Use a piece of paper to put a few crystals of KCL03 into the test tube, add about 1ml of concentrated HCL solution to the test tube, and then quickly take a piece of filter paper impregnated with 0.05N KL solution and starch paste and place it on the mouth of the test tube. The result is that in a test tube, there is a yellow-green gas rising and a piece of filter paper from white to black. Explain the phenomena that occur and write the reaction equation to prove it.