View source for Water cycle

[[Category: Physics]]
== Explanation ==
The water cycle is the process of continuous phase changes of [[water]] on a planet like [[Earth]]. It explains how liquid water moves from the [[ocean]] to the [[atmosphere]] as [[water vapour]] and how it falls back to the land and continues the cycle. The cycle begins with the water in its [[liquid]] [[phase|phase of matter]] mostly on the surface of the ocean, where it is heated by the Sun. Liquid water, when its [[temperature]] increases, it [[evaporation|evaporates]] faster and changes to the gaseous phase called water vapour. This warm vapour rises in the [[atmosphere]] while it expands and cools with the [[altitude]]. When cooled enough, the water vapour loses [[energy]]. The atmosphere has other particles and dust floating around too. So these particulates suspended in the atmosphere acts as [[nucleation]] sites and lead to the [[condensation]] of the water vapour back into liquid water droplets. These suspended liquid droplets in the atmosphere is just known as [[cloud]]s. When the droplets increase in size, they fall back to the surface of the planet due to [[gravity]] as [[rain]], also known as [[precipitation]]. The water finds a reservoir and then flows back to the oceans to continue the cycle.

== Frequently Asked Questions ==
=== Why rain water is not salty when it is evaporated from the ocean that has salty water? ===
The salty ocean water that contains sodium and chloride ions do not have enough energy to evaporate at the standard temperature at the surface of the ocean. Therefore, water that evaporates from the oceans as vapour will only contain fresh water molecules in it. The same happens in [[distillation]] of water.

=== What causes acid rain? ===
[[Acid rain]] is caused when the rainwater that falls from the clouds reacts with the compounds like [[nitrogen oxide]] and [[sulfur dioxide]] in the air. These compounds decrease the [[pH]] value of the rain and make it more acidic in nature. Naturally, rain is a little acidic as it reacts with [[carbon dioxide]] in the atmosphere to form [[carbonic acid]]. But it is due to the industrial revolution and other human activities like [[fossil fuel]] burning, the concentration of the other pollutants like sulfur dioxide and nitrogen oxide have increased to cause rain that is highly acidic in nature. Other sources of these pollutants include natural events like [[lightning]] and volcanic eruptions.