Strong acids can be distinguished from weak acids by any of the following methods, EXCEPT

Analysis

The question asks us to identify which of the listed methods cannot be used to distinguish between a strong acid and a weak acid. To solve this, we must understand the fundamental difference between strong and weak acids.

Concept Explanation

• Strong Acids: These dissociate (ionize) completely in water to produce a high concentration of hydrogen ions ($H^+$). Examples include $HCl$, $H_2SO_4$, and $HNO_3$.
• Weak Acids: These only partially dissociate in water, resulting in a lower concentration of hydrogen ions. Examples include $CH_3COOH$ (ethanoic acid).

Why the methods work (or don't):

• Conductivity (A): Strong acids have more ions in solution, so they conduct electricity much better than weak acids of the same concentration.
• pH Measurement (C): Since pH is defined as $-\log[H^+]$, the higher concentration of $H^+$ in a strong acid results in a lower pH compared to a weak acid of the same concentration.
• Heat of Reaction (D): The heat of neutralization for a strong acid with a strong base is constant (approx. $-57.3 \text{ kJ/mol}$). However, for a weak acid, some energy is consumed to complete the dissociation of the acid, making the heat of reaction different.

Step-by-Step Evaluation

• Option B (Litmus paper): Litmus paper is an indicator used to determine if a substance is acidic or basic. Both strong acids and weak acids will turn blue litmus paper red. Because both types of acids produce the same color change, litmus paper cannot be used to differentiate the strength of the acids.

Conclusion

While conductivity, pH, and heat of reaction are quantitative measures that depend on the degree of dissociation (and thus distinguish strength), litmus paper is a qualitative test that only identifies acidity, not the degree of ionization.

Exam Tip

When you see questions about "distinguishing" substances, always ask yourself: "Does this method provide a measurement of concentration or degree of ionization?" If the method only gives a "Yes/No" (like litmus paper), it usually cannot distinguish between degrees of strength.