Lesson Summary Students predict and test whether bag of objects from previous lessons will float or sink in salt water. Through the investigation, students discover that the buoyant force in salt water is greater than fresh water. The increased buoyant force is caused by the added weight of the salt in salt water. Students now discover that some objects that sink in fresh water will float in salt water. Teacher Background Salt water is denser than fresh water because of the weight of the dissolved salt. This greater density of the salt water, as compared to fresh water, increases the buoyancy of objects. Liquids with greater density than fresh water push up on objects with a greater buoyant force. In this lesson, the buoyant force of salt water is great enough to cause both acrylic cylinders and the nylon bolt to float, even though they sink in fresh water. The concentration of salt in the water needs to be very high in order for the acrylic cylinders to float.  Due to less concentration of salt within individual containers, some students may discover the acrylic cylinders sink. This can lead to a good discussion about possible reasons for the discrepancy and to suggest that students investigate the effect on adding more salt. Some students may suggest adding more water, which will dilute the concentration of salt. Set-up/Management Tips Count the bag of objects to ensure they are all there. This investigation uses plastic cups, instead of plastic tanks. If water has been saved in containers, it can be reused for this investigation. Set up multiple testing Literacy Support Scientific Vocabulary The following words are key vocabulary words that will be introduced in this lesson and reinforced throughout the unit: Dilute Continue to use vocabulary introduced in previous lessons.