Question

An acid is regarded as strong if it:

• A. is highly soluble in water
• B. turns blue litmus red
• C. has a strong action on skin.
• D. is completely dissociated in an aqueous solution.

Answer 1

The correct option is D

is completely dissociated in an aqueous solution.

The explanation for the correct option:

(D) is completely dissociated in an aqueous solution.

1. An acid is defined as a substance that releases one or more ${\mathrm{H}}^{+}$ions in an aqueous solution.
2. These ions exist as hydronium $\left({\mathrm{H}}_3{\mathrm{O}}^{+}\right)$ ions.

Reaction involves:

$$\begin{gathered} \mathrm{HA}\left(\mathrm{l}\right)\stackrel{\mathrm{water}}{\to }{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{A}}^{-}\left(\mathrm{aq}\right)[\mathrm{Here}\mathrm{HA}\mathrm{is}\mathrm{an}\mathrm{acid}] \\ {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right) \end{gathered}$$

The relative strength of acids is compared in terms of the degree of dissociation or degree of ionization.$\mathrm{Degree}\mathrm{of}\mathrm{dissociation}\left(\mathrm{\alpha }\right)=\frac{\mathrm{Number}\mathrm{of}\mathrm{molecules}\mathrm{of}\mathrm{acid}\mathrm{existing}\mathrm{as}\mathrm{ions}}{\mathrm{Total}\mathrm{number}\mathrm{of}\mathrm{molecules}\mathrm{of}\mathrm{acid}}$

1. If acid is strong, this means that almost all the molecules are existing as ions.
2. The value of $\mathrm{\alpha }$ for strong acid is near 1.
3. Hence, an acid is regarded as strong if it is completely dissociated in an aqueous solution.

Explanation for the incorrect options:

(A) is highly soluble in water

Generally strong acids are highly soluble in water but there are some weak acids which are also highly soluble in water.

Example: acetic acid $\left({\mathrm{CH}}_3\mathrm{COOH}\right)$,vinegar etc.

(B) turns blue litmus red

Acids turns blue litmus red. Hence weak acids also turn blue litmus red.

(C) has a strong action on skin.

1. Acids have strong action on the skin.
2. When acid comes into contact with skin, it can cause chemical burning, scarring, redness, itching, irritation, etc.
3. Weak acids also affect the skin in the same way.

Therefore, Acid is considered as strong if it dissociates completely in its ions and a strong acid gives more number of ${\mathrm{H}}^{+}$ ions when it is dissolved in water or in an aqueous solution.

Hence option (D) is correct.