A student takes four test tubes marked a, b, c and d containing NaCl, Na₂CO₃, NaHCO₃ and NaOH respectively. He adds acetic acid to the test tubes one by one and every time he immediately brings a lighted match-stick near to the mouth of the test tube. He observes that the match-stick extinguishes in the cases of

When acetic acid ($CH_3COOH$) is added to the test tubes:

1. Test tube a ($NaCl$): No chemical reaction occurs that produces a gas. Thus, the lighted match-stick continues to burn.
2. Test tube b ($Na_2CO_3$): Acetic acid reacts with sodium carbonate to produce sodium acetate, water, and carbon dioxide ($CO_2$) gas:

$$2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2\uparrow$$

1. Test tube c ($NaHCO_3$): Acetic acid reacts with sodium hydrogencarbonate to produce sodium acetate, water, and carbon dioxide ($CO_2$) gas:

$$CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2\uparrow$$

1. Test tube d ($NaOH$): A simple neutralization reaction occurs between the acid and the base, producing salt and water without releasing any gas:

$$CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$$

Since carbon dioxide ($CO_2$) is a non-combustible gas and does not support combustion, it extinguishes the lighted match-stick brought near the mouths of test tubes b and c.

• Option A (a and b) is incorrect because $NaCl$ in test tube a does not react to release $CO_2$.
• Option C (c and d) is incorrect because the reaction with $NaOH$ in test tube d does not produce any gas.
• Option D (d and a) is incorrect because neither test tube d nor test tube a produces $CO_2$ gas.