A higher level of hemoglobin in the blood increases the blood’s density. This is the basis for a simple test that can be used to see if a prospective blood donor has a high enough hemoglobin level to donate safely. A drop of blood is placed in a copper sulfate solution, and the time for the drop to sink to the bottom is measured. If this time is too long, the hemoglobin level is too low. Explain how this test works.