1 Water changes state continuously

• Within the range of temperatures found on Earth, water can exist in all three states of matter: solid (ice, snow, hail), liquid (rain, rivers, oceans) and gas (water vapour).
• Water on or near Earth's surface is in a constant state of change between these three states, and it moves continuously from one place to another along many different pathways: from the sea to the air, from the air to the land, from the land back to the sea, and through living organisms.

• This continuous movement forms an almost closed system called the water cycle in nature (the hydrological cycle).
• It describes the continuous movement of water around the planet.

Why "almost closed"?

• (A favourite exam question.) Because the total amount of water on Earth stays essentially constant: water is neither created nor destroyed on a large scale by the cycle; it is only transferred between reservoirs (ocean, atmosphere, land, ice, living things) and changes state.
• The system exchanges energy with the outside (it receives energy from the Sun) but practically no matter.
• Only tiny amounts of water vapour escape from the top of the atmosphere into space, and tiny amounts are added from space or from volcanic activity, so we say "almost" closed rather than completely closed.

• The cycle is not just a plumbing diagram.
• The book emphasises that the water cycle is a system that can alter Earth's surface physically, chemically and biologically:

• Physically — flowing water erodes rock, carries sediments, shapes valleys and deltas (for example the Nile Delta), and freezing water breaks rocks.
• Chemically — water dissolves minerals, causes rocks to weather, and carries dissolved salts to the sea.
• Biologically — water supports life, enters and leaves organisms, and controls where ecosystems can exist.