Explain why it is dangerous to mix solutions in a test tube by placing a finger or thumb over the opening of the tube and inverting it

This is a dangerous practice since the solutions in the test tube may be corrosive, poisonous, or both. For instance, you may have a strong base or acid in the test tube and mixing the contents of the test tube by covering the opening and inverting it may seriously burn your thumb. Of you could have a compound that easily passes through the skin such as DMSO which can carry other chemicals through the skin barrier. An example of a poison that can pass through skin is dimethylmercury. In her case, she accidentally got a couple of drops on her latex gloves. As it turns out dimethylmercury was able to pass through her gloves and skin in about 15 seconds. She died less than a year after her accidental exposure.