Question

Define an acid in terms of ions.

Answer 1

1. Acid

Acids are chemical compounds that are commonly sour in taste and can form aqueous solutions which can change the colour of blue litmus paper to red.

2. Acid in terms of ions

Acids are species that are proton donors or hydrogen ions (${\mathrm{H}}^{+}$) when they are dissolved in aqueous solutions and can accept electrons. Mostly, acids contain hydrogen atoms that are dissociated in water to form a ${\mathrm{H}}^{+}$ ion and an anion. An example is given below:

$$\begin{gathered} \mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right) \\ \mathrm{Hydrochloric}\mathrm{acid} \end{gathered}$$

The pH of acid depends on ${\mathrm{H}}^{+}$ ion. When the concentration of ${\mathrm{H}}^{+}$ ion is high, the pH is lower. The lower the pH, the more the strength of the acid.

Examples of acids are : HCl (Hydrochloric acid), ${\mathrm{H}}_2{\mathrm{SO}}_4$ (Sulphuric acid), ${\mathrm{HNO}}_3$ (Nitric acid).