Build a Hydrometer
Milk carton or graduated cylinderSaltSmall lead sinker or BBs
Clear plastic strawMetric rulerGum
Fresh waterPermanent marker
Students will understand how a hydrometer works by building a model and conducting experiments.
- Tell students that a hydrometer is an instrument that floats in a liquid to determine density. It floats higher in more dense liquids and lower in less dense liquids. Review density, if necessary.
- Seal one end of a plastic straw with chewed gum.
- Drop one or two BBs into the straw and place the straw in a container filled almost to the top with fresh water. Add one or more BBs until the straw floats upright in the water with at least one cm of straw above the water.
- Mark the level of the water on the straw with the permanent marker.
- Remove the hydrometer from the water.
- Ask the students if they think the straw will float higher or lower in the water when salt is added to the water.
- Add salt to the water at least two times, about 20g each time, testing density each time. Is there a pattern emerging?
- What would you expect if even more salt were added?