Final answer:
Copper utensils become blackish after some days due to tarnishing caused by the reaction of copper with oxygen and sulfur compounds in the air.
Step-by-step explanation:
Copper utensils and sheets become blackish after some days due to a process called tarnishing. Tarnishing occurs when copper reacts with substances in the air, such as oxygen and sulfur compounds. Over time, a layer of copper oxide and copper sulfide forms on the surface of the copper, giving it a blackish color.