A balance is used to weigh chemicals. The chemicals are always in some form of container and never placed directly on the balance. It is important not to move a balance because they have been calibrated for the exact position they are in. Some balances have plastic housing with small doors to keep air currents from affecting the measurement. Close these doors whenever the balance is in use.
To use a balance to determine the weight of a chemical, first put the empty container that the chemical will be in on the balance. Once you have a reading, press the TARE or ZERO button on the balance. Remove the container from the balance (never add chemical to a container while it is on the balance). Reweigh after adding chemical to get the weight of only the chemical. It is important to keep the balance clean.