By Emmalise Mac | Updated Aug 30, 2022
When a piece of calcium metal is introduced into a 6 M hydrochloric acid solution, two distinct, vigorous reactions occur. Understanding the fundamental acid‑base behaviour of HCl in water is essential to predict the outcomes when Ca is added.
Hydrochloric acid is a strong acid that fully dissociates in aqueous solution:
HCl → H⁺ + Cl⁻
Protons associate with water to form hydronium ions:
H⁺ + H₂O ⇌ H₃O⁺
In a 6 M solution, the concentration of H₃O⁺ is high, yielding a very low pH. Water molecules remain present, allowing further reactions.
Calcium metal reacts with hydronium ions and water, producing calcium hydroxide and hydrogen gas:
Ca + 2 H₃O⁺ → Ca(OH)₂ + H₂↑
This exothermic process releases heat and visible hydrogen bubbles. The freshly formed Ca(OH)₂ coats the surface of the solution as a white precipitate.
Simultaneously, calcium ions combine with chloride ions to form calcium chloride, a soluble salt:
Ca²⁺ + 2 Cl⁻ → CaCl₂ (aq)
CaCl₂ remains dissolved in the solution; no visible precipitation occurs.
These reactions collectively shift the equilibrium, reduce hydronium concentration, and modestly raise the pH, which can be confirmed by litmus paper.
