Which laboratory instrument is commonly used to prevent leaks or spills in a pipette?

The choice of a stopper as the correct answer highlights its role in sealing openings in laboratory glassware, including pipettes. A stopper can effectively prevent leaks or spills by blocking the opening of a pipette when not in use or when transporting liquids. This is particularly important in maintaining the integrity of the sample being handled and ensuring safety in the laboratory environment.

When a pipette is not being actively used to dispense liquid, a stopper can be inserted to create a tight seal, thus stopping any liquid from escaping. This feature allows for safer handling of chemicals, particularly volatile solvents or hazardous substances that need to be contained.

Other instruments like coverslips are typically used in microscopy for securing samples on a slide, while hot plates are used for heating substances. A stopcock is a valve that controls the flow of a liquid or gas, but it does not serve the purpose of sealing openings in a pipette in the same way that a stopper does. Therefore, the use of a stopper is essential for preventing spills and ensuring accurate liquid handling in laboratory settings.