Egg shells are made up of CaCO₃.When CaCO₃ is treated with HCI CO₂ gas is liberated which turns lime water milky. CaCO₃(s)+2HCl(dil)-CaCl₂(aq) + H₂O(l) + CO₂(g) When CO₂ is passed through lime water(Ca(OH)₂), the following reaction takes place:Ca(OH)(aq) + CO₂(g) +CaCO₃(s) + H₂O(l)Formation of CaCO, turns it milky. Hence solution contain an acid.