Your dry ice appears to float because it's actually skimming across the surface of the water on a cushion of its own gas. Think of it like a tiny, cold hovercraft!
Here's a breakdown of why this happens:
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Sublimation: Dry ice is solid carbon dioxide (CO2). Unlike regular ice (frozen water), it doesn't melt. Instead, it sublimates, meaning it transforms directly from a solid into a gas.
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Gas Layer Formation: When dry ice is placed in water (especially warm water), the sublimation process speeds up dramatically. This rapid sublimation creates a layer of gaseous CO2 between the dry ice and the surface of the water.
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Reduced Friction: Hovercraft Effect: This layer of CO2 gas reduces friction between the solid dry ice and the liquid water. The dry ice then skims or glides across the surface, seemingly "floating." It behaves similarly to a hovercraft that rides on a cushion of air. Because it is never completely still on top of the water due to this effect, the dry ice will appear to "vibrate."
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Density Considerations: While dry ice is denser than water, the effect of the gas layer lifting it reduces the contact area and overcomes the density difference, allowing it to skim. If the sublimation slowed down enough for a prolonged period of time, or the contact area grew significantly, then the dry ice would eventually sink.
In essence, the escaping gas from the bottom of the dry ice creates a barrier, allowing it to skim across the water's surface with very little friction, giving the illusion that it's floating.
