Ask a Physicist Answers
On Saturday, I accidentally left a full cup of diet soda sitting in my car. It was originally full of ice, which of course melted over the course of a day and a half. When I got into my car on Monday morning, the straw was just over half full of soda, extending about 2" above the full level of soda in the cup. This condition held constant for quite some time, even when the cup was lifted and jiggled/disturbed. Once I had taken a tiny sip, the level of soda in the straw went back down to where I would have expected to find it (at the same level as the soda in the cup). Given that this was a standard lid (with an opening at the top for the straw that is certainly not airtight, nor was the top opening of the straw airtight at all) I could not imagine how the soda in the straw was being pushed or pressured to a higher level than the soda in the cup. The weather over the weekend was cold, but not freezing, averaging in the low 50's. Why did the liquid level in the straw from my soda rise above the liquid in the cup after being left in my car for the weekend?
Answer: I would guess that over the course of a day and a half, evaporation caused the fluid in the straw to become less dense. Diet soda is less dense than water, and as the water evaporated out of the straw, leaving more concentrated diet soda behind, it would become even less dense than usual. The fluid in the cup would have experienced somewhat less evaporation because it's not directly exposed to air. As a result the comparatively more dense fluid in the cup would push the less dense column of fluid up the straw. Once you take a sip, you have removed the lower density fluid, and the level goes back to normal. BTW, the situation would have been reversed if you'd had regular (non-diet) soda in your cup because sugary soda syrup is denser than water.
BS Physics from the University of Maryland
Former Applications Physicist at the Super Conducting Super Collider (SSC)
Lisa from Michigan