So when someone asks what is your pH, they are asking where on the scale is your acid or basicity?
Sorry if this is really not that important, I'm just trying to understand what the lingo really means.
Yes that's it, it is a number that tells you if water is acidic, neutral or basic.
Let's dive down a bit deeper to decode the lingo (or just ignore everything below this line, if you are happy with the above):
In any vessel containing water you have an equilibrium reaction going on: H
2O + H
2O <=> H
3O
+ + OH
- (sometimes that gets simplified to H
2O <=> H
+ + OH
-).
Without any added acids or bases, you have in equilibrium same amounts of H
3O
+ and OH
-, in concentrations of 10
-7 mol/L: [H
3O
+] = [OH
-] = 10
-7 mol/L (the notation [a] means "concentration of a", "mol" is the unit that chemists use to count molecules, and "Liter" is used as the SI-unit for volumes, 10
-7 is the same as 0.0000001, but a bit more convenient, it tells you immediately that there are 7 zeros involved rather than having to count them).
When you add an acid to the water, this equilibrium changes, and you get more H
3O
+ and less OH
-, but in the new equilibrium the product of both concentrations still remains 10
-14 (always). You get e.g. [H
3O
+] = 10
-10 mol/L and [OH
-] = 10
-4 mol/L.
The concentration of H
3O
+ is used as a measure for how acidic (if [H
3O
+] > 10
-7 mol/L) or basic (if [H
3O
+] < 10
-7 mol/L) the watery solution is.
Now, it's bit of a mouthful to always speak of [H
3O
+] = 10
-x mol/L - all you are really interested in, is the "x". To get that, you have to take the logarithm (to the base of 10) of [H
3O
+] in the units mol/L, and multiply with -1 to (usually) get a positive number (just out of convenience). And then call it pH, i.e. pH = x.
Or in the full formula: pH = -lg([H
3O
+]/(1 mol/L)).
In neutral water without any added acids or bases, you have therefore pH=7, which is just another notation for [H
3O
+] = 10
-7 mol/L.
If you add an acid like muriatic acid (HCl), it will release H
+ which will grab some H
2O to form more H
3O
+, i.e. [H
3O
+] > 10
-7 mol/L, or pH < 7 (remember the "-" in the pH-formula). Some of the OH
- will also grab some H
+ to from H
2O to ensure the product of [H
3O
+] and [OH
-] remains 10
-14.
A base like caustic soda (NaOH) will release some OH
-, some of this excess OH
- will grab some H
+ from H
3O
+ and form H
2O, ensuring that the product of [H
3O
+] and [OH
-] remains 10
-14, resulting in [H
3O
+] < 10
-7 mol/L, or pH > 7 (there are also different types of bases that don't release OH
-, but directly grab some H
+ from H
3O
+).
Hope that clarifies a bit what pH means - and that I didn't just create confusion. In the end, pH is just a measure for how many H
3O
+ molecules are in the water, and that's what makes it acidic - H
3O
+ activates (although not directly as far as I know, it's a bit more complicated) the "sour" taste receptors on your tongue.