Chapter 2 · Question 3

How do metal carbonates and metal hydrogencarbonates react with acids? Describe with the examples of sodium carbonate and sodium hydrogencarbonate reacting with dilute HCl. How is the evolved gas identified?

Answer Revealed

Direct Answer:

Metal carbonates and hydrogencarbonates react with acids to produce the corresponding salt, carbon dioxide gas, and water. Sodium carbonate with HCl:

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

. Sodium hydrogencarbonate with HCl:

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

. The evolved CO₂ is identified by passing it through lime water (

\text{Ca(OH)}_2

solution), which turns milky due to the formation of insoluble calcium carbonate:

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

Simple Explanation

When you add acid to baking soda (sodium hydrogencarbonate) or washing soda (sodium carbonate), the mixture fizzes as carbon dioxide gas bubbles out. Both produce the same salt (sodium chloride, common table salt), water, and CO₂. To confirm the gas is CO₂, bubble it through lime water — a clear liquid that turns milky white. That milkiness is caused by tiny particles of calcium carbonate (chalk) forming. If you keep bubbling CO₂ for too long, the milkiness actually disappears because the chalk dissolves again.

Exam-Ready Structure

The reaction of metal carbonates and hydrogencarbonates with acids is a key chemical property of acids, demonstrated through Activity 2.5 in the NCERT textbook. 1. General reaction: Metal carbonate/hydrogencarbonate + Acid

\rightarrow

Salt + Carbon dioxide + Water. Carbon dioxide is always a product, which can be confirmed by the lime water test. 2. Activity 2.5 setup: Two test tubes A and B are labelled. About 0.5 g of sodium carbonate (

\text{Na}_2\text{CO}_3

) is added to test tube A, and about 0.5 g of sodium hydrogencarbonate (

\text{NaHCO}_3

) is added to test tube B. About 2 mL of dilute HCl is added to each tube. 3. Observations: Effervescence (fizzing) in both tubes due to the evolution of CO₂ gas. The gas is passed through lime water using a delivery tube, as illustrated in Figure 2.2. 4. Reactions: - Sodium carbonate:

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

- Sodium hydrogencarbonate:

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

5. Lime water test for CO₂:

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

. The lime water turns milky due to the formation of white, insoluble calcium carbonate. 6. Important extension: On passing excess CO₂ through the milky solution, the milkiness disappears because calcium carbonate reacts with excess CO₂ and water to form soluble calcium hydrogencarbonate:

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

. Limestone, chalk, and marble are all different forms of calcium carbonate.

Key Points

Metal carbonate + acid $\rightarrow$ salt + CO₂ + water; same for hydrogencarbonate
Na₂CO₃ + 2HCl $\rightarrow$ 2NaCl + H₂O + CO₂; NaHCO₃ + HCl $\rightarrow$ NaCl + H₂O + CO₂
Lime water test: CO₂ turns lime water milky — $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$
Excess CO₂ dissolves the milkiness: $\text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca(HCO}_3)_2$
Limestone, chalk, and marble are different forms of calcium carbonate

Common Mistakes

Confusing the lime water test (CO₂) with the pop sound test (H₂) — each gas has a specific confirmatory test
Writing the product as sodium oxide instead of sodium chloride — the salt contains the anion of the acid

How do metal carbonates and metal hydrogencarbonates react with acids? Describe with the examples of sodium carbonate and sodium hydrogencarbonate reacting with dilute HCl. How is the evolved gas identified?