Do you really understand the pH scale? I didn't. Basically, when you see any reference to acid or alkali content, you see a number that refers to this scale.
The pH scale ranges from 0 to 14. 7 is in the middle, so it is neutral, being neither acid or alkaline. The lower the number, the more acidic a substance is. The higher the number, the more alkaline that substance would be.
Each whole number represents a factor of ten. So, something that is pH 4 is ten times more acidic than something pH 5. Lemon juice at pH 2 is not twice as acidic as tomato juice at pH 4, but 100 times as acidic!
It might seem silly that the lower a number is, the more acidic it is, but there is a reason for this. If you really want to know why it’s like that, follow this link to a page that will explain the geeky stuff about the pH scale. Warning: You may feel the urge to start wearing a pocket protector after reading that information!
Looking at the scale, you see that battery acid is about as acidic as it gets with a pH of 0. Notice gastric acid (read stomach acid), is almost as acidic as battery acid! Notice also that cola is more acidic than vinegar.
Distilled water is 7 or neutral. Between 8 and 9 is seawater and also your small intestine is alkaline at about pH 9. Between 13 and 14 is oven cleaner.
Products at either extreme, below 1 or above 13, are extremely corrosive. Some examples are hydrochloric acid at the low end and caustic soda (lye) on the high end. Household cleaners like drain opener or oven cleaner, that you would think are acids, are really very alkaline. Very alkaline substances are just as dangerous as very acidic ones.