So why is 7 the neutral value on this scale?
Wouldn’t it been easier to have it as 0 so every negative number was considered acid and so on?
In: Chemistry
The pH scale is not built around a neutral of 7. The neutral of 7 is more or less an accident. It’s not even always true – the 7 thing is just a property of water.
pH is a measure of how densely packed hydrogen ions are in the liquid. It’s a logarithmic scale so 7 is very very different from 14.
Its not a linear scale. And therefore, it doesnt go from 0 to 14 (or rather – 7 to 7 as u suggest) but rather, it goes from 10^0 to 10^14. This is logarithmic scale.
Thats the easiest i can deconstruct this :
pH = | log(conc of H+ ions) |
pH is literally the negative log of the concentration of hydrogen (H+) in water at that condition. The thing is, even in basic conditions, you have a lot of it. 7 is only neutural because at that value (in water) it’s equally balanced by the amount of (OH-).
There’s a lesser used scale of pOH which uses an “inverted” value. And, pH can go outside of 0 and 14, since it’s a logarithmic expression of concentration, not an arbitrary scale.
It’s not arbitrary, it’s the value of pH that we measure in pure water.
pH describes the concentration of hydrogen ions (H+) dissolved in a water solution. A solution with a pH of 1 will have about 0.1 gram of H+ per liter (note that number is written with 1 zero in it), a solution with a pH of 4 will have 0.0001 grams of H+ per liter (4 zeros), and pH of 11 corresponds to 0.00000000001 grams of H+ (11 zeros).
Water molecules (H2O) have two hydrogen atoms and one oxygen atom, and they sometimes spontaneously break up into H+ and OH-. In pure water, it so happens that at any given moment about 0.0000001 grams of hydrogen are in the form of loose H+ ions, so pure water has a pH of 7.
* A note for pedants: I’m sweeping a bunch of minor technical details involving activity, concentration, and atomic weight under the rug in the name of ELI5.
Easier … more seductive …
It’s based on the count of ions (OH¯, H₃O⁺) and 7 happens to be the neutral spot. It was once believed that one can’t go beyond 0 .. 14, but I heard that some chemicals can be even more aggressive.
Usually we talk about acids containing water and then the value has a direct meaning – it’s useful for people dealing with it and good enough for normal people to not need a different scale.
You could have just looked up what pH is. Then you wouldnt have that question. Some ppl here have the patience of Angels with questions like this.
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