Why can't milk of magnesia (Mg(OH)2), act as a buffer?

It is not a weak base. A buffer consists of

A. a weak acid and its salt OR
B. a weak base and its salt.
Mg(OH)2 does not meet either criteria. It is not a weak base and there is no salt.

Milk of magnesia, which is a suspension of magnesium hydroxide (Mg(OH)2) in water, cannot act as a buffer because it does not contain both a weak acid and its conjugate base or a weak base and its conjugate acid.

To understand why this is important, let's first talk about what a buffer is. A buffer is a solution that can resist changes in pH when small amounts of acid or base are added to it. It contains a weak acid and its conjugate base, or a weak base and its conjugate acid. The weak acid and its conjugate base can react with small amounts of added base to prevent a significant increase in pH, while the weak base and its conjugate acid can react with small amounts of added acid to prevent a significant decrease in pH. This ability to "buffer" the pH is what makes buffers important in maintaining the stability of many biological systems.

In the case of milk of magnesia, Mg(OH)2 is a strong base. It reacts completely with water to form hydroxide ions (OH-) and magnesium ions (Mg2+). Since it is a strong base, it does not have a weak acid counterpart that could act as a conjugate acid, which is necessary to form a buffer system.

Therefore, milk of magnesia cannot act as a buffer because it lacks the required components, a weak acid and its conjugate base, or a weak base and its conjugate acid.