Preboiling with Salt: Testing Experiment

Aim

More examination of preboiling.

Prior Knowledge

Description of the Experiment

The preboiling thing is very strange. The temperature difference between the top and the bottom of the beaker should account for a 3% change in bubble volume, yet the bubbles shrink by extremely large factors to almost nothing. We thought maybe that there was rapid condensation back to liquid phase enhanced by some sort of turbulence effects as the bubble drags through the water. But if this is so, then if we raise the overall boiling point of the water by adding salt I should be able to reduce this effect. Well, that is our prediction when we add salt to the water, the effect should be reduced. But... See below...

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Questions

So, you see that infact salt enhanced the preboiling effect quite markedly. Either the mechanism by which the vapour returns to the water is more complex than we thought or we probably misinterpretted how adding salt would affect the whole system. We are at a loss here.

Maybe the dominant factor governing bubble size may be surface tension. We will try and investigate this further some time.

Another thought is that although the vapour pressure has gone up, the number of impurities in the water (due to all the undissolved salt floating around) may actually encourage the condensation process. But still, very mysterious...