Answer:
4
Step-by-step explanation:
Do not heat a test tube with the cork covering its open end. This is because this will result in buildup of pressure and ultimately lead to an explosion which could lead to injury. Instead, heat the test tube at an angle (with the mouth facing away from you or your neighbor) and close to a window so the emanating fumes are not directed at you. Heat the test tube gently without having to put it directly into the flame.