# What does it mean when a substance has a pH greater than 7?

May 31, 2017

Well, at ${25}^{\circ} \text{C}$ and $\text{1 atm}$, the substance is expected to be basic. In other conditions, you'll have to try it and tell me.

At ${25}^{\circ} \text{C}$ and $\text{1 atm}$, the autoionization constant of water is:

${K}_{w} = \left[{\text{H"^(+)]["OH}}^{-}\right] = {10}^{- 14}$

and thus, $\left[{\text{H"^(+)] = ["OH}}^{-}\right]$ would result in $\text{pH} = 7$, since

"pH" = -log["H"^(+)]

and ["H"^(+)] = sqrt(K_w) = 10^(-7) "M" when $\left[{\text{H"^(+)] = ["OH}}^{-}\right]$.

When $\text{pH} > 7$, it follows that ["H"^(+)] < 10^(-7) "M", i.e. that the solution is basic... at ${25}^{\circ} \text{C}$ and $\text{1 atm}$. This reflects the fact that there is less ${\text{H}}^{+}$ than ${\text{OH}}^{-}$ in solution, and ${\text{OH}}^{-}$ influences the basicity.

How would you describe a solution with $\text{pH} < 7$ at ${25}^{\circ} \text{C}$ and $\text{1 atm}$?

May 31, 2017

As mentioned by Truong-Son N, this is assuming that the substance is at room temperature (${25}^{\circ}$ C) and room pressure (1 atmosphere).
pH actually refers to the concentration of ${H}^{+}$ ions in a solution. Each step down the pH scale is an increase of x10 in concentration. This refers to acid strength because ${H}^{+}$ ions are released into solution when an acid is dissolved in water and the stronger the acid, the higher the proportion of acid molecules that are ionised (split into ions - including ${H}^{+}$).