An oxide layer on aluminum is ____, while on iron it is ____.

• A. White, reddish
• B. Black, reddish
• C. Reddish, grey
• D. White, black

Answer: (B)

Oxide films form on metals when they react with oxygen, and their color comes from the oxide material itself and how thick the film is. Aluminum tends to develop a very thin, protective oxide (aluminum oxide) that is usually light-colored, often appearing white or gray. Iron, when it rusts, forms iron oxide that is typically reddish-brown. The color you see can shift with thickness or with surface treatments, so under some conditions an aluminum oxide film can look dark or black, while iron oxide stays reddish in common rust situations. The key idea is that oxide color reflects the oxide type and its thickness, which is why aluminum and iron oxide films can appear different.