Unit 4, Lesson 20

The process of "watering down" an acidic or basic solution is called dilution. It simply entails adding water to an acid or base to, in an acid's case, raise the pH toward 7, or in a base's case, lower the pH toward 7. 7 is considered "neutral" for any substance. Diluting a substance makes it weaker, or less concentrated, since adding water lowers the molarity. When the molarity of an acid or base is lowered, the pH increases by 1. This is because the pH scale is logarithmic, and a tiny change in pH makes for a huge change in concentration. For example, changing an acid from a pH of 3 to a pH of 4 makes that acid 10x less acidic. Changing it to pH 5 is 100x more acidic. pH 6, 1000x more acidic, and so on.

Though diluting an acid or base is meant to make it more neutral, one cannot achieve full neutrality. Rather, they can get very close. An acid can only be diluted to a pH between 6 and 6.9. A base can only be diluted to a pH around 7.1 (or 8, because any pH above 7 is basic).

Problems: 

2.) Explain why you can't turn an acid into a base by diluting with water. You can't eliminate the acid completely--you can only neutralize it, because water is neutral. However, if you were diluting with a base, you could change the acid, because the pH would gradually raise until it was high enough to be a base.

4.) How much water do you need to add to 10 mL of a solution of HCl with a pH of 3 to change it to a pH of 6? 999 mL of water.
pH of 3 = .0010 M
pH of 4 = .00010 M (10x less acidic) (+9 mL of water)
pH of 5 = .000010 M (100x less acidic) (+99 mL of water)
pH of 6 = .0000010M (1,000x less acidic) (+999 mL of water)

...I don't know if that's correct. I'm a bit fuzzy on this myself.