It is true that some substances require water to exhibit acidic or alkaline properties, but this is not always the case. Water is a universal solvent, and many acids and alkalis are soluble in it. When dissolved in water, these substances dissociate into ions, which are electrically charged particles. The concentration of these ions in solution determines whether the solution is acidic or alkaline. For example, hydrochloric acid (HCl) is a strong acid, and when it dissolves in water, it completely dissociates into hydrogen ions (H+) and chloride ions (Cl-). The high concentration of hydrogen ions in the solution makes it acidic. On the other hand, sodium hydroxide (NaOH) is a strong alkali, and when it dissolves in water, it completely dissociates into sodium ions (Na+) and hydroxide ions (OH-). The high concentration of hydroxide ions in the solution makes it alkaline.

There are also some acids and alkalis that can react with water to produce hydrogen ions and hydroxide ions. For example, carbon dioxide (CO2) is an acidic oxide, and when it reacts with water, it forms carbonic acid (H2CO3). Carbonic acid is a weak acid, and it dissociates into hydrogen ions and bicarbonate ions (HCO3-).

In some cases, it is not necessary to have water present to see if something is acidic or alkaline. Some acids and alkalis are corrosive and can directly attack the skin or other materials. For example, sulfuric acid (H2SO4) is a strong acid, and it can cause severe burns if it comes into contact with the skin. Sodium hydroxide is a strong alkali, and it can also cause severe burns.

Therefore, while water is often necessary to observe the acidic or alkaline properties of a substance, there are some exceptions where this is not the case.