Double Displacement That Produces a Gas

Introduction

An acid and a carbonate often produce bubbles of carbon dioxide along with a salt and water. The full equation can be related to double displacement, but the gas-forming step makes the chemistry more than a static exchange of ion partners. Correct products and coefficients are especially important because three products must be accounted for.

Core explanation

For calcium carbonate and dilute hydrochloric acid, the balanced equation is CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). Calcium pairs with chloride in the salt. Two acid hydrogens and the carbonate oxygen atoms give water and carbon dioxide. Count Ca 1, C 1, O 3, H 2 and Cl 2 on each side. The gas label (g) explains bubbling and possible mass loss from an open vessel.

At a formula-pattern level, acid H⁺ meets carbonate to form carbonic acid, while the metal cation pairs with the acid anion. Carbonic acid can then break down: H₂CO₃ → H₂O + CO₂. Combining the steps gives the familiar three-product equation. This is why acid-carbonate chemistry is often taught near double displacement but also includes a decomposition-like gas-evolution step.

The essential ionic equation for a carbonate is CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g) in the simple strong-acid aqueous representation. Left charge −2 + 2(+1) = 0, matching the neutral products. Left atoms are C 1, O 3 and H 2; right CO₂ and H₂O contain the same. For solid CaCO₃, the net ionic equation must retain the solid as a reactant: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g). Do not split CaCO₃(s) into aqueous ions as though it were already dissolved.

Sodium carbonate in solution reacts with hydrochloric acid as Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g). Here the carbonate starts dissolved, so the first ionic form applies after sodium and chloride spectators cancel. Both calcium and sodium examples give the same CO₂ and water products but different salts and initial carbonate states.

Hydrogencarbonate, HCO₃⁻, is related but has a different hydrogen count. Sodium hydrogencarbonate plus HCl is NaHCO₃ + HCl → NaCl + H₂O + CO₂, balanced one-to-one. Do not use a coefficient two for HCl by copying the carbonate-ion equation; HCO₃⁻ already contains one hydrogen.

Bubbles alone do not identify CO₂, because many reactions and physical changes can release gas. The reactants and an appropriate gas test support identification. In an open system, the remaining liquid-and-solid mass may fall as CO₂ leaves, while the total mass of all products remains conserved.

Step-by-step reasoning

1. Identify carbonate or hydrogencarbonate and the acid-derived salt anion. 2. Build the salt formula from the metal cation and acid anion. 3. Add water and CO₂, then balance acid hydrogens and all other atoms. 4. Use states to distinguish solid from dissolved carbonate and select the corresponding net ionic form.

Visual explanation

Draw a carbonate group meeting two acid hydrogen counters. One O joins the two H to make H₂O; the C and remaining two O form CO₂ bubbles. The metal counter pairs with the acid anion in the dissolved salt.

Real-world analogy

Two groups exchange members, but one newly formed group immediately separates into a liquid part and a gas part. The final result has more products than a simple two-pair swap. The intermediate carbonic acid helps explain this extra step.

Real-world example

An acid spill on limestone can produce carbon dioxide effervescence and dissolved calcium salt under suitable conditions. The chemical equation accounts for the gas and material changes. The surface area and acid concentration affect the observed rate, but they do not change the ideal atom balance for the specified reaction.

Why?

Why are two HCl needed for CaCO₃? Calcium chloride contains two Cl atoms, and the carbonate-to-water-plus-CO₂ change needs two hydrogen atoms. Two HCl units supply both requirements at once.

Common misconception

“The carbonate group remains CO₃ in a product salt.” In the acid-carbonate reaction, its carbon and oxygen are redistributed into CO₂ and H₂O. Count each element across the three products rather than treating carbonate as an unchanged spectator block.

Worked example

Balance Na₂CO₃ + HCl → NaCl + H₂O + CO₂. Two sodium atoms require 2NaCl, which requires 2HCl. Those two hydrogens form H₂O. Oxygen count: three in Na₂CO₃, two in CO₂ and one in H₂O. Final: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.

Quick check

1. What gas forms when calcium carbonate reacts with dilute hydrochloric acid? Answer: CO₂(g), carbon dioxide, along with calcium chloride and water in the balanced equation.

Exam focus

Include all three product types: salt, water and CO₂. Balance carbonate and hydrogencarbonate examples separately. Keep a solid carbonate intact in a net ionic equation and audit charge as well as atoms.

Advanced insight

Gas escaping can drive a reaction by removing a product from the liquid system, but carbon dioxide also dissolves and participates in acid-base equilibria. The simple arrow captures a common observable outcome; quantitative treatment depends on CO₂ pressure, pH and concentrations.

Summary

Acid-carbonate chemistry combines ion exchange with carbonic-acid breakdown to release CO₂. Full equations produce a salt, water and gas. Correct salt formulas, carbonate state, hydrogen counts and charge balance make the prediction reliable.

Practice questions

1. Balance CaCO₃ + HCl → CaCl₂ + H₂O + CO₂. Answer: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. 2. Give a net ionic equation for dissolved carbonate with acid. Answer: CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g). 3. Why does NaHCO₃ + HCl need only one HCl in the stated gas-forming equation? Answer: Hydrogencarbonate already contains one hydrogen, so one HCl supplies the second hydrogen for water and one chloride for NaCl.