Question

Why is solid carbon dioxide called dry ice?
(A) When heated it changes into liquid
(B) When heated it does not changes to gas
(C) When heated it does not change to liquid but directly changes into gas.
(D) When heated no changes occurs

Answer 1

Hint: We should be aware of certain uses of solid carbon dioxide which may help us in remembering and understanding it better. It is also used as a cooling agent, used to preserve food for a long time etc.

Complete step by step answer:
The dry ice is a cold white dense mist produced by solid carbon dioxide in air. It is primarily used as a cooling agent because its temperature is lower than that of water and mainly it does not leave behind the residue. This makes something special than normal ice as it leaves behind residue on foods or things. The solid carbon dioxide that is dry ice is used widely for the purpose of preserving food especially when mechanical cooling is not available.
Let's first understand the process of sublimation.
The process of conversion of a solid to gas is known as sublimation. When the atmospheric pressure is less than the vapour pressure of the compound, this does not suit the condition of melting of solid but this more likely suits the boiling point of anything. That is we can adjust the boiling point by changing the atmospheric pressure but this does not work the same in case of melting point. At low temperature, for certain substances the boiling point might move to a temperature that is lower than melting point, here where sublimation comes into picture.
Over certain range of temperature and pressure the carbon dioxide remains as liquid that is between $-{{54}^{\circ }}C$ to\[-{{30}^{\circ }}C\] at $1Pa$ to $1000Pa$. At $-{{54}^{\circ }}C$ in 5.3atm pressure that is at high temperature the carbon dioxide solidifies. Dry ice sublimes at $-{{78}^{\circ }}C$ that is in terms of kelvin is 194.65K in earth atmosphere pressure. Sublimes means, on heating it does not change to liquid instead directly converted to gas. This is the reason why solid carbon dioxide is called dry ice.

Thus, option C is the correct answer.

Note: As it sublimes it is used as a cooling agent. There is no point where it melts to liquid so it is used to preserve food especially when the mechanical cooling is not available. When the atmospheric pressure is less than the vapour pressure of the compound here where the sublimation takes place.