What is Water?
Water is a chemical compound made of two hydrogen atoms bonded to one oxygen atom. We write its formula as H₂O. It is the most common liquid on Earth and covers about 71% of the planet's surface. Despite looking simple, water has a set of properties that are truly extraordinary — and without them, life as we know it would not exist.
At room temperature, pure water is:
- Colourless — it has no colour
- Odourless — it has no smell
- Tasteless — pure water has no flavour
- Transparent — light passes through it easily
These basic properties make water easy to identify. But what makes water truly special goes much deeper.
The Three States of Water
Water is one of the few substances we encounter naturally in all three states of matter:
| State | Name | Temperature (at sea level) |
|---|---|---|
| Solid | Ice | Below 0 °C |
| Liquid | Water | 0 °C to 100 °C |
| Gas | Steam (water vapour) | Above 100 °C |
Water melts at 0 °C (its melting point) and boils at 100 °C (its boiling point). These values are so useful that the original Celsius temperature scale was actually defined using these two fixed points.
When water changes from liquid to gas, the process is called evaporation (or vaporisation). When steam cools and turns back into liquid, that is condensation. When liquid water freezes into ice, that is solidification, and the reverse is melting.
Anomalous Expansion — Why Ice Floats
Most substances contract (get smaller) when they cool and expand when they heat up. Water does the same — but only down to 4 °C. Below 4 °C, something unusual happens: water starts to expand as it cools further and freezes.
This means ice is less dense than liquid water. That is why ice cubes float in your drink, and why frozen lakes have a layer of ice on top rather than at the bottom.
Real world: Lakes in Winter
When a lake freezes in winter, the ice forms on the surface and acts like an insulating blanket. The water underneath stays liquid and does not freeze solid, even when the air above is far below 0 °C. Fish and other aquatic animals survive the winter because of this anomalous expansion of water. If ice were denser than liquid water (like most solids are denser than their liquid), lakes would freeze from the bottom up and most aquatic life would die.
High Specific Heat Capacity
Water can absorb a large amount of heat before its temperature rises significantly. Scientists describe this as having a high specific heat capacity. In simple terms, water heats up slowly and cools down slowly compared to most other substances.
This property has enormous consequences for our climate:
- Oceans store heat during summer and release it slowly in winter, keeping coastal regions mild and preventing extreme temperature swings.
- Our bodies are about 60–70% water, which helps us maintain a stable internal temperature even as the environment around us changes.
Coastal cities like Lisbon or Dublin have much milder winters than inland cities at the same latitude, like Madrid or Warsaw. The nearby ocean — full of water with its high specific heat capacity — acts as a giant temperature buffer.
Surface Tension
Have you ever seen an insect walking on water? Or noticed that water forms droplets on a waxy leaf? These are examples of surface tension — the tendency of water's surface to behave like a thin, elastic film.
Surface tension happens because water molecules are attracted to each other. Molecules in the middle of the water are pulled in all directions equally. Molecules at the surface are only pulled inward and sideways — there is nothing above them — so the surface is pulled tight.
Real world: Water Striders
Water strider insects (also called pond skaters) use surface tension to walk on water. Their long, thin legs spread their weight over a large area, and the surface tension of the water supports them without breaking. You can test surface tension yourself by carefully placing a small steel needle on the surface of calm water — it floats even though steel is much denser than water.
Water as the Universal Solvent
Water is often called the universal solvent because it can dissolve more substances than almost any other liquid. Sugar, salt, most acids and bases, gases like oxygen and carbon dioxide — all dissolve in water.
This property is vital for:
- Biology — nutrients and minerals dissolve in water and are transported through the bloodstream
- Chemistry — many chemical reactions take place in water solution
- Industry — water is used as a solvent in manufacturing, cleaning, and food production
Water dissolves substances so well because its molecules are slightly charged (polar), allowing them to pull apart the particles of other substances.
Summary Table
| Property | Value / Description |
|---|---|
| Chemical formula | H₂O |
| Melting point | 0 °C |
| Boiling point | 100 °C |
| Colour | Colourless |
| Density of ice vs liquid | Ice is less dense — it floats |
| Specific heat capacity | Very high — heats and cools slowly |
| Surface tension | High — insects can walk on it |
| Solvent ability | Dissolves more substances than almost any liquid |
Practice Problems
Why does ice float on liquid water instead of sinking?
Show answer
A city near the ocean has much milder winters than a city at the same latitude that is far inland. Which property of water explains this difference?