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Bubbles and Henry’s Law February 28, 2009

Posted by Chris Sullivan in Technical Diving.
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When my family moved from Australia to Canada in 1967, we noticed that a glass of water left out overnight would contain bubbles by morning. We didn’t notice this when we lived in Australia. My father, a chemical engineer,  mentioned to me once that he was talking to a colleague about this phenomenon, and told him to try leaving a glass of hot water out instead. Sure enough, no bubbles. He never explained to me why this happened, and I never asked, but much later I wondered why this occurred.

We learn about Henry’s law in diving. This states that at a given temperature, the amount of gas that dissolved in a liquid will be proportional to the partial pressure of each of the gases which surround the liquid when in equilibrium. Dissolved gas is what gives us various physiological problems in diving, including the decompression sickness, so it’s important to learn about it. The concept of equilibrium is important here – the gas doesn’t go into and out of solution right away. If it did, a beer would go flat as soon as it was opened, much to the disappointment of many divers I know. Similarly, it takes time for the body to expunge the excess nitrogen (and/or helium) from the bloodstream as the diver surfaces.

Temperature is important as well. For the bloodstream, it’s hopefully reasonably constant, but for beer and glasses of water, temperature plays an important role in the ability of the liquid to dissolve gas. Colder liquids can absorb more gas (which is one reason we like cold beer). The math behind this is considerably more complex than Henry’s law (which holds temperature constant). The Wikipedia reference above has an explanation of how it works, if you’re interested. Suffice to say that cold water holds more gas than warm.

So when we moved to Canada, the cold tap provided water that was considerably colder than room temperature, since it came from outside and the rooms had central heating. Cold water holds more gas than warm, so when we left the glass of water overnight some of the air came out of solution in the form of bubbles, many of which were still clinging to the sides of the glass when we got up in the morning. In Australia, with warmer outside temperatures and no central heating, this didn’t happen, as the water and air temperatures were much nearer to each other.

Hot water left overnight cools down, so air is absorbed into solution, and no bubbles appear. If that glass was put into the refrigerator for long enough, then brought back out into the room, bubbles would form as gas would be absorbed by the water while it was chilled.

Go ahead and try this at home!

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