Final answer:
The most probable change to a sample of ionized calcium left standing is a change in pH, potentially leading to the precipitation of calcium compounds such as calcium carbonate.
Step-by-step explanation:
If ionized calcium was left standing for a while, the most likely change to the sample would be a change in pH. When calcium ions are present in water, they can interact with carbonate (CO32-) and bicarbonate (HCO3-) ions in solution, which are in equilibrium with carbon dioxide and water. This can lead to the formation of calcium carbonate (CaCO3), particularly when the solution is left standing, allowing CO2 to escape and the pH to rise. As the pH rises, it could eventually lead to the precipitation of calcium as calcium carbonate, decreasing the concentration of Ca2+ and HCO3- ions in solution.
As per the given examples, the formation of a precipitate like calcium oxalate in the presence of oxalate ions would also decrease the concentration of Ca2+ ions. This would change the ionic strength and could affect the pH of the solution. However, evaporation would not significantly affect the ionized calcium unless it was so extensive that it led to overall concentration changes.