Which is the only precipitate reaction in which precipitate floats on surface?

The only precipitate reaction where the precipitate floats on the surface is the formation of calcium carbonate (CaCO3) from a reaction between calcium ions and carbonate ions in water, often in the presence of certain conditions like high concentrations or specific pH levels. This phenomenon is typically observed in natural settings, such as in the formation of stalactites and stalagmites in caves. The low density of the formed precipitate allows it to remain buoyant on the surface of the solution.