Why must litmus paper be wet before being dipped in dry gases?

Jerrycurl12

6 years ago

Dry litmus paper has no water content in it. So, when we take the pH of the solution, the water is not in contact with the solution whose pH is no be measured.
Moist litmus paper has water content in it. The litmus paper is moistened with water before taking its pH.The moist litmus paper is then dipped in the solution to take its pH.
The difference is best understood with an example:
HCl is not acidic in the gaseous form. HCl is dissociated in the form of H+ and Cl​- to behave as an acid. So, when we take the pH of dry gas, it will not be acidic. But when it comes in contact with water, it behaves as acid.

Aliyuus2

6 years ago

You are talking about a moist red litmus paper that is used to detect ammonia gas?

Moist means to provide water as a medium.

Ammonia itself does not exhibit alkaline properties since it has not been ionized yet. So you need to provide water for the following ionization:

NH3+H2O → NH4+ + OH-

That hydroxide ion is for the alkaline properties, so we need to use moist litmus paper to test for alkalinity or acidity.

Nne_chi

6 years ago

Litmus is an indicator.For solutions,we just need to dip the litmus paper into the solution as there is an abundance of water.For gases,the gases need to dissolve in the water already on the damp litmus paper to cause the H+ concentration to change, triggering the color change,if any.

Modudharniel16

6 years ago

Generall