Please rub the surface of the test tube dry before doing the experiment.在进行试验之前请先把试管的表面擦干。Adding acid to the test tube starts a chemical process which leads to the formation of crystals.把酸加入试管就会产生化学反应，最后形成晶体。When acid is present, the chemical in the test tube turns red.存在酸的话，试管内的化学物就会变成红色。