Understanding the Reaction: Washing Soda and Hydrochloric Acid

Let's explore what happens when washing soda (sodium carbonate) reacts with dilute hydrochloric acid.

Identifying the Reactants

• Washing Soda: This is chemically known as sodium carbonate. Its chemical formula is usually written as $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$ for the decahydrate form, or just $\text{Na}_2\text{CO}_3$ for the anhydrous form. It is a base-like salt.
• Dilute Hydrochloric Acid: This is a strong acid with the chemical formula $\text{HCl}$.

The Chemical Reaction

When an acid reacts with a carbonate, it typically produces a salt, water, and carbon dioxide gas. This is a classic type of reaction in chemistry.

The balanced chemical equation for the reaction between sodium carbonate and hydrochloric acid is:

\(\text{Na}_2\text{CO}_3\text{(aq)} + 2\text{HCl}\text{(aq)} \rightarrow 2\text{NaCl}\text{(aq)} + \text{H}_2\text{O}\text{(l)} + \text{CO}_2\text{(g)}\)

Observing the Products

Based on the chemical equation, the products are:

• Sodium Chloride ($\text{NaCl}$): This is a salt, commonly known as table salt. In this reaction, it will dissolve in the water, forming an aqueous solution.
• Water ($\text{H}_2\text{O}$): This is a liquid, also present in the dilute acid and released during the reaction.
• Carbon Dioxide ($\text{CO}_2$): This is a gas.

What We Observe

The formation of carbon dioxide gas ($\text{CO}_2$) is easily observable. When $\text{CO}_2$ is produced rapidly in a liquid, it causes bubbles to form and rise. This rapid bubbling is called effervescence. Since the reaction between a strong acid like $\text{HCl}$ and a carbonate like $\text{Na}_2\text{CO}_3$ is vigorous, the effervescence is brisk.

Analyzing the Options

Let's look at why the other options are incorrect:

• Option 1: White-coloured precipitate is formed. The salt formed, sodium chloride ($\text{NaCl}$), is highly soluble in water. Therefore, no white precipitate is formed in this reaction.
• Option 3: $\text{H}_2$ gas comes out with pop sound. Hydrogen gas ($\text{H}_2$) is typically produced when reactive metals react with acids (e.g., Zinc with $\text{HCl}$). The test for hydrogen gas is that it burns with a 'pop' sound. This reaction, involving a carbonate and an acid, produces carbon dioxide ($\text{CO}_2$), not hydrogen ($\text{H}_2$).
• Option 4: Only neutralisation reaction occurs, no gas is released. Carbonates are considered basic salts, and their reaction with acids is a form of neutralisation. However, this specific type of neutralisation reaction involving carbonates (or bicarbonates) always produces carbon dioxide gas, unlike the neutralisation reaction between a simple acid and a simple base (like $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$). Gas is definitely released.

Therefore, the correct observation is the release of $\text{CO}_2$ gas with brisk effervescence.

Revision Table: Reactions of Carbonates with Acids

Additional Information on Effervescence and Gas Tests

Effervescence: This term specifically describes the escape of gas dissolved in a liquid or produced during a reaction in solution. It's the visible bubbling you see, like in a fizzy drink.

Test for Carbon Dioxide ($\text{CO}_2$): The presence of carbon dioxide gas can be confirmed by passing the gas through lime water (a solution of calcium hydroxide, $\text{Ca(OH)}_2$). The lime water turns milky or cloudy due to the formation of insoluble calcium carbonate ($\text{CaCO}_3$).

\(\text{CO}_2\text{(g)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O}\text{(l)}\)

If excess carbon dioxide is passed, the milkiness disappears as the insoluble calcium carbonate converts to soluble calcium bicarbonate ($\text{Ca(HCO}_3\text{)}_2$).

\(\text{CaCO}_3\text{(s)} + \text{H}_2\text{O}\text{(l)} + \text{CO}_2\text{(g)} \rightarrow \text{Ca(HCO}_3\text{)}_2\text{(aq)}\)