Water: Earth's Most Important Compound

Introduction

Water is so common that it is easy to think of it as ordinary. In fact it is one of the strangest substances known. It covers about 71% of Earth's surface, makes up roughly 60% of an adult human body and is the only substance found naturally on Earth as a solid, a liquid and a gas. Many of its properties are unusual for such a small molecule, and those very properties make life possible. This page looks at what water is made of and why it behaves as it does.

Core explanation

Composition and formula. Water is a compound of hydrogen and oxygen. Each molecule contains two hydrogen atoms joined to one oxygen atom by covalent bonds, giving the formula H₂O . By mass, water is about 11% hydrogen and 89% oxygen, because an oxygen atom is sixteen times heavier than a hydrogen atom. Water can be formed when hydrogen burns in oxygen:

hydrogen + oxygen → water 2H₂ + O₂ → 2H₂O

Shape and polarity. The water molecule is bent, with an angle of about 104.5° between the bonds. Oxygen attracts the shared electrons more strongly than hydrogen, so the oxygen end is slightly negative and the hydrogen ends slightly positive. The molecule is polar , and neighbouring molecules attract each other strongly through hydrogen bonds .

Unusual properties. Because of hydrogen bonding, water behaves very differently from other small molecules:

- High boiling point. Water boils at 100 °C at normal pressure. Similar-sized molecules such as methane (−162 °C) are gases at room temperature. Without hydrogen bonding, water would be a gas and Earth would have no liquid oceans. - Ice floats. Most substances are denser as solids. Water is densest at about 4 °C; ice (about 0.92 g/cm³) is less dense than liquid water (1.00 g/cm³), because hydrogen bonds hold the molecules in an open hexagonal lattice. - High specific heat capacity. It takes about 4200 J to warm 1 kg of water by 1 °C, much more than for most substances. Water warms up and cools down slowly. - Excellent solvent. Its polar molecules surround and dissolve many ionic and polar substances, so natural water always contains dissolved materials. - High surface tension. Strong attractions at the surface allow some insects to walk on water.

Formulae

Formula of water: H₂O. Formation from its elements: 2H₂ + O₂ → 2H₂O. Relative formula mass: (2 × 1) + 16 = 18.

Step-by-step reasoning

To calculate the percentage of hydrogen by mass in water:

1. Relative formula mass of H₂O = (2 × 1) + 16 = 18. 2. Mass of hydrogen in one formula unit = 2 × 1 = 2. 3. Percentage of hydrogen = (2 ÷ 18) × 100 ≈ 11%. 4. The rest, about 89%, is oxygen.

Visual explanation

Picture a water molecule as a large red sphere (oxygen) with two small white spheres (hydrogen) attached like mouse ears at an angle. Around it, other molecules line up so that each small white sphere points towards the red sphere of a neighbour, forming a network of dotted lines that represent hydrogen bonds.

Real-world analogy

Water molecules are like people at a crowded party who keep holding hands with their neighbours. It takes a lot of energy to pull everyone apart, which is why water needs so much heating to boil. Ice is like the same people standing in an orderly ring with arms stretched out, taking up more space.

Real-world example

In winter, a lake freezes from the top down. The floating ice insulates the water below, which stays at about 4 °C at the bottom, so fish survive until spring. If ice sank, lakes could freeze solid.

Why?

Why do coastal places have milder climates than inland ones? The sea's high specific heat capacity means it warms slowly in summer and cools slowly in winter, so it keeps nearby land cooler in summer and warmer in winter.

Common misconception

"Water is a mixture of hydrogen and oxygen." Water is a compound: the elements are chemically bonded in a fixed ratio of 2 : 1, and water's properties are completely different from those of either gas.

Worked example

Question: Hydrogen sulfide, H₂S, has a larger molecule than water but boils at −60 °C. Why does water boil at a much higher temperature?

Reasoning: Boiling separates molecules. The stronger the attractions between molecules, the higher the boiling point. Water forms strong hydrogen bonds; H₂S does not form them to any significant extent.

Answer: Hydrogen bonds between water molecules need much more energy to overcome, so water boils at 100 °C.

Quick check

1. What is the formula of water, and what elements does it contain? Answer: H₂O; hydrogen and oxygen.

Exam focus

Learn water's formula, its boiling and melting points (100 °C and 0 °C at standard pressure) and the fact that ice is less dense than water. When explaining unusual properties, name hydrogen bonding and link each property to an effect on life or climate.

Advanced insight

Pure water is also very slightly ionised: a tiny fraction of molecules split into H⁺ and OH⁻ ions. At 25 °C the concentration of each is 1 × 10⁻⁷ mol/dm³, which is why pure water has a pH of 7 and why it conducts electricity only extremely weakly.

Summary

Water, H₂O, is a compound of two hydrogen atoms and one oxygen atom, about 11% hydrogen by mass. Its bent, polar molecules form hydrogen bonds, giving water a high boiling point, a high specific heat capacity, strong solvent power and the rare property that ice floats. These properties shape climate and make life on Earth possible.

Practice questions

1. Explain why water is described as a compound rather than a mixture. Answer: Hydrogen and oxygen are chemically bonded in a fixed ratio and cannot be separated by physical methods. 2. Give two unusual properties of water caused by hydrogen bonding. Answer: Any two of: high boiling point, ice less dense than liquid water, high specific heat capacity, high surface tension. 3. Why is it important for aquatic life that ice floats? Answer: Ice forms an insulating layer on top, so the water beneath stays liquid and organisms can survive. 4. Calculate the relative formula mass of water. Answer: (2 × 1) + 16 = 18. 5. Explain why the sea helps keep coastal temperatures steady. Answer: Water has a high specific heat capacity, so it changes temperature slowly and moderates the air temperature nearby.