Here's why:

* Acids are molecules: They have a specific chemical structure, often containing one or more hydrogen atoms.

* Dissolving in water: When acids dissolve in water, they release hydrogen ions (H+). This is what makes them acidic.

* Hydrogen ions determine acidity: The higher the concentration of hydrogen ions in a solution, the more acidic it is.

Think of it like this:

Acids are like a container of hydrogen ions. When you dissolve the acid in water, you're opening the container and letting the hydrogen ions flow out. The more hydrogen ions that are released, the stronger the acid.

So, while hydrogen ions are a defining characteristic of acids, they aren't the source of acids themselves. The acid molecule is the source, and the hydrogen ions are released upon dissolving.