Final answer:
In a copper bowl, never add acid to egg whites, as the chemical reaction can impede the bowl's ability to stabilize the egg whites and may lead to contamination.
Step-by-step explanation:
When using a copper bowl to whip egg whites, never add acid. The chemical reaction between copper and acid can reduce the bowl's ability to stabilize egg whites and may also cause the copper to corrode, potentially contaminating the food with copper ions. Copper bowls are preferred for whipping egg whites because the copper ions from the bowl help to form a more stable foam when the egg whites are beaten, resulting in fluffier and more consistent peaks. However, adding acid can interfere with this stabilization process.