When does a positive test for an acid occur?

A positive test for an acid occurs when a substance donates protons (H⁺ ions) in a solution, typically resulting in a decrease in pH. This can be confirmed using indicators such as litmus paper, which turns red in the presence of acids, or through specific chemical reactions that produce characteristic changes. Additionally, a positive test may involve detecting the presence of acidic functional groups in organic compounds through various qualitative analysis methods.