Yes, it is possible for a substance to act as both an acid and a base. This concept is known as amphoterism.
Amphoterism refers to the ability of a compound or ion to react as an acid or a base depending on the context or the chemical environment in which it is placed. This behavior is observed in certain substances that have the capability to donate or accept protons (H⁺ ions) depending on the conditions.
For example, water (H₂O) is an excellent example of an amphiprotic molecule. In an acidic solution, water can act as a base by accepting a proton (H⁺ ion):
HCl + H₂O → H₃O⁺ + Cl⁻
In a basic solution, water can act as an acid by donating a proton:
NH₃ + H₂O → NH₄⁺ + OH⁻
Similarly, some ions and compounds can exhibit amphoterism. One common example is the hydroxide ion (OH⁻). In an acidic solution, OH⁻ can accept a proton and act as a base. In a basic solution, it can donate a proton and act as an acid.
Amphoterism is also observed in certain metallic hydroxides, such as aluminum hydroxide (Al(OH)₃) or zinc hydroxide (Zn(OH)₂). These compounds can act as bases by accepting protons in acidic solutions and as acids by donating protons in basic solutions.
Overall, amphoterism demonstrates the ability of certain substances to exhibit both acidic and basic properties depending on the specific chemical conditions or reactions in which they are involved.