These will inform you of any hazards and precautions of which you should be aware. Know what chemicals you are using. Carefully read the label twice before taking anything from a bottle. Chemicals in the lab are marked with NFPA hazardous materials diamond labels. Learn how to interpret these labels.
Excess reagents are never to be returned to stock bottles.
If you take too much, dispose of the excess. Many common reagents, for example, alcohols and acetone, are highly flammable. Do not use them anywhere near open flames. Always pour acids into water. If you pour water into acid, the heat of reaction will cause the water to explode into steam, sometimes violently, and the acid will splatter.