The Chemistry of Dry Ice

 

††††††††††† Thereís more than meets the eye when dry ice is added to water. Within a certain pressure and temperature range, solid carbon dioxide (dry ice) does not melt. Instead it sublimates or passes directly to the gas phase.

††††††††††† If a small amount of a pH indicator known as bromothymol blue is added to tap water, a blue-green colour results. (see picture to the left). When dry ice is added, a significant amount of carbon dioxide dissolves in water, creating carbonic acid.

H2O + CO2 - ->H2CO3 .The indicator soon responds to the presence of H+; the indicatorís molecular structure changes so that its electron energy levels are no longer the same, altering its response to light. To make a long story short, it reflects different a color, as shown to the right and in the picture below.

Why is there a cloud above the water and around the beaker. Isnít carbon dioxide gas invisible? It is, but the carbon dioxide gas molecules are cold enough to cause the water vapour in the air to condense into very small droplets. Hence, we have a cloud. Although dry ice is at a temperature of

-78oC, cold enough to cause major frostbite, the cloud itself seems to be at room temperature. The reason for this is that although sublimation is an endothermic process, the condensation of water is an exothermic one, so overall the environment experiences very little temperature change. This of course implies that with the amount of dry ice weíre using and with the amount of condensation we are causing, the DHís cancel out, and none of our students below are shivering.

††††††††††† Also notice that the cloud tends to flow over the top and then down the beaker. Why?

CO2's molar mass is 44g/mole. For ideal gases, density is directly proportional to molar mass. Since air (78% N2, 21% O2 and 1% mix of Ar and H2O) has an average molar mass of only [0.78(28) + 0.21(32 + 0.005(40) + 0.005*(18) ] =29 g/mole, it allows CO2 to sink.

††††††††††† Another unexpected result occurs when dry ice is added to limewater [Ca(OH)2]. Because of the large amount of CO2 that is dissolved in water upon sublimation, the initial reaction that causes cloudiness:

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††††††††††††††††††††††††††††††††††† CO2 + Ca(OH)2 - -> CaCO3(s) + H2O, does not end the story. CO2 in water, as mentioned earlier, forms carbonic acid. This eventually dissolves the CaCO3, which caused the cloudiness, so eventually the solution becomes clear again:

††††††††††††††††††††††††††††††††††† H2CO3(aq) + CaCO3(s) - -> Ca(HCO3)2(aq)