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Name: Kevin W.
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: September 2003


Question:
To whom it may concern: I was reading a book on the Internet and the character's ship had water compression systems. The book also said that water could not actually be compressed. When I asked my chemistry teachers at school, they said that they did not know much about it. My question here is: Can water be compressed at all? Why or why not?



Replies:
All substances, gases, solids, and liquids, including water, are compressible. Some substances just require more pressure per change in volume to compress than others. The numerical value of the compressibility depends both on the temperature and on the pressure too. That is how hard you have to squeeze at one pressure to obtain a certain fractional decrease in the volume is different than at some other pressure. The same for two different temperatures. Under most "normal" lab conditions however solids and liquids are usually considered to be incompressible.

Vince Calder


Technically speaking, water can be compressed, but very little even at high pressures. For practical design purposes, water is considered an incompressible fluid, that is, its density does not change with pressure. The reason anything is compressible is due to how close the atoms are packed together. If you think about air, it is highly compressible because there is considerable spacing between the atoms, so it is relatively easy to force the atoms closer together. When you have a liquid, the atoms are much closer together and considerable pressure is required to make them any closer. Solids will compress some as well, but it takes significant pressure. Temperature also plays a factor in density (and thus compressibility), just ask your chemistry teacher about the ideal gas law. I am not sure what your book was using a "water compression system", but hydraulic systems are used almost everywhere to transmit power across a system. What a hydraulic system does is put a liquid (usual an oil of some type) under pressure, route that pressurized liquid through pipes or hoses, and releases the pressure at a place where work needs to be done, usually through a motor or cylinder that transfers the pressure into work. You might want to research on hydraulic systems (hydraulic lifts, hydraulic elevators, etc.) to get a better understanding on how energy can be transferred around in a hydraulic system.

Hope this helps and thanks for using NEWTON.

Christopher Murphy, P.E.


Kevin,

Water is said to be "practically" incompressible. This statement is true for most considerations at low pressures. But at higher pressures and as temperatures vary, there will be changes in density which must be taken into account when doing any engineering calculations. I hope that this is a help.

Sincerely,

Bob Trach



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