Preparing Salts with Metals or Carbonates

Introduction

An acid need not react only with a hydroxide to form a salt. Suitable metals can replace acid hydrogen and release H₂, while carbonates consume acid and release CO₂ along with water. These routes provide useful product clues and can allow an insoluble excess solid to be removed before a soluble salt is crystallised.

Core explanation

Zinc reacts with dilute hydrochloric acid in the idealised equation Zn + 2HCl → ZnCl₂ + H₂. Zinc atoms become Zn²⁺, and acid-derived hydrogen forms hydrogen gas. The two chloride ions balance the zinc ion in ZnCl₂. Bubbles are evidence of gas production, but visible bubbling alone does not identify the gas. A suitable hydrogen test or other evidence is needed to distinguish it from carbon dioxide or an unrelated gas.

Not every metal reacts with every acid in the same simple way. A metal must be chemically capable of the relevant displacement under the stated conditions. Copper does not liberate hydrogen from ordinary dilute nonoxidising HCl in the simple reactivity-series model, while zinc can. Strongly oxidising acids may react by different pathways and produce other gases; therefore the generic “metal + acid → salt + H₂” rule has a defined scope. Use the actual metal, acid and conditions given in a problem.

Carbonates follow a different characteristic pattern. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The carbonate ion consumes acid and a carbonic-acid intermediate breaks down to water and carbon dioxide. Hydrogen carbonate behaves similarly with a one-proton demand per HCO₃⁻: NaHCO₃ + HCl → NaCl + H₂O + CO₂. The gas here is CO₂, not H₂. Its production can be distinguished by a suitable test; for instance, carbon dioxide passed into limewater can form a calcium carbonate precipitate under the familiar school-test conditions.

These reactions can assist soluble salt preparation if the metal or carbonate reagent is an insoluble solid and an excess can be filtered away. For example, excess calcium carbonate can react with acid until no more carbonate is consumed, and unreacted solid can be filtered from the dissolved calcium salt solution. The filtrate can then be concentrated for crystal recovery if the salt's properties suit that approach. Care is needed in interpretation: bubbles may stop because acid is exhausted, or because a surface is blocked or a reaction is slow. The balanced equation and observable evidence should both be considered.

An acid–metal reaction also has a redox interpretation: metal is oxidised and hydrogen ions are reduced. An acid–carbonate reaction is generally acid–base/gas formation without the same change in oxidation states. Both produce salts, but the electron and proton bookkeeping differ. This distinction helps prevent calling every bubbling acid reaction a metal displacement or every salt-forming reaction a neutralisation to water only.

Step-by-step reasoning

1. Identify whether the second reactant is a suitable metal, a carbonate, or a hydrogen carbonate. 2. Predict the salt from the metal cation and acid anion using charge balance. 3. Add H₂ for a suitable acid–metal displacement, or H₂O + CO₂ for a carbonate route. 4. Balance atoms and check any gas identity against the type of reactant and conditions. 5. For preparation, decide whether any excess solid can be filtered and whether the salt remains dissolved.

Visual explanation

Draw two reaction branches from an acid bottle. The metal branch points to “salt + H₂ bubbles” with Zn + 2HCl as a model. The carbonate branch points to “salt + H₂O + CO₂ bubbles” with CaCO₃ + 2HCl. Put the gas labels in separate boxes to emphasize that bubbling is not by itself a gas identification.

Real-world analogy

Two different machines can both deliver a boxed product but release different by-products. Acid–metal and acid–carbonate reactions can both yield salts, yet one can release hydrogen and the other carbon dioxide. The analogy highlights product pathways; actual gas identities follow balanced chemistry rather than machine labels.

Real-world example

Calcium carbonate in limestone reacts with acid and releases carbon dioxide. This can be observed as fizzing, while dissolved calcium salt remains if the reaction mixture permits it. The same chemistry helps explain why carbonate-containing building materials can be attacked by acidic solutions.

Why?

Why does acid plus carbonate yield water as well as carbon dioxide? Protonated carbonate can form carbonic acid, which readily gives CO₂ and H₂O in the overall reaction. The product pattern conserves carbon and oxygen while accounting for the acid protons.

Common misconception

“Any gas bubbling from acid is hydrogen.” Acid–carbonate reactions produce CO₂; suitable acid–metal reactions produce H₂ in the simple model. Gas identity requires reactant reasoning and, when asked experimentally, an appropriate confirmatory test.

Worked example

Write and balance the reaction of magnesium carbonate with dilute nitric acid. Magnesium carbonate contains Mg²⁺ and CO₃²⁻; nitrate is NO₃⁻, so the salt is Mg(NO₃)₂. The carbonate pattern adds water and CO₂. The balanced equation is MgCO₃ + 2HNO₃ → Mg(NO₃)₂ + H₂O + CO₂. Two acid formula units provide the two nitrates and two hydrogens needed for water. The released gas is carbon dioxide, not hydrogen.

Quick check

1. What gas is expected from CaCO₃ reacting with dilute HCl, and what salt forms? Answer: Carbon dioxide is expected, and the charge-balanced salt is calcium chloride, CaCl₂.

Exam focus

Use the reactant type to select the correct gas product, then balance the salt formula from charges. State limits of the generic metal-displacement rule. In preparation questions, explain filtration only when excess reactant is an insoluble solid.

Advanced insight

Carbonate reactions can be coupled to CO₂ dissolution and bicarbonate equilibria, so the observed amount of gas may depend on whether the system is open and on solution conditions. In a closed vessel, some generated CO₂ remains dissolved or in the headspace; the balanced overall equation still gives the ideal total carbon accounting.

Summary

Suitable metals react with certain acids to make a salt and hydrogen, while carbonates make a salt, water and carbon dioxide. The salt formula follows ion charges and the full equation supplies mole ratios. Bubbles suggest gas formation but must not be treated as proof of gas identity by themselves.

Practice questions

1. Balance zinc with dilute sulfuric acid in the simple displacement model. Answer: Zn + H₂SO₄ → ZnSO₄ + H₂; zinc sulfate and hydrogen are the products. 2. Balance sodium hydrogen carbonate with HCl. Answer: NaHCO₃ + HCl → NaCl + H₂O + CO₂, with one acid proton consumed per hydrogen carbonate ion. 3. Why is Cu + dilute HCl not routinely written as CuCl₂ + H₂? Answer: Copper does not displace hydrogen from ordinary dilute nonoxidising HCl under the simple reactivity-series conditions.