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Suck back, also called suck-back, suckback or suck back effect is the tendency for a liquid connected to an enclosed container or vessel to be suddenly pulled into it, due to the creation of a vacuum in the enclosed flask.
This effect can occur due to two causes:
- Physical cause: A closed flask with a single delivery tube, like an enclosed test tube with a single gas outlet, is strongly heated, causing the air within it to expand. This expansion can be seen bubbling out of the delivery tube and through the water. If the heat is removed, or becomes less intense, then the air in the test tube contracts and sucks cold water back along the delivery tube. If the glass is not heat resistant, this can often have disastrous consequences.
- Chemical one: gasses that are highly soluble in water, like hydrogen chloride or ammonia, will rapidly and aggressively absorb any water they enter in contact with, provided the flask containing the gas is fully enclosed, and it's only connected to the water with a single hose. This phenomenon has also been observed in gasses that react with water.
Preventing suck back
A simple way of preventing the suck back effect is to use a funnel-and-beaker trap contraption. It's simple and effective, providing one maintains the water level stable and uses the right type of funnel.