Final answer:
Oil and water do not mix evenly but form separate layers because they are immiscible, with oil being nonpolar and water being polar, resulting in oil floating on top of the water due to its lower density.
Step-by-step explanation:
The statement that oil and water always mix evenly is false. Oil and water are known to be immiscible, meaning they do not mix to a notable extent. In a container, these substances form separate layers due to the nature of their molecules; water molecules are polar, leading to strong hydrogen bonding between them, while oil molecules are nonpolar and do not have this type of interaction with water molecules.
Therefore, when oil and water are combined, the less dense oil forms droplets and floats on top of the water. This characteristic can often be observed in everyday situations such as a puddle with a thin layer of oil spread over it or in a salad dressing made with oil and vinegar, which is watery.