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Name: Kassie
Status: student
Grade: 6-8
Location: OR
Country: N/A
Date: October 2006

Question:
If the gas from dry ice is colorless and clear, why does it form clouds of white vapor, especially when you blow on it?



Replies:
Kassie, The gas from dry ice is carbon dioxide. Dry ice is solid carbon dioxide and is converted directly into gas without going through a liquid phase in a process called sublimation. The cloud of vapor that you see when you blow on the dry ice is actually water vapor that is condensing into fog. This happens because the -78 degree C dry ice is converted into still very cold gaseous carbon dioxide. It is still colder than the condensation point of water, so the humidity in the air is turned into, effectively, a cloud.

Matt Voss


Kassie,

What we think of as "air" is actually a combination of many different gases. One of the gases in air is water vapor. Quite a bit of water vapor can be held in gaseous form if the temperature is high. However, when the temperature drops, some of the water vapor will be converted to liquid form, come together, and form small droplets suspended in air. These droplets tend to reflect or bend light a lot more than gaseous water vapor, and we see them as a white smokey substance. So, it is not the dry ice that is turning smokey white, but rather the air around it that has been made cold because of the dry ice.

Your exhaled breath also has a lot of water vapor. Coming out of your lungs, this water vapor is generally warm and can stay in a gaseous form. However, when it gets cold, the water vapor rapidly becomes liquid and the droplets of liquid water form. This is why you can see your breath in winter, and this is why blowing on dry ice makes a lot more smokey white substance appear.

Greg (Roberto Gregorius)


What you see is water vapor condensed as a "fog" by the cold carbon dioxide evaporating from the "dry ice". You will notice that the "cloud" tends to hug the ground. This is because the cold carbon dioxide is denser than the surrounding warmer air.

Vince Calder


Dry ice is very cold. The gas coming from it is still very cold and causes any water vapor it comes in contact with to condense into tiny water droplets -- essentially clouds.

Your breath is warm and moist. When warm, moist air from your mouth meets with the cold carbon dioxide gas a LOT of water vapor forms clouds.

Greg Bradburn



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