Why ninhydrin test is positive for all proteins?

The ninhydrin test is positive for all proteins because it reacts with free amino groups found in the amino acids that make up proteins. When ninhydrin is applied, it forms a colored complex, typically purple or blue, indicating the presence of amino acids. Since all proteins consist of amino acids linked by peptide bonds, the test detects these amino groups, leading to a positive result for any protein. This makes the ninhydrin test a general method for assessing the presence of proteins in a sample.