Is the ‘neutral’ of the pH scale based on something scientifically objective, or simply what is ideal for humans?
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At neutral pH the amount of -OH ion and H+ ion are the same. Which means that neither ion can push the solution to a more acidic or basic pH.
All the levels of pH and pOH are based on logarithmic scale, so there is a math/scientific basis for the scale itself
Acidity is determined by the number of H+ and OH- ions in solution. At low pH there are more H+ ions, at high pH there are more OH- ions, and at the middle of the scale, a neutral solution has equal amounts of both.
If you were to have a container of pure H2O, it will not stay pure H2O, but instead will have some amount of Hydroxide Ions (HO) and Hydrogen Ions (H) that are in the water. These two things are in perfect balance in this scenario, but an imbalance of one or the other would make the water acidic or basic.
It’s based on the nature of pure H20 water. Naturally, some small portion of H20 molecules will separate into H+ and OH- atoms/molecules and form a balance with each other and the surround water molecules, called an “equilibrium” in chemistry. In this context “equilibrium” doesn’t mean perfectly 50/50, it means however much H+ there is and however much OH- there is, that balance, will be kept stable by the chemistry of the liquid.
Many other liquids will form a similar equilibria but will favor the H+ side or the OH- side and the pH scale is essentially a mathematical expression of how strongly that compound favors the H+ or OH- side of the balance.
Why is the neutral pH (it stands for “potential of hydrogen”, or the basically the concentration of hydrogen ions, which are what causes acidity) 7 and not, for example, 8 or 10?
Because of the logarithm of hydrogen ions’ concentration in one mol of pure water is -7 (the concentration is 1/10000000). This is definitionally the point where there is an equal amount of OH ions as well (with it being pure water, the only way to have H ions is to have an equal amount of OH ions).
If “neutral” means pH of 7, then it is arbitrary. Water will then be basic or acidic depending solely on temperature! But if one instead defined neutral as having the same amount of H^+ as OH^-, then pure water is neutral at all temperatures (okay, only up to very high temperatures). Defining neutrality at having an equal amount is not random or arbitrary.
In the end, pH (respectively electronegativity) describes how much a substance is willing to give a proton (resp. electron) to another one. Protons and even more so electrons are the most basic objects in chemistry and are essential for how most of it works. So having special nomenclature for both is a good idea.
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