Nitrogen dioxide is a dark brown gas and is denser than air.

 

A gas jar containing nitrogen dioxide is sealed with a glass plate and is then inverted on top of a gas jar containing air.

 

 

The glass plate is removed. Which one of the following correctly describes the colours inside the gas jars after a long period of time?

 

A)  brown in both upper and lower jars

 

B)  dark brown in lower jar and colourless in upper jar

 

C)  dark brown in lower jar and light brown in upper jar

 

D)  light brown in lower jar and dark brown in upper jar

 

 

Answer: A

 

When gases are mixed, given sufficient period of time, a uniform/ homogeneous mixture will be obtained.

 

Common misconception: shouldn't the denser gas sink to the bottom jar?

 

In a gas, the particles are moving so rapidly and randomly that the effect of density is negligible.

 

Just imagine a chaotic crisis situation where everyone is running for their lives, a group of friends will not be able to stay together and become lost in the crowd.

 

HCl is very soluble in water, whereas Cl₂ is only slightly soluble in water. Both gases can be dried using concentrated sulfuric acid.

 

Which diagram represents the correct method of obtaining pure dry Cl₂ from a mixture of damp Cl₂ and HCl?

 

 

Answer: B

 

- For a gas to be passed through a solution/ drying agent, the in-tube must be submerged into the solution → C and D shows an incorrect setup.

 

- In the correct setup (B), the mixture of gases is passed into water → HCl is dissolved and only Cl₂ is passed into the 2nd tube.
In the 2nd tube, damp Cl₂ is passed into conc. sulfuric acid (drying agent) → water is removed and dry Cl₂ is obtained.

 

- Setup in A is erroneous as the gas after drying with conc. sulfuric acid is passed into water again.