Chemistry Form 4: Understanding of Acid and Bases

Acid is a chemical substance that dissolves in water to produce hydrogen ions, H⁺ ions. This causes the concentration of hydrogen ion in the solution to increase and the pH to drop.

According to the Arrhenius definition, when hydrogen chloride which is a gas HCl is dissolved in water, it dissociates to produce hydrogen ions and chloride ions in solution. This making HCl an acid when dissolves in water.

Without presence of water, the acid will not dissociate to produce hydrogen ions and it will not show acidic properties.

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Also, a base is a chemical substance that dissolves in water to produce hydroxide ions, OH^- ions. This causes the concentration of hydroxide ion in the solution to increase but decrease in concentration of hydrogen ion. This is why the pH increases when a base is added to water.

According to Arrhenius definition, sodium hydroxide, NaOH is a base because when it dissolved in water, the NaOH dissociates to produce hydroxide ions.

Without presence of water, the base will not dissociate to produce hydroxide ions and it will not show basic properties.

                                                 Acid                                       Base

Operational Definition	When added to water pH < 7	When added to water pH > 7
Arrhenius Definition	When added to water dissociates to release a hydrogen ion (H+)	When added to water dissociates to release a hydroxide ion (OH-)

The Swedish Chemist, Svante Arrhenius (1859-1927). The Arrhenius definition of acids and bases is an extension of his theory of electrolyte solution, which formed his 1884 doctoral thesis.