When marble, primarily composed of calcium carbonate (CaCO₃), is added to dilute hydrochloric acid (HCl), a chemical reaction occurs. This reaction produces carbon dioxide gas (CO₂), water (H₂O), and calcium chloride (CaCl₂). The balanced equation for this reaction is:
Reaction of Marble with Hydrochloric Acid
CaCO₃ (s) + 2 HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)
Observation with Lime Water
The evolved carbon dioxide gas is then passed through lime water, which is a dilute solution of calcium hydroxide (Ca(OH)₂). Initially, lime water is clear and colorless. When CO₂ is bubbled through it, a reaction occurs, resulting in the formation of calcium carbonate, which is insoluble in water and causes the lime water to turn milky. The equation for this reaction is:
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)
Excess Carbon Dioxide
If an excess of carbon dioxide is passed through the lime water, the initial milky appearance will disappear. This happens because the excess CO₂ reacts with the calcium carbonate formed earlier to produce soluble calcium bicarbonate, which is colorless in solution. The equation for this reaction is:
CaCO₃ (s) + CO₂ (g) + H₂O (l) → Ca(HCO₃)₂ (aq)
Summary of Changes
- Initial reaction produces CO₂ gas.
- Passing CO₂ through lime water turns it milky.
- Excess CO₂ clears the lime water by forming soluble calcium bicarbonate.
These observations illustrate the chemical behavior of calcium carbonate in acid and its interaction with lime water, showcasing the principles of acid-base reactions and gas solubility.
