The Hydrologic Cycle and Essential Properties of Water

The Water Cycle

The Earth was formed 4,500 million years ago. At first, it was a great ball with hundreds of volcanoes on its surface. The magma of these volcanoes was loaded with water vapor, which rose to the surface by continuous volcanic eruptions.

Then the Earth cooled, water vapor condensed and fell as rain, thus beginning the cycle of water. The rain filled depressions in the Earth and created the oceans. Since then, the same amount of water has circulated again and again on the Earth; i.e., the total amount of water on the planet does not change.

The circulation and conservation of water on Earth is called the water cycle or hydrologic cycle. In this cycle, the water is purified.

Water moves constantly in nature, appearing in three states: solid (like snow or ice), liquid (like running water), and gas (like water vapor).

What is the Water Cycle?

The Sun, heating the water in lakes, rivers, and oceans, causes it to turn into vapor that rises into the air. This process is called evaporation. Plants absorb water through their roots for nourishment. This water is shifted upward, reaching the leaves and flowers, where it evaporates, goes into the air as water vapor, and forms clouds. This process is transpiration.

When the air cools, water droplets form or snow falls to Earth as rain: the steam has turned into liquid. This is condensation.

Once the water reaches the ground, it can fall directly into rivers, lakes, and seas or flow through the earth to reach them. That water eventually returns to the atmosphere by evaporation. Then the cycle begins again.

Did You Know? (Glossary)

Magma

In geology, an existing mass of molten rock inside the Earth under very high pressure and temperature. Magma is solidified by cooling. The term comes from Greek magma ("paste, ointment").

Depression

A hollowing or sinking of a surface or part of a body.

Properties of Water

The chemical formula for water is H₂O, which means it is formed by one oxygen atom and two hydrogen atoms attached to it. The oxygen atom is almost two and a half times larger than each hydrogen atom.

Water is the only compound on Earth that, at normal temperatures, exists in three states: solid, liquid, and gas.

A block of ice melts at 0 °C, turning into liquid water: that is the water's fusion temperature (or melting point). Water turns to steam at a temperature of 100 °C: this is the boiling point. These values are taken as fixed points to calibrate thermometers expressed in degrees Celsius (°C).

When water is pure and at 25 °C, it is colorless, odorless, and tasteless. Most of the water on Earth is in liquid form. This is due to the Earth's temperature, which is warm because of its proximity to the Sun.

Key Densities

• Pure water: 1000 kg/m³
• Seawater: 1025 kg/m³
• Ice: 910 kg/m³

Ice floats on liquid water, allowing the existence of life to thrive.