For generations, home cooks have added salt to boiling water when preparing hard-boiled eggs. What Really Happens When an Egg Cracks in Boiling Water? But during boiling, eggs are often subject to thermal stress or microfractures in the shell. By plunging freshly boiled eggs into ice-cold water, the egg white contracts and peels away more easily from the membrane. In high-heat environments, proteins become remarkably sensitive to changes in ionic concentration—meaning what you add to the cooking water can have measurable effects.