Which solution has the greatest number of hydroxide ions? baking soda, pH=9 milk pH=6 tomato juice, pH=3.5 A vinegar, pH=2

The pH scale measures how acidic or basic a substance is. The scale ranges from 0 to 14. A pH less than 7 indicates an acidic solution, a pH of 7 indicates a neutral solution, and a pH greater than 7 indicates a basic solution. The higher the pH, the more basic the solution is, and the more hydroxide ions it contains. In this case, baking soda has the highest pH of 9, indicating it is the most basic and therefore contains the greatest number of hydroxide ions.