# Why does the gas from dry ice flow down?

I am reading Dan Broughton's answer in replying to the question "why does the gas from dry ice flow down" here.

According to the answer, the fog is water's color and water is condensed and suspended in the CO2. Do you agree with this answer? If so then should water have color? I see some documents saying that it is nearly colorless.

Why does the gas from dry ice flow down?

Dry ice is simply the common term used for solid carbon dioxide (CO2), so the gas coming off of the dry ice is CO2 gas. CO2 gas has a density of 1.96 kg/m^3 and air (78% N, 21% O2) is 1.29 kg/m^3. Since the CO2 is more dense, it sinks in air.

But CO2 is colorless, so what makes it appear as a white mist? As the gas sublimes (change directly from solid to gas), it comes off of the dry ice at -78.5 C (-109.2 F). Being well below the freezing point of water, any moisture in the air surrounding the dry ice condenses and is suspended in the CO2, creating the cloud/fog effect that we can see.

These two phenomena combined are what create the fog that flows along the ground.

• Well, yes, the answer is mostly correct. Water doesn't have color, nor does the fog. – Ivan Neretin Apr 15 '18 at 22:42
• Also, being colder and denser, even a plain air fog coming off a cold object (e.g. dry ice in a plastic bag, vented away from the experimenter) will sink. – DrMoishe Pippik Apr 15 '18 at 22:58
• Colorless as in invisible as gas at all. – Mithoron Apr 15 '18 at 23:17
• The quoted answer is correct. It just omits to mention scattering and reflection from water/ice minute particle. This is the core of your question. Why cola is dark and its foam is white is another question related to this, just in this latter example both forms are visible. – Alchimista Apr 16 '18 at 8:30

The gas flows downwards because it is more dense. It is more dense both because $\ce{CO2}$ is heavier than $\ce{N2}$ and $\ce{O2}$ and because the gas is colder than the ambient air.