Litmus test question?

Question

Why does the mixture of ammonium nitrate with aqueous sodium hydroxide need warming? And why must the litmus paper be moist?

Answer 1

Actually, the reaction goes quite nicely at room temperature.

NH4+ + NO3- + Na+ + OH- --> NH3 + HOH + Na+ + NO3-

As you can see sodium ion and nitrate ion did nothing.  The net ionic equation is:

NH4+ + OH- <==> NH3 + HOH

Actually its an equilibrium.  But in this case the equilibrium lies far to the right, meaning that it forms a lot of ammonia gas (which is dissolved in water) and water.

The solution is warmed so that the solubility of ammonia gas is decreased.  The solubility of gases is the reverse of most solids.  As T increases solubility of gases decrease.  By warming the solution, more NH3 comes out of solution and you can test it (and smell it) with the wet litmus paper.  The NH3 will dissolve in the water to produce a small amount of OH-.  

NH3 + HOH <==> NH4+ + OH-

As mentioned above, the equilibrium (in this case, it is the reverse) will lie far to the left and a relatively small amount of OH- will be produced, but it will be enough to turn litmus blue.

Answer 2

The mixture needs warming because it requires greater energy than is available to it at room temperature for the reaction betwenn the substances it contains to occur. By heating the substances, you add energy to the system and thus 'allow' the reaction to take place.

The litmus must be moist because ammonia (a GAS) will be produced in the reaction and must therefore be dissolved in liquid before it will be able to change the litmus paper's colour. (As you probably know, ammonia is highly soluble in water).