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Question: What is a substance with a pH of 4 called?...

What is a substance with a pH of 4 called?

Explanation

Solution

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⁡[H3O+]pH=-\log [{{H}^{+}}]=-\log [{{H}_{3}}{{O}^{+}}]
Where [H+][{{H}^{+}}] or [H3O+][{{H}_{3}}{{O}^{+}}] represent the concentration of H+{{H}^{+}} or H3O+{{H}_{3}}{{O}^{+}} ions.
A solution that has more hydrogen ions H+{{H}^{+}} or hydronium ions H3O+{{H}_{3}}{{O}^{+}} than pure water is considered to be acidic.
In an aqueous solution, H+{{H}^{+}} or a proton cannot exist, hence the water molecules exist in the form of H3O+{{H}_{3}}{{O}^{+}} and OH−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
2H2O(l)⇌H3O+(aq)+OH−(aq)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=[H3O+][OH−]K=[{{H}_{3}}{{O}^{+}}][O{{H}^{-}}]
The product of concentrations of H3O+{{H}_{3}}{{O}^{+}} or H+{{H}^{+}} and OH−O{{H}^{-}} is always 1×10−14M1\times {{10}^{-14}}M.
So, we can say that
K=[H3O+][OH−]=10−14K=[{{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+]=[H3O+]=[OH−]=10−7M[{{H}^{+}}]=[{{H}_{3}}{{O}^{+}}]=[O{{H}^{-}}]={{10}^{-7}}M
The concentration of H+{{H}^{+}} ions or H3O+{{H}_{3}}{{O}^{+}} ions in water is 10−7{{10}^{-7}}M.
Now, in a solution whose pH is 4, the concentration of H+{{H}^{+}} ions will be

& 4=-\log [{{H}^{+}}] \\\ & [{{H}^{+}}]={{10}^{-4}}M \\\ \end{aligned}$$ 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.