**Ammonia and Its Properties**
Ammonia (NH3) is a highly important compound used in various industrial and household applications. It is a colorless gas with a pungent odor and is composed of one nitrogen atom bonded to three hydrogen atoms. Ammonia is highly soluble in water and forms a solution called aqueous ammonia.

**Acidity and Alkalinity**
Acidity and alkalinity are properties of substances that determine their ability to donate or accept protons (H+ ions). Acids are substances that donate protons, while bases are substances that accept protons. The pH scale is commonly used to measure the acidity or alkalinity of a substance, ranging from 0 (highly acidic) to 14 (highly alkaline).

**Action on Litmus Paper**
Litmus paper is a commonly used indicator to determine the acidity or alkalinity of a substance. It contains a dye that changes color depending on the pH of the solution it comes in contact with. Red litmus paper turns blue in the presence of alkaline solutions and remains red in acidic or neutral solutions.

**Dry Ammonia and Litmus Paper**
Dry ammonia gas, being a weak base, does not have any effect on litmus paper. It does not release hydroxide ions (OH-) in the absence of water, which are responsible for the alkaline properties of ammonia. Therefore, dry ammonia does not change the color of litmus paper, as it lacks the necessary hydroxide ions to react with the dye in the paper.

**Ammonia Solution and Litmus Paper**
In contrast, when ammonia gas dissolves in water, it forms an aqueous solution of ammonia. In this solution, ammonia molecules react with water molecules to produce hydroxide ions (OH-). The presence of hydroxide ions makes the solution alkaline and gives it the ability to turn red litmus paper blue.

**Explanation**
When ammonia gas is dissolved in water, it reacts with the water molecules according to the following equation:
NH3 + H2O → NH4+ + OH-
Here, ammonia (NH3) reacts with water (H2O) to form ammonium ions (NH4+) and hydroxide ions (OH-).

The hydroxide ions produced in the solution are responsible for the alkaline nature of the solution. They can accept protons from acidic solutions, effectively neutralizing them. When the red litmus paper comes in contact with the ammonia solution, the hydroxide ions react with the dye in the paper, causing the color to change from red to blue.

In summary, dry ammonia does not have any effect on litmus paper because it does not contain hydroxide ions. However, when ammonia is dissolved in water to form an aqueous solution, it produces hydroxide ions, giving the solution alkaline properties that can turn red litmus paper blue.