Chemistry Matching: Indicators and Reactions Explained

This question requires matching chemical substances and reactions (Column I) with their corresponding observations (Column II). Let's break down each match:

Column I Analysis

1. Phenolphthalein in $NaOH$:
   • Phenolphthalein is a common acid-base indicator.
   • It remains colorless in acidic solutions but turns pink in alkaline (basic) solutions.
   • Sodium hydroxide ($NaOH$) is a strong base.
   • Therefore, when phenolphthalein is added to $NaOH$, it will show a pink colour. This matches observation 'b'.
2. $CaCO_3$ + dilute acid:
   • $CaCO_3$ represents calcium carbonate, commonly known as limestone or chalk.
   • When calcium carbonate reacts with a dilute acid (e.g., hydrochloric acid, $HCl$), a chemical reaction occurs.
   • This reaction produces carbon dioxide gas ($CO_2$), water ($H_2O$), and a salt (e.g., calcium chloride, $CaCl_2$).
   • The evolution of $CO_2$ gas is a characteristic observation. This matches observation 'a'. The reaction can be represented as: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$
3. Neutralization reaction:
   • A neutralization reaction is a fundamental concept in acid-base chemistry.
   • It occurs when an acid and a base react with each other.
   • The typical products of a neutralization reaction are a salt and water.
   • For example, the reaction between $HCl$ (acid) and $NaOH$ (base) produces $NaCl$ (salt) and $H_2O$ (water): $HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$
   • This matches observation 'c'.

Final Matching Summary

Based on the analysis of each item:

• Phenolphthalein in $NaOH$ results in a pink colour (1 matches b).
• $CaCO_3$ + dilute acid leads to $CO_2$ gas evolution (2 matches a).
• A neutralization reaction produces salt + water (3 matches c).

Therefore, the correct matching is 1-b, 2-a, 3-c.