Lesson Summary Students continue to investigate how the buoyancy of objects is affected by fresh and salt water. In this lesson, they construct a device that can be used to determine how heavy a liquid is compared with fresh water. This tool is called a hydrometer. The hydrometer floats in salt water and fresh water, assisting students to develop explanations for the differences they see in the levels of water. They make predictions about how high the hydrometer will float in salt and fresh water, and compare the results. Also, they extend their ideas about hydrometers through reading and discussion. Teacher Background The hydrometer is a tool that can be used to compare the densities or relative weights of equal volumes of different liquids. A hydrometer is clearly marked to indicate the point at which it floats in fresh water. By measuring how high the hydrometer floats in a liquid other than fresh water, an observer can tell immediately if the liquid is heavier or lighter than fresh water.  The hydrometer floats higher in salt water than in fresh water, indicating the salt water is heavier than fresh water. Set-up/Management Tips Graduated cylinders from lesson 10 will be used, make sure they are still labeled with masking tape. Separate clay into smaller sections (about the size of a marble). Ahead of time, for construction of the hydrometer, mark the middle point of each plastic straw using a permanent marker. (see p. 121 of teacher manual) For an extension, students can explore other liquids to test. (see teacher manual p. 124) Literacy Support Scientific Vocabulary The following words are key vocabulary words that will be introduced in this lesson and reinforced throughout the unit: Hydrometer Plimsoll Mark (from Student Investigations Book)