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Suck back
This can occur when we are strongly heating a test tube and collecting a gas over water. When the test tube is strongly heated the air within it expands, and 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. A Pyrex test tube can withstand limited thermal shock - it should easily cope with cooling from 100 degrees centigrade rapidly to room temperature. However, the glass in a strongly heated test tube is at several hundred degrees centigrade, and it is unlikely that the glass will survive the sudden cooling by the cold water. There is a high risk that the glass will shatter, and this shows the benefit of wearing eye protection:
If you see the water rising up the delivery tube make sure you do not pull the Bunsen away as this will make the situation worse. Though as can be seen in the following video, strong heating does not always prevent suck back. The solution to suck back is to take the clamp stand and lift the apparatus clear of the water in the trough. Always avoid holding the Bunsen under the heated test tube - if it breaks it will drop on your hand!
Suck back also occurs when we are trying to dissolve a highly soluble gas in a solvent. For example, dissolving ammonia or hydrogen chloride gas in water. Although this time there is no explosion risk, water is sucked back into the reactant container.The solution to this problem is to replace a simple delivery tube with an inverted funnel inserted just under the surface of the solvent. As the solvent sucks back into the large volume of the funnel, the solvent level drops beneath the lip of the funnel so that further suck back is not possible.
