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I know that water can exist in various states (liquid, solid, ...) and can be in various places (clouds, oceans, ground, ...). What I want to know is whether or not the total number of water molecules on planet Earth is steadily increasing or decreasing and why? For example, is the amount of water now on planet Earth the same as it was 1000 years ago or a million years ago?

Based on answers from allure and farcher, I realize that my original question was not exactly what I wanted to know about so I have revised my question.

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The number of water molecules is certainly not constant, because chemical reactions can create or destroy water. A simple example is metabolism of Glucose, which creates water.

$C_6H_{12}O_6 + 6O_2 \rightarrow 6 CO_2 + 6 H_2O + \mathrm{Energy}$

The reverse reaction (destroying water) happens during photosynthesis.

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  • $\begingroup$ Thanks for the information. I have revised my question. $\endgroup$ Feb 19 at 9:57
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You might have observed in a Science class at school a simple example of the number of water molecules changing , the electrolysis of water.
An electric current is passed through water and oxygen gas is collected at one electrode and twice the volume of hydrogen gas is collected at the other electrode.
The reverse process occurs in the power unit of a car whose fuel is hydrogen gas.

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  • $\begingroup$ Thanks for the information. I have revised my question. $\endgroup$ Feb 19 at 9:58
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Is the total number of water molecules on Earth exactly constant ?

No, because of the various chemical reactions that consume or produce water molecules.

Is the total number of water molecules on Earth approximately constant ?

Yes, to within a small fraction of one percent.

Is the total number of water molecules on Earth steadily increasing or decreasing ?

No, otherwise the effect would be significant over large enough time spans, and we would have geologic evidence that the Earth in the past was much drier or wetter than it is now.

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The new question you've asked is one of geology, which means the details are a bit fuzzy. It is known that water is lost over time from the atmosphere. It is believed that "... the Earth lost at least one ocean of water early in its history, between the Hadean and Archean eons." However, we don't really have a firm grasp at the moment on what created the water. A common theory is that it arived on icy asteroids, but recent studies suggest the water may have come from hydrogen inside the Earth' mantle. The details are tricky because it is believed that 10 oceans worth of water may exist inside the Earth, but there's very little data available on the topic.

So, since we don't really understand the water creation process, its tricky to figure out whether it's in balance on a thousand year or million year timescale. On the scale of billions of years, we're reasonably confident that Earth started out without water and it occurred over time, so by definition the amount of water grew on that time scale.

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Every time you turn on your gas heating you create water due to the combustion reaction $CH_4 + 4 O_2 \rightarrow CO_2 + 2 H_2 O$. Similar when you burn wood or drive your car. There are other reactions that convert water to other molecules such as plants creating sugars during photosynthesis. The water obtained by combusting fossil fuels in our homes and cars (or naturally by combustion in forest fires etc) is obtained from the materials made by photosynthesis so there is a certain amount of recycling going on there in the very long term. Overall it is very hard to tell if there is a net gain or loss over millions of years but I would guess there is a very slow net loss due to accumulation of dead plant and animal matter in soils and underground and a small amount lost from the outer atmosphere to space.

However, most of the water we consume from rivers is normally naturally conserved and returned in the form of rain.

When we say there is a water shortage and we must conserve water we are really talking about conserving the energy and resources used to clean up the water again and distribute it.

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    $\begingroup$ Throughout most of human history, we have relied on naturally occurring processes to clean up and recycle water for us. We did not expend any energy apart from the energy needed to lift the clean water out of a well or a stream, and we did not exploit any resource other than the water itself. Nearly all of our agriculture still functions that way. $\endgroup$ Feb 19 at 15:42
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    $\begingroup$ Why do charities ask for donations to provide clean water to children in Africa when you say water is free? It is because as populations expand and spill over to more arid lands there is an increasing need for desalination plants and water cleanup facilities, deep wells and pumps and pipe systems and reservoirs etc. If the population grows tenfold, we wont be able to rely on waiting for the rain. Why is the water table falling in Mexico city? Why does Gaza need water pumped from Israel when they live right by the sea? $\endgroup$
    – KDP
    Feb 19 at 15:49
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    $\begingroup$ Children in Africa (and also in the U.S.A., don't ask me how I know) used to drink untreated surface water and well water and rain water with no ill consequence. It's only because of over population and climate change that we are suddenly faced with the need to begin spending energy and resources to get the fresh water that nature used to provide for free. $\endgroup$ Feb 19 at 16:00
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    $\begingroup$ I know too. I used to live in Africa ;-) ..and yes about population. $\endgroup$
    – KDP
    Feb 19 at 16:01
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In addition to all the other answers, the earth is constantly being bombarded by small meteors, many of which will contain small amounts of water.

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