Question

Which of the following will not be stable in acid?

• A. $NaHCO_3$ and $Na_2CO_3$
• B. NaOH and $NaHCO_3$
• C. NaOH and $Na_2CO_3$

Answer 1

Answer: (A)

1

Answer 2

The term "stable in acid" means that the substance does not undergo a chemical reaction when mixed with an acid. Conversely, "not stable in acid" means the substance reacts with acid.

Let's analyze the stability of each compound in the presence of an acid:

1. Sodium Bicarbonate ($NaHCO_3$): This is a salt of a weak acid (carbonic acid, $H_2CO_3$) and a strong base (sodium hydroxide, $NaOH$). When $NaHCO_3$ comes into contact with an acid, it reacts to produce carbonic acid, which is unstable and quickly decomposes into water and carbon dioxide gas. The reaction is: $NaHCO_3(aq) + H^+(aq) \rightarrow Na^+(aq) + H_2CO_3(aq)$ $H_2CO_3(aq) \rightarrow H_2O(l) + CO_2(g)$ Overall: $NaHCO_3(aq) + H^+(aq) \rightarrow Na^+(aq) + H_2O(l) + CO_2(g)$ Since it reacts and produces gas, $NaHCO_3$ is not stable in acid.

2. Sodium Carbonate ($Na_2CO_3$): Similar to $NaHCO_3$, $Na_2CO_3$ is also a salt of carbonic acid and a strong base. It reacts with acids to produce carbon dioxide gas. The reaction is: $Na_2CO_3(aq) + 2H^+(aq) \rightarrow 2Na^+(aq) + H_2CO_3(aq)$ $H_2CO_3(aq) \rightarrow H_2O(l) + CO_2(g)$ Overall: $Na_2CO_3(aq) + 2H^+(aq) \rightarrow 2Na^+(aq) + H_2O(l) + CO_2(g)$ Since it reacts and produces gas, $Na_2CO_3$ is not stable in acid.

3. Sodium Hydroxide (NaOH): This is a strong base. It readily reacts with acids in a neutralization reaction to form salt and water. The reaction is: $NaOH(aq) + H^+(aq) \rightarrow Na^+(aq) + H_2O(l)$ Since it reacts, NaOH is not stable in acid.

Now, let's evaluate the given options based on this understanding:

• 1) $NaHCO_3$ and $Na_2CO_3$: Both $NaHCO_3$ and $Na_2CO_3$ react with acids, characteristically producing $CO_2$ gas. Therefore, this pair will not be stable in acid.
• 2) NaOH and $NaHCO_3$: Both NaOH and $NaHCO_3$ react with acids. Therefore, this pair will not be stable in acid.
• 3) NaOH and $Na_2CO_3$: Both NaOH and $Na_2CO_3$ react with acids. Therefore, this pair will not be stable in acid.

All three options contain compounds that are not stable in acid. However, in multiple-choice questions, if multiple options seem literally correct, one often looks for the "best" or "most characteristic" answer. The reaction of carbonates and bicarbonates with acid to produce $CO_2$ gas (effervescence) is a very common and characteristic indicator of their instability in acid. Option 1 presents a pair where both compounds share this specific type of instability (gas evolution).