為何液體不能被壓縮?

Liquids are NOT incompressible. They are only hard to compress as compared with gases. For example, the bulk, which is a physical quantity characterizing the compressibility of a substance, for water is 2.2 x10^9 Pa (i.e. 2.2 GPa). But for air, it is 10^5 Pa (i.e. 100 kPa). That means, in order to reduce the same fractional volume, we need to exert a pressure of 22,000 times in water than in air. The physics reason behind is that molecules in gases are almost not bound by intermolecular forces and they are separated at large distance. Hence, there is lots of space between gas molecules that makes compression more easily. On the other hand, molecules in liquids are closely packed and are held together by strong intermolecular force, compressing a liquid is thus more difficult.

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嚴格地說液體是可以壓縮的；在海洋底部存在被壓縮的海水。
壓縮液體的關鍵是温度，降低温度使液體的體積收縮，但是温度太低液體就變成固體； 用機械式的方法加壓，液體的空間減少，分子間互踫的機率增加，以致温度上升，使液體的電子動能增加，體積反有增大的傾向，增減相抵以至體積不變，加壓的機械能轉為熱能； 如繼續不斷的加壓，温度上升，最後液體化成氣體。