How does a laboratory water pump work

The water pump is another example of the Bernoulli effect, and is an everyday piece of equipment in most laboratories, for example being used during Buchner filtration. It comprises a piece of rubber tubing to connect the flask to be evacuated to a pump. Inside the pump, a rapid flow of water past one end of a small aperture inside the head decreases the pressure of the adjacent gas, so the pressure inside the pump soon decreases. 

Gas passes from the flask to the pump where the pressure is lower. The change in Gibbs function associated with these pressure changes is given by 

It should be clear from Equation (4.39) that gas movement in the opposite direction, from low pressure (/ (finai)) to high (/Amman) would cause AG to be positive, thereby explaining why the process of gas going from low pressure to high never occurs naturally. Stated another way, compression can only occur if energy is put into the system so, compression involves work, which explains why pumping up a car tyre is difficult, yet the tyre will deflate of its own accord if punctured. 

These highly oversimplified explanations ignore the effects of turbulent flow, and the formation of vortices. 

The minimum pressure achievable with a water pump equals the vapour pressure of water, and has a value of about 28 mmHg. 

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