Physical and Chemical Properties of Water
Water (H2O) is a compound of two hydrogen atoms and one oxygen atom whose physical properties include a boiling point of 100 °C, a freezing point of 0 °C and a maximum density near 4 °C, and whose chemical properties include reacting with metals, oxides and its own splitting by electrolysis. In brief: pure water is colorless, odorless and tasteless, it is neutral with a pH of 7 at 25 °C, and it expands when it freezes, which is why ice floats.
Water is the most familiar compound on Earth, and many of its properties are unusual. They make life possible and shape the climate. This article lists the physical and chemical properties of water, explains where they come from, includes a data table, works through a heat calculation and gives a short history of how water was shown to be a compound.
Why is the water molecule special?
A water molecule is bent, with the oxygen atom in the middle and the two hydrogen atoms at an angle of about 104.5°. Oxygen attracts the shared electrons more strongly than hydrogen does, so the oxygen end has a slight negative charge and the hydrogen ends have a slight positive charge. The molecule is therefore polar.
Polar molecules attract each other. The attraction between the positive hydrogen of one water molecule and the negative oxygen of another is called a hydrogen bond. These bonds are individually weak but numerous, and they explain most of the unusual properties of water.
What are the physical properties of water?
- Appearance: pure water is clear, colorless, odorless and tasteless in small amounts. Thick layers look blue.
- High boiling and melting points: for such a small molecule, water boils at a high temperature, because hydrogen bonds must be broken.
- High specific heat capacity: it takes 4.18 J to warm 1 g of water by 1 °C, which is higher than most common liquids. Water therefore warms and cools slowly and moderates the climate near oceans.
- High latent heats: large amounts of energy are needed to melt ice or to evaporate water.
- Surface tension: the surface behaves like an elastic skin, which allows small insects to walk on water.
- Good solvent: many ionic and polar substances dissolve in water.
Key physical data for water
| Property | Value |
|---|---|
| Melting and freezing point | 0 °C |
| Boiling point | 100 °C |
| Density at 4 °C (maximum) | about 1.000 g/cm3 |
| Density of ice at 0 °C | about 0.917 g/cm3 |
| Specific heat capacity (liquid) | 4.18 J/(g·°C) |
| Latent heat of fusion | about 334 J/g |
| Latent heat of vaporization | about 2,260 J/g |
| pH of pure water at 25 °C | 7 |
Why does ice float?
Most substances are denser as solids than as liquids, but water is an exception. In ice, hydrogen bonds lock the molecules into an open, hexagonal structure with empty spaces. This takes up about 9% more volume than the same mass of liquid water, so ice is less dense and floats.
The consequences are important. Lakes freeze from the top down, and the layer of ice insulates the water below, which lets fish survive the winter. Water pipes can burst in a freeze because the expanding ice pushes against the pipe walls.
What are the chemical properties of water?
- Reaction with active metals: sodium reacts vigorously with water: 2Na + 2H2O → 2NaOH + H2. Hydrogen gas is released. Less reactive metals such as iron react only with steam or very slowly.
- Reaction with metal oxides: calcium oxide forms calcium hydroxide: CaO + H2O → Ca(OH)2.
- Reaction with nonmetal oxides: carbon dioxide forms a weak acid: CO2 + H2O ⇌ H2CO3.
- Decomposition by electrolysis: an electric current splits water into hydrogen and oxygen: 2H2O → 2H2 + O2. The volume of hydrogen collected is twice that of oxygen.
- Amphoteric behavior: water can act as both a weak acid and a weak base, and a very small fraction of molecules form hydronium and hydroxide ions.
- Hydration: water forms hydrates with some salts, such as blue crystals of copper(II) sulfate pentahydrate.
Worked example: heating water
How much energy is needed to warm 250 g of water from 20 °C to 100 °C and then boil away 50 g of it?
- Warming. Q1 = m × c × ΔT = 250 × 4.18 × 80 = 83,600 J.
- Boiling 50 g. Q2 = m × L = 50 × 2,260 = 113,000 J.
- Total. 83,600 + 113,000 = 196,600 J, or about 197 kJ.
Boiling a small portion needs more energy than heating the whole sample by 80 °C, which shows how much energy is stored in the change of state.
How was the composition of water discovered?
For many centuries water was regarded as one of the basic elements. In 1781 Henry Cavendish burned hydrogen in air and obtained water, and in the 1780s Antoine Lavoisier interpreted the result correctly: water is a compound of hydrogen and oxygen, not an element. In 1800 William Nicholson and Anthony Carlisle decomposed water into the two gases using electricity, and the formula H2O was established in the 19th century after the work of Gay-Lussac, Humboldt and others on combining volumes of gases.
Which mistakes are common?
Mistake 1: Saying that water is always densest as a solid
Water is densest as a liquid at about 4 °C. Ice is less dense than liquid water.
Mistake 2: Believing that water conducts electricity well in pure form
Pure water is a poor conductor. Tap water and salt water conduct because of dissolved ions.
Mistake 3: Thinking that boiling water decomposes it
Boiling is a physical change. Water vapor is still H2O.
Mistake 4: Confusing hydrogen bonds with bonds inside the molecule
The bonds between hydrogen and oxygen within a molecule are covalent and strong. Hydrogen bonds are weaker attractions between separate molecules.