Exercise 4.4: Building a Hydrometer

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Experiment Time: Building a Hydrometer

Aim: to build a hydrometer

Prediction: Before you begin, predict if the straw will float higher or lower in the water when salt is added to the water.

· Milk carton or graduated cylinder
· Clear plastic straw
· Fresh water
· Salt
· Metric ruler
· Permanent marker
· Small lead sinker or BBs
· Gum

1. Seal one end of the plastic straw with chewed gum.
2. 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.
3. Mark the level of the water on the straw with the permanent marker.
4. Remove the hydrometer from the water.
5. Add salt to the water at least two times, about 20g (1 tsp.) each time, testing density each time.

Check Your Understanding


Use the ruler to measure the distance between the marks on the straw. Record your results on a table.


1. Describe the pattern that you notice. What is the relationship between salt and how the straw floats in the water?
2. What would you expect if even more salt were added?