Final answer:
Soap contains a higher concentration of hydroxide ions, as it has a pH greater than 7 and is classified as a base, in contrast to milk, lemon juice, and water which are either neutral or acidic.
Step-by-step explanation:
Among the food items listed, soap contains water solutions with a higher concentration of hydroxide ions. This is because soap typically has a pH greater than 7, which classifies it as a base. Bases are known to have a higher concentration of hydroxide ions compared to hydronium ions. On the pH scale, substances with higher pH values are more basic, and therefore have more hydroxide ions. Milk, lemon juice, and water would have lower concentrations of hydroxide ions because milk is only slightly basic, lemon juice is acidic, and pure water is neutral.