Ca2+ (aq) + 2(HCO-3) (aq) <-> CaCO3 (s) + CO2 (aq) + H2O (l) -> CaCO3 (s) + CO2 (g) + H2O (l)
where:
aq = dissolved in water; g = gas; l = liquid; s = solid
In words:
Hard water means that it contains much lime in the form of Calciumbicarbonate Ca(HCO3)2; that is why there are much Calcium- (Ca2+) and Hydrocarbonate- (HCO3) ions present.
Under special circumstances the equilibrium turns more to the right, Calciumcarbonate (CaCO3) arises in solid state, plus water H2O and dissolved Carbondioxide CO2; the latter evaporates.
The solid Calciumcarbonate (CaCO3) is usually called Boiler Scale or after the German term in aqueduct studies Sinter. The proces is called inscrustation.