Here's why:

* Substance: Muriatic acid is a chemical compound with a specific chemical formula (HCl). It's a single, pure substance.

* Solution: Muriatic acid is a solution of hydrochloric acid (HCl) in water. It's not a pure compound but a mixture of HCl dissolved in water.

In summary: While muriatic acid is a mixture of HCl and water, it is considered a substance because it's a specific chemical compound (HCl) that is the main component of the mixture.