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Water Incompressible?

Is water incompressible or not? And if not, how much can you compress it?


In physics exam papers you sometimes see "For the purposes of this experiment, assume water is incompressible". This is often a reasonable assumption, but... is it true?

Well, you can't get a cutprice pump and use it to put two gallons of water into a one gallon container. Air, you can do that with. Water, not so. But then, people question whether water is compressible at all.

The question of the incompressibility of water depends on whether you want to be absolute, or to have a balanced view based on observations in life. This won't affect the compressibility of water, but there are different ways of looking at it. I'll explain:

Do you think steel is incompressible? This puts the whole "water incompressible?" question in focus. Steel is obviously incompressible in the sense that you can get a big block of it and jump up and down on it and it doesn't "give". However, steel must be compressible to some extent or it wouldn't make very good springs, and then there's the matter of what the railway companies have to do about the railway tracks getting longer in hot weather because of the heat.

In the sense that "steel is incompressible", it's also true that "water is incompressible", but these things only approximate to being true. The approximation fits with everyday life experiences.

Air, notably, is compressible. You can get an air pump and put gallons and gallons of it into a car tyre and there you have it, a car tyre full of compressed air. You can also get diving cylinders which contain almost a whole room full of breathable air which divers can carry about underwater without having to carry around a whole room of space. The fact is, you can't do this with water. Even the people who say "water is compressible" can't compress it enough to make a difference that's much help in terms of carrying loads of water around in small containers.

Scientifically, there is a measurement of incompressibility. It's known as "Bulk Modulus". One way to explain bulk modulus is to say that the bulk modulus of a material is the amount of pressure which, if you applied 1% of it, would reduce the volume of the material by 1%.

Air has a bulk modulus of about 1 atmosphere, 15 pounds per square inch, and at 10 atmospheres you can get ten times as much air into a strong container. Water also has a bulk modulus, (ie water is not incompressible). The bulk modulus of water is about 22,000 atmospheres.

At the bottom of the deepest depth of the sea, the Mariana Trench, the pressure is about 1100 atmospheres. This is a heck of a lot of pressure, 7 tons per square inch, and the number of submarines that have ever been built to withstand that pressure is... about five or six.

Someone asked "If you took a very strong 1 gallon container and took it down to the bottom of the deepest sea and then put the lid on, and then brought it back up again, how much water would it contain?". The answer is about 1.05 gallons, or 5% more than if it had been filled up with water at the surface.

So, to put that in perspective: The deepest sea, with the highest pressure of water on earth, can compress water by 5%.

This puts a clear distinction to some answers to some notable questions:

* Water IS compressible, in an absolute sense.

* The amount of pressure required to achieve any noticeable compression is such that any notions of inventions that can have "compressed water" being carried about with any efficiency, are impractical.

In terms of an answer to the question "Is water incompressible?", the true answer is "Water is not incompressible". If the question were phrased "Can you compress water?", it starts to sound like a practical engineering question, and the answer is generally "not really".

Yes, you can assume that water is incompressible for most scientific experiments and questions in exam papers. For example, take a litre of water and put it in a vertical tube of area 1cm2, what's the height? About 10 metres. The difference is so small it can be neglected.

The compressibility factor of water can be neglected for most purposes, but there are some applications where it makes a difference. For example, if the water in the earth's oceans were completely incompressible, the water height would be about 100ft higher than it is currently. That's not the same as "what if all the ice melted". If the ice melted, the volume of water would have to go somewhere, and it can't be just compressed and hidden away. So, sea level would rise. Also see the myth of carbon sequestration

Pressures of thousands of atmospheres are not achievable without some advanced engineering. If you're in the business of making diamonds then it makes water look quite compressible. (Diamond has a bulk modulus of 44,000 atmospheres).

On some other planets, (and worlds which the International Astronomical Union says are not planets), there is water much deeper than on earth, and the pressure is therefore much greater. It will be interesting to see what bizarre things are discovered in some future space missions. It is currently speculated that Europa, one of the moons of Jupiter has water 100Km deep. Although the gravity is less than on earth, the pressure is still likely to be quite high.

A few "Water Compressibility" experiments and observations:

Water, and ice, give a remarkable impression of incompressibility when pipes are burst because of frost. It becomes a matter of home insurance because the water/ice is stronger than the metal, and ice is bigger than water

Many years ago, when I was a child, I did a childish experiment on a fishtank. I connected the ornamental diver up to a header tank of water, and put small amounts of ink in the water so I could see it coming out of the diver's helmet (where the air would usually come). Jets of inky water came out and progressed horizontally and upwards if hot water was applied, but horizontally and downwards if cold water was applied. At the time I started to question the teachers' statement that water was absolutely incompressible, because obviously warm water was lighter than cold water!


About this explanation: Many websites find it difficult to explain the "water incompressible" question. It's the the question of why you never see baby pigeons. The reason it's difficult to explain whether water is compressible or not is that most websites take an opinion about it, starting off by saying "water is incompressible", or "water is compressible" and then trying to explain it all from there. The trouble is, whichever viewpoint is taken, it's possible to criticise it. For example, if you say water is compressible, you have difficulty explaining why you can't compress it in a practical way. On the other hand, if you say water is incompressible, you have difficulty because there are scientific ways of showing water can be compressed by small amounts using a lot of pressure.

So, on this page I have explained in which ways water is compressible, and in which ways it is incompressible. If anyone has constructive comments or suggestions, you can contact me and the page can be refined.