Explain why a scientist must be cautious when a new chemical that has many potential uses is synthesized.
A scientist must be cautious when making, analyzing, or handling a new chemical substance because you can never know when a new substance will have any toxic effects until it has been tested. Many chemicals used by most scientists in small or moderate doses under controlled conditions are relatively benign and have little to no toxicity associated with them. But you cannot use this an excuse for lack of proper handling procedures for new, untested chemicals. Most chemical toxins cannot be readily identified by their structure alone. They must go through a series of standard tests to determine both acute (short term) or chronic (long term) toxic effects. This testing can take a long time and be relatively expensive.
Since one cannot determine toxicity just from the structure alone, one must assume that a new, untested chemical is dangerous and must take the proper precautions when handling the material. This includes proper personal protective equipment (gloves, glasses, and lab coat) and also adequate isolated ventilation (like a hood system or a respirator). There was a famous incident in the chemistry world several years ago where a college professor died due to exposure to a very small amount of dimethyl mercury. The toxic effects of mercury are well know but standard metal mercury is not readily absorbed into the human body through contact. Dimethyl mercury is much more readily solubilized and absorbed through the skin, and as such is much more acutely dangerous to people when handling it.