What Is Dry Ice and Why Does It Make Fog?

It looks like magic. It makes spooky fog that pours down instead of floating up. And it never leaves a puddle. So what is dry ice actually doing?

Dr Matt Agnew unpacks the weird science of dry ice, what sublimation is, why the fog pours downward like a waterfall, and why dry ice behaves so completely differently from the ice in your freezer.

What you'll learn:

• Normal ice is frozen water... dry ice is frozen carbon dioxide, the gas we breathe out
• Dry ice skips the liquid step entirely... straight from solid to gas with no puddle
• Scientists call that process sublimation, one of the most interesting phase changes in science
• The fog is mostly not carbon dioxide at all... it's water vapour from the surrounding air getting chilled so fast it becomes visible
• Cold air and carbon dioxide are both heavier than regular warm air, so the fog pours downward like a fog waterfall
• Dry ice is essentially a natural fog machine used in theatres, film sets and Halloween displays worldwide

Key Science Ideas:

• Sublimation: When a solid skips the liquid stage and goes straight to gas
• Carbon dioxide: The gas we breathe out, and what dry ice is made of
• Water vapour: Invisible water in the air that becomes visible when it gets suddenly cold
• Phase change: When matter moves between solid, liquid and gas states
• Density: Why cold air and carbon dioxide both sink rather than rise

Safety note: Dry ice is extremely cold and must only be handled by adults wearing insulated gloves. It should only be used in well ventilated spaces. This is a watch and learn episode.

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