If you think back to high school chemistry class, you may recall that pH is an abbreviation for potential hydrogen. A pH number measures from 0 to14 how acidic or alkaline a liquid is -- anything above 7 is alkaline and anything below 7 is acid. Water has a pH level of 7 -- it's neutral, meaning it has the same amount of acids and alkalis, which balance each other out.
When you're thinking about liquids in terms of their pH levels, going up or down one number on the scale represents a tenfold change in the acidity or alkaline nature of a liquid. For example, the pH level of milk is around 6. Because the pH level of water is 7, milk is 10 times more acidic than water. Vinegar has a pH of about 3, making it 40 times more acidic than water [source: Carpi].
On the other end of the spectrum, milk of magnesia is a base with a pH of 10, which is why it's soothing to an acidic stomach. Lye has a pH of 13 and can dissolve sticky substances like fat. As you can see, substances at either end of the pH spectrum can be corrosive -- but if you combine these substances, they neutralize and create salt water. They balance because they're at opposite ends of the scale [source: Ophardt].
Now that you've had your general chemistry lesson, take a look at the next page to learn what pH-balanced means for your skin cleanser.