"an acid is a substance that forms hydrogen ions when dissolved in water." this is an example of-

"An acid is a substance that forms hydrogen ions when dissolved in water." this is an example of Arrhenius theory .

Explanation:

Any substance that, after ionisation, dissolves in liquid and produce hydrogen ions called Arrhenius acid (HCl, HCN,etc). Whereas, a substance that dissolves in aqueous releases hydroxide ions called Arrhenius base (NAOH, KOH, etc).

Arrhenius theory elaborates why acids have same properties: The internal properties of acids are determined by the presence of  ions formed by dissolving acids in aqueous. These also explain why acid neutralises bases and vice versa. Acids give an ion and the bases give an ions.

Example:

Where, (aq) stands for aqueous which means in the presence of water that is, water acts as a solvent.