Water that has a pH of 5 means it is slightly acidic. The pH scale is a scale from 0 to 14, with 7 representing neutral, right in the middle. Water, pure water, that is, has a pH of 7. It is neither acidic nor basic. substances with numbers lower than 7 lean towards being acids, while substances with numbers higher than 7 lean towards being bases. If bean seeds germinate best in water that is slightly acidic, perhaps the weak acid in the water is performing some duty that assists the seed in its efforts to sprout forth. Perhaps it eats away at the tough outer coat, which houses the delicate first leaves, called cotyledons, inside. Maybe it provides better nourishment to the fledgling bean seedling, making conditions for it to germinate more likely to take place. Whatever, the reason, there is a reason why there is a need to have water at a slightly acidic pH of 5, where it delivers a few well-timed hydrogen ions (H+) in the right place at the right time.