Final answer:
Oil contaminated with water will have a milky appearance due to the formation of an emulsion where water droplets are suspended in the oil, while other contaminants like dirt, air, and chemicals do not typically cause this appearance.
Step-by-step explanation:
Oil that has been contaminated by water will have a milky appearance. This phenomenon occurs because oil and water do not mix. Oil is composed primarily of hydrocarbons, which are organic compounds consisting of carbon and hydrogen atoms. Given oil's composition, water contamination in oil can create an emulsion, wherein water droplets become suspended in the oil, resulting in a cloudy or milky appearance. In contrast, contaminants such as dirt, air, and chemicals do not typically present with a milky appearance when mixed with oil.
When water contaminates oil, it can happen due to a variety of reasons, including water entering storage tanks, oil spills, and condensation inside containers. Regardless of how the water contamination occurs, the presence of water in oil is a sign of contamination that needs to be addressed to avoid potential damage to machinery or engines using the oil.