neighbors, which is how liquid water forms a constantly shifting network.

A single hydrogen bond is weak — roughly a twentieth as strong as an O–H covalent bond — and in liquid water they break and re-form within trillionths of a second. But there are so many of them that their combined effect is large. It's why water stays liquid at temperatures where similar small molecules are gases.

The same network is behind these familiar properties:

Heat. Before molecules can move faster, hydrogen bonds have to stretch and break, so water soaks up a lot of energy per degree of temperature rise. That's why oceans moderate climate and why heating water takes real energy — the specific heat of water calculator puts numbers on it.

Ice floats. When water freezes, hydrogen bonds lock the molecules into an open hexagonal lattice with more empty space than the liquid has. Ice is therefore less dense than liquid water, and freezing expands the volume by about 9%. Liquid water is densest near 4 °C, which is why lakes freeze from the top down — and why a frozen pipe can burst.

Why water dissolves so much

Water dissolves ionic compounds and other polar substances because its polar molecules can surround and stabilize them. Drop table salt into water: the negative oxygen ends of water molecules crowd around sodium ions (Na⁺), the positive hydrogen ends crowd around chloride ions (Cl⁻), and the crystal is pulled apart ion by ion. Chemists call the shell of water molecules around each ion hydration.

Sugar dissolves by a different route. It isn't ionic, but its –OH groups form hydrogen bonds with water. Oil is the opposite case: its molecules are nonpolar, so water has nothing to grip, and water molecules would rather bond with each other. The oil gets pushed out into its own layer. Chemists sum it up as “like dissolves like.”

This is why water earns the nickname universal solvent. It overstates things — water doesn't dissolve everything — but it explains why every natural water source carries dissolved material. Dissolved calcium and magnesium are what make water “hard,” and you can convert hardness between units with the hardness ppm to gpg calculator. Dissolved oxygen, a gas held in water in small amounts, is what fish breathe; the dissolved oxygen calculator covers that side.

Self-ionization, pH and why “neutral” isn't always 7

Water also reacts with itself, a little. Now and then a hydrogen ion (a proton) hops from one water molecule to a neighbor, leaving a hydronium ion and a hydroxide ion:

The product of the two ion concentrations is a constant called the ion product of water:

That is where pH 7 comes from. The pH scale runs from 0 to 14 and measures hydronium concentration: below 7 is acidic, above 7 is basic. Add an acid and hydronium rises; add a base and hydroxide rises. Either way, the product of the two stays fixed at a given temperature.

Few ions also means pure water is a poor conductor of electricity. Tap water conducts well because dissolved minerals supply ions, which is what the conductivity to TDS calculator uses to estimate dissolved solids.

Making and splitting water

Burn hydrogen in oxygen and you get water, plus energy — about 286 kJ for every mole (18 g) of liquid water formed:

Run it backward with electricity and water splits into its elements. This is electrolysis:

By mass the split is different: about 1 part hydrogen to 8 parts oxygen, because each oxygen atom is roughly sixteen times heavier than a hydrogen atom. The water electrolysis calculator works out the gas volumes for a given amount of water.

Chemistry by the numbers

Chemists count in moles. One mole is 6.022 × 10²³ molecules, and a mole of water has a mass equal to its molar mass, about 18.015 g. To convert any mass of water into moles:

A liter of water has a mass of very close to 1,000 g, so it holds roughly 55.5 mol — about 3.3 × 10²⁵ molecules. The molar mass of water calculator and the moles of water calculator do these conversions for any amount.

Common misconceptions

Boiling doesn't break water apart. The bonds inside each molecule are covalent and strong. Boiling only overcomes the hydrogen bonds between molecules, which is why steam is still H₂O. Ice, liquid water and steam are one substance in three arrangements.

Hydrogen bonds aren't the bonds inside H₂O. The O–H bonds in a molecule are covalent. “Hydrogen bond” names the much weaker attraction between separate molecules.

Not every water molecule is identical. A small share of natural hydrogen is deuterium, a heavier isotope, roughly 1 atom in 6,400. Water made with it is called heavy water (D₂O). At natural levels the effect on everyday properties is negligible.

The bottom line

Water's chemistry follows from one simple structure. Two hydrogens and one oxygen form a bent molecule; oxygen's stronger pull on electrons makes it polar; polarity produces hydrogen bonds; and hydrogen bonds account for the high boiling point, the large heat capacity, the strong surface tension, floating ice and the ability to dissolve salts and sugars. A tiny fraction of molecules ionize, which sets neutral pH at 7 at 25 °C. Keep the structure in mind and the properties stop looking like a list of trivia.