Why does ice float up to the top after staying at the bottom for a while in soda?

Question

I noticed this recently when I poured myself a glass of soda and I want to know why this is. When I pour in the soda sometimes the ice will stay at the bottom not floating up for a while even after it is completely submerged but there is a point where once I add a little bit more water where the ice just floats up. This seems really counterintuitive to me as the ice is already under water an inch or so but once I add just a little bit more it floats up to the top.

To explain what I see in the simplest way possible:

• I put ice into a cup the cup is shaped like a rounded out V with a flat bottom.

• I pour soda into the cup and for a while the ice is a solid inch or so under water, staying there.

• I add a little bit more soda and all of a sudden the ice floats up to the top.

Any ideas as to why this happened? The ice is fully submerged so shouldn't adding more water make it want to stay down more?

My current working theory is that the CO2 bubbles lower the density of the soda so the ice can sink, but at a certain point the CO2 bubbles are cooled just enough so they take up less space and the ice floats back up.

So,

• Pour in soda with room temperature bubbles (ice is denser).
• After a bit, ice cools bubbles down so they take up less space and soda is denser so ice floats back up.

At least that's what I think. I'm not sure at all about this as I only have 2 college physics courses under my belt.

Answer 1

There are a couple reasons that home ice might not float in water/soda.

Less likely is that the ice and the bottom of the glass form a seal similar to a suction cup. Buoyancy depends on the pressure of the fluid being able to push up on the object. But if the bottom is sealed, this pressure is not available. As the pressure rises though, eventually some will intrude. The seal breaks and the ice rises.

More common is that when the ice is added to the glass, some water is on the surface. But if the ice is cold enough, it can freeze the water and it "sticks" to the glass. Now to move the ice the buoyancy has to overcome not only the weight of the cube, but also the cohesive forces from the ice to the vessel.

Both of these cases will be weak and will be overcome if sufficient pressure is applied (more fluid), or even without any fluid changes as the ice melts and the shapes change.

You can test if this is correct. If the difference were in the density of the fluid, you should be able to make an ice cube sink at some point. But the two scenarios above would only happen when ice is added to an empty glass and fluid is later added. The ice is buoyant, just not sufficient to overcome the extra forces. If gently prodded, it should float. As that would have no effect on either the density of the fluid or the ice, that should indicate that density changes are not involved.

Answer 2

I actually liked Rome's original explanation, which seemed rather reasonable here. Until I found some journal articles on actual densities for soda water at various dissolved molar concentrations under 10 degrees Celsius. In all cases the density is greater in Carbonated water, most likely due to the Van Der Waals interactions with water molecules and Carbon Dioxide (more likely at lower temperatures), given that CO2 has a molecular mass of 44g, compared with 18 for water.

Reference: https://escholarship.org/content/qt6dn022hb/qt6dn022hb.pdf

So the conundrum is one that I have wondered too. The addition of water is also not required. In general, I find that the ice rises at 10 seconds after the soda is poured +/- five seconds. It does appear that the rise is initiated, at least in part, by the adhesion of liberated CO2 bubbles around the edges of the cubes, since the effect is more marked with larger cubes that irregular crushed ice (with a larger available surface area, these tend to rise faster).

It is possible that adhesion to the bottom of the glass is a factor and would explain why the ice does not sink again when it reaches the surface. Upon contact with the surface, a proportion of the bubbles attached to the cubes are dislocated (presumably by the sudden deceleration). This suggests that the resistance to buoyancy is more related to surface contact being broken than any substantial change in density.