If hydroxyl ions are allowed to concentrate in stagnant water, the water can become strongly _____.

When hydroxyl ions are concentrated in water, they increase the pH level of that water, making it more basic. Hydroxyl ions (OH-) are indicative of a higher concentration of bases in a solution. Consequently, as these ions accumulate in stagnant water, they create a basic environment which is characterized by a pH greater than 7. This is relevant in the context of corrosion and water chemistry, as basic conditions can affect the stability and behavior of various materials submerged in or exposed to the water.

In this case, the presence of an abundance of hydroxyl ions shifts the balance towards basicity rather than acidity or neutrality. Neutral water has a pH of 7, while an acidic solution would show a predominance of hydrogen ions (H+) and a lower pH. Therefore, the strong basicity resulting from the concentration of hydroxyl ions is the primary factor that supports the conclusion that the water can become strongly basic.