Understanding the Action of Dry HCl Gas on Litmus Paper

The question asks about the effect of dry hydrochloric acid gas (HCl gas) on dry litmus paper. To answer this, we need to understand what makes an acid acidic and how litmus paper works as an indicator.

What is Litmus Paper?

Litmus paper is a type of pH indicator. It is commonly used to test whether a solution is acidic or basic. There are two main types:

• Blue litmus paper: Turns red in acidic solutions.
• Red litmus paper: Turns blue in basic solutions.

In neutral solutions, neither blue nor red litmus paper changes colour.

Why is HCl Gas Acidic?

Hydrochloric acid is known to be a strong acid. However, its acidic properties are only exhibited when it is dissolved in water. In the presence of water, HCl gas undergoes ionization:

$\text{HCl(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_3\text{O}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

or more simply represented as:

$\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

The acidity comes from the presence of hydrogen ions ($\text{H}^+$) or hydronium ions ($\text{H}_3\text{O}^+$) in the aqueous solution. These ions react with the litmus indicator, causing the colour change.

Effect of Dry HCl Gas on Dry Litmus Paper

The question specifically mentions dry HCl gas and dry litmus paper. For HCl gas to show acidic properties, it needs to ionize, which requires water. Since both the HCl gas and the litmus paper are dry, there is no water present for the HCl gas to ionize and produce $\text{H}^+$ ions.

Without the presence of $\text{H}^+$ ions, the dry HCl gas does not behave as an acid and therefore cannot react with the indicator in the dry litmus paper to cause a colour change.

Think of it this way: the acidic part of HCl is hidden when it's just the dry gas molecule. Water is like the key that unlocks its acidic behavior by breaking it apart into ions.

Analyzing the Options

Let's look at the given options in light of this understanding:

1. Blue litmus paper turns red: This would happen if the HCl was acting as an acid (i.e., if water were present). Since it's dry, this is incorrect.
2. Blue litmus paper turns white: Litmus paper does not typically turn white in acidic or basic conditions. This is incorrect.
3. Blue or red litmus paper does not change its colour: This aligns with our understanding that dry HCl gas does not exhibit acidic properties in the absence of water, and thus cannot change the colour of dry litmus paper.
4. Red litmus paper turns blue: This would happen if the substance were basic. HCl is an acid (when in water), not a base. This is incorrect.

Therefore, the correct action of dry HCl gas on dry litmus paper is no colour change.

Revision Table: Dry HCl Gas and Litmus Action

Additional Information on Acids, Bases, and Indicators

• Acids: Substances that produce $\text{H}^+$ ions (protons) when dissolved in water (Arrhenius definition). They taste sour and can corrode materials. Examples: $\text{HCl}$, $\text{H}_2\text{SO}_4$, $\text{HNO}_3$.
• Bases: Substances that produce $\text{OH}^-$ ions (hydroxide ions) when dissolved in water (Arrhenius definition). They taste bitter and feel soapy. Examples: $\text{NaOH}$, $\text{KOH}$, $\text{Ca(OH)}_2$.
• Indicators: Substances that change colour depending on whether they are in an acidic, basic, or neutral environment. Litmus is a common example. Others include phenolphthalein, methyl orange, etc.
• Importance of Water: Many substances behave differently in their dry state compared to when they are in an aqueous solution. Ionic compounds often need to be dissolved to conduct electricity, and many acids require water to exhibit their acidic properties due to the need for ionization.

This concept highlights the crucial role of water in allowing substances like HCl to demonstrate their characteristic chemical properties.