Final answer:
Water alone didn't clean the oily salad bowl because oil is nonpolar and doesn't mix with the polar water molecules. Detergents are needed to remove oil because they contain surfactants that can bind to oil and water, allowing for effective cleaning.
Step-by-step explanation:
The reason water alone was not effective in cleaning the oily salad bowl is due to the chemistry of oil and water. Oil is nonpolar and hydrophobic, meaning it does not mix with water, which is polar. To effectively clean oil from surfaces, detergents or soaps are necessary because they contain surfactants. Surfactants have both hydrophobic and hydrophilic parts which allow them to attach to the oil and make it miscible with water, thus effectively removing the oily film. This is why dishes washed only with water can still have an oily residue; the water can't properly cling to the oil and remove it.