Certain common pieces of equipment such as instrumentation and balances have to be shared with your classmates. Leaving such equipment dirty or broken is not only rude and selfish, but will be penalized severely. To avoid problems remember this simple rule:
Leave it ready to go for the next user.
What does this mean? It means that you will clean up after yourself or face a laboratory penalty. These penalties will increase exponentially if you persist in bad habits such as leaving chemicals on the balances. If you use something, it is your responsibility to make sure that the next user can proceed unencumbered and without fear of contacting an unknown chemical. I don't care if the balance was dirty when you started -- when you are done with it MUST be clean. Likewise, if you notice that we are about to run out of a reagent, tell your instructors before we run out. It is extremely rude (and bad laboratory practice) to return an empty bottle of reagent to the shelf.