Question

What is a substance with a pH of 4 called?

Answer 1

Hint: To solve this question, we first need to know what is pH. In an aqueous solution, the acidity or the basicity can be specified using a pH scale. The pH scale denotes the potential or the "power of hydrogen".

Complete answer:
For an aqueous solution, the pH of a solution can be determined by the following formula
\[pH=-\log [{{H}^{+}}]=-\log [{{H}_{3}}{{O}^{+}}]\]
Where $[{{H}^{+}}]$ or $[{{H}_{3}}{{O}^{+}}]$ represent the concentration of ${{H}^{+}}$ or ${{H}_{3}}{{O}^{+}}$ ions.
A solution that has more hydrogen ions ${{H}^{+}}$ or hydronium ions ${{H}_{3}}{{O}^{+}}$ than pure water is considered to be acidic.
In an aqueous solution, ${{H}^{+}}$ or a proton cannot exist, hence the water molecules exist in the form of ${{H}_{3}}{{O}^{+}}$ and $O{{H}^{-}}$ ions. This exchange of protons between water molecules is known as the autoionization of water. This can be represented by the following equation
\[2{{H}_{2}}{\mathrm O}(l)\rightleftharpoons {{H}_{3}}{{O}^{+}}(aq)+O{{H}^{-}}(aq)\]
The equilibrium constant for this equation can be given by
\[K=[{{H}_{3}}{{O}^{+}}][O{{H}^{-}}]\]
The product of concentrations of ${{H}_{3}}{{O}^{+}}$ or ${{H}^{+}}$ and $O{{H}^{-}}$ is always $1\times {{10}^{-14}}M$.
So, we can say that
\[K=[{{H}_{3}}{{O}^{+}}][O{{H}^{-}}]={{10}^{-14}}\]
Since in pure water, hydronium and hydroxide ions are formed in a 1:1 ratio, so, we can say that
\[[{{H}^{+}}]=[{{H}_{3}}{{O}^{+}}]=[O{{H}^{-}}]={{10}^{-7}}M\]
The concentration of ${{H}^{+}}$ ions or ${{H}_{3}}{{O}^{+}}$ ions in water is ${{10}^{-7}}$M.
Now, in a solution whose pH is 4, the concentration of ${{H}^{+}}$ ions will be
\[\begin{align}
  & 4=-\log [{{H}^{+}}] \\
 & [{{H}^{+}}]={{10}^{-4}}M \\
\end{align}\]
Since the concentration of hydrogen ions ${{H}^{+}}$ is more than that of pure water, the solution is acidic.
Hence the substance with a pH of 4 is called an acidic substance.

Note:
It should be noted that as the concentration of the hydrogen ions ${{H}^{+}}$ or the hydronium ions ${{H}_{3}}{{O}^{+}}$ increases in the solution, the acidity of the solution increases.
Similarly, if the concentration of $O{{H}^{-}}$ increases in the solution, the basicity of the solution increases.