Question

Why does a sealed baggie of dry ice lose weight over time at room temp, even before it pops and the gas escapes? I thought it might be due to buoyancy, perhaps? As the bag expands, it displaces air. But if that is the case, the why would it matter since CO₂ is actually MORE dense than air, right? Please help me understand.

Answer 1

The weight loss in a sealed bag of dry ice occurs due to sublimation, where the solid CO₂ turns directly into gaseous CO₂ and escapes from the bag. This process happens before the bag visually pops and releases the gas, and is not related to changes in buoyancy.

Step-by-step explanation:

A sealed bag of dry ice loses weight over time at room temperature not because of buoyancy, but due to sublimation. Sublimation is the process where a solid turns directly into a gas without becoming a liquid first.

Dry ice, which is solid carbon dioxide (CO₂), sublimes into CO₂ gas at temperatures above -78°C, even at room temperature.

The CO₂ molecules escape through tiny pores or imperfections in the bag's seal, leading to a loss of mass from the bag before it visually appears to let gas out when it pops.

It's also worth noting that although CO₂ is more dense than air, the concept of buoyancy doesn't apply here because the CO₂ isn't mixing with the air inside the bag until it escapes.

Therefore, the decrease in weight isn't due to a change in density or buoyancy of the gas within the bag but is solely due to the mass of CO₂ that leaves the bag as it sublimes.