Is muriatic acid mixture or substance?

Muriatic acid is actually a substance, not a mixture. It’s basically another name for hydrochloric acid in a really strong, liquid form. People usually use it for cleaning or in pools, but it’s super strong, so you have to be careful. It’s not mixed with other chemicals when you buy it, though sometimes people dilute it with water before using. So yeah, it’s a single substance with strong chemical properties.