Identifying the Incorrect Chemical Statement

We are asked to identify the incorrect statement among the given options regarding the properties of substances and pH values.

Let's analyze each statement carefully:

Analysis of Statement (I): Bleaching powder is a pale yellow powder.

Bleaching powder, chemically known as Calcium Oxychloride ($\text{CaOCl}_2$), is generally a pale yellow powder with a strong smell of chlorine. This statement aligns with the common description of bleaching powder.

Conclusion for Statement (I): This statement is generally considered correct.

Analysis of Statement (II): Dry $\text{NH}_3$ gas turns red litmus blue.

Ammonia ($\text{NH}_3$) is a basic gas. Basic substances turn red litmus paper blue. However, for litmus paper to change color, the substance must be in a dissolved state (usually in water) because the color change reaction involves the ions of the substance interacting with the indicator. Dry gases do not contain ions that can interact with the dry litmus paper. Therefore, dry $\text{NH}_3$ gas will not cause a color change in dry red litmus paper. Moisture is essential for the reaction to occur.

Conclusion for Statement (II): This statement is incorrect because dry ammonia gas requires moisture to show its basic nature on litmus paper.

Analysis of Statement (III): The pH of rainwater is nearly 7.

Pure water has a pH of 7. However, natural rainwater is not pure. It dissolves carbon dioxide from the atmosphere, forming carbonic acid ($\text{H}_2\text{CO}_3$), which is weakly acidic. This makes natural rainwater slightly acidic, with a typical pH value around 5.6. A pH of nearly 7 would imply neutral water.

Conclusion for Statement (III): Factually, this statement is generally considered incorrect as typical rainwater pH is around 5.6.

Analysis of Statement (IV): The pH of acid rain is nearly 5.6.

Acid rain is defined as precipitation (rain, snow, fog, etc.) that is significantly more acidic than normal rainwater. Normal rainwater has a pH of about 5.6 due to dissolved $\text{CO}_2$. Acid rain is typically defined as having a pH lower than 5.6, often ranging between 4 and 5, but can be much lower. Therefore, saying the pH of acid rain is nearly 5.6 is incorrect; 5.6 is the threshold below which rain is considered acidic, not the typical pH of acid rain itself.

Conclusion for Statement (IV): Factually, this statement is generally considered incorrect as acid rain pH is typically below 5.6.

Summary and Final Determination

Based on our analysis, statements (II), (III), and (IV) appear incorrect based on scientific facts. However, statement (II) is unequivocally incorrect because the reaction with litmus requires moisture, which is absent in dry $\text{NH}_3$ gas. The inaccuracy of statements (III) and (IV) depends on the interpretation of "nearly". Given the common understanding of the litmus test and the pH of dry gases, statement (II) is the most definitively incorrect statement among the options.

Therefore, the incorrect statement is (II).

Let's look at the options based on this conclusion:

• Option 1: All of the above statements are correct (Incorrect, as at least II is incorrect).
• Option 2: I), II) and III) (Incorrect, as I is correct and it doesn't just include II).
• Option 3: Only II) (This aligns with our conclusion that only statement II is incorrect).
• Option 4: I), II) and IV) (Incorrect, as I is correct and it doesn't just include II).

Thus, the statement that is INCORRECT is statement (II).

Revision Table: Key Concepts Revisited

Additional Information on Acid-Base Properties and pH

Acids and bases are fundamental chemical substances. Acids release hydrogen ions ($\text{H}^+$) in water, while bases release hydroxide ions ($\text{OH}^-$) or accept $\text{H}^+$ ions.

• pH: The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It is defined as the negative logarithm of the hydrogen ion activity ($a_{\text{H}^+}$), often approximated as the molar concentration of $\text{H}^+$ ions: $\text{pH} = -\text{log}_{10} [\text{H}^+]$.
• Litmus Paper: Litmus paper is a simple indicator. Red litmus paper turns blue in basic solutions, and blue litmus paper turns red in acidic solutions. It is derived from lichens. The color change is a chemical reaction between the indicator molecules and the $\text{H}^+$ or $\text{OH}^-$ ions present in the solution. If water is absent, ionization often doesn't occur sufficiently to cause a noticeable color change.
• Acid Rain Formation: Acid rain is primarily caused by emissions of sulfur dioxide ($\text{SO}_2$) and nitrogen oxides ($\text{NO}_x$) from burning fossil fuels. These pollutants react with water molecules, oxygen, and other chemicals in the atmosphere to form sulfuric and nitric acids. These acids then mix with precipitation and fall to the ground as acid rain.