What Happens When Eggshell Reacts with Nitric Acid?

Equation: CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂↑

Word equation: Calcium carbonate + Dilute nitric acid → Calcium nitrate + Water + Carbon dioxide

Observations:

1. Brisk effervescence (fizzing) — CO₂ gas is released.
2. The eggshell slowly dissolves in the acid.
3. The gas evolved turns lime water milky → confirms CO₂.
4. No colour change (both reactants and products are colourless).
5. The solution gets warm slightly (exothermic reaction).

Why nitric acid is special here: Unlike sulphuric acid, nitric acid does not form an insoluble product. Ca(NO₃)₂ is soluble in water, so the eggshell dissolves completely.

Eggshell composition (approximate): • Calcium carbonate (CaCO₃): ~94% • Magnesium carbonate (MgCO₃): ~1% • Calcium phosphate (Ca₃(PO₄)₂): ~1% • Organic matrix (protein): ~4%

CaCO₃ is the principal component, which is why eggshell reacts with acids in the same way as limestone, chalk, and marble — all forms of CaCO₃.

General reaction of carbonates with dilute acids: MCO₃ + 2HX → MX₂ + H₂O + CO₂↑ (where M = metal, X = acid anion)

With HCl: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ With H₂SO₄: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂↑ (reaction slows — CaSO₄ is insoluble)

The CO₂ produced can be confirmed by the limewater test:

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (limewater turns milky/white)

Steps:

1. Place eggshell pieces in a test tube.
2. Add dilute nitric acid.
3. Pass the evolved gas through limewater.
4. Limewater turns milky → confirms CO₂.

This experiment demonstrates:

1. Acidic nature of nitric acid — reacts with carbonates.
2. Basic nature of CaCO₃ — it neutralises the acid.
3. CO₂ is produced in acid-carbonate reactions.
4. Double displacement / acid-base neutralisation with gas evolution.

This reaction has practical relevance: acid rain (containing HNO₃ and H₂SO₄) dissolves limestone buildings and shells by the same chemistry.