What is the precipitation form of calcium when a sample is added to oxalic acid?

When calcium is treated with oxalic acid, the expected reaction involves the formation of calcium oxalate. This compound, represented as CaC2O4, is a salt formed from the reaction between calcium ions (Ca²⁺) and oxalate ions (C2O4²⁻). Oxalic acid, being a diprotic acid, can dissociate to release oxalate ions in solution, which then combine with the calcium ions that may be present.

Calcium oxalate is well-known for being a sparingly soluble compound, which means it precipitates out of the solution when calcium ions encounter oxalate ions under the right conditions. This tendency to form a precipitate is often utilized in analytical chemistry for the quantitative determination of calcium.

The other options listed do not represent the correct reaction or precipitate formed with oxalic acid. For instance, compounds like Ca(OH)2 and CaCO3 involve different reactions with hydroxide ions and carbonate ions, respectively, while CaSO4 results from the reaction with sulfate ions. Thus, the most appropriate and chemically accurate answer regarding the precipitation form of calcium when added to oxalic acid is indeed calcium oxalate, CaC2O4.