Students Learn About Object Density in this Third Grade Science Project


You can study object density in water. Tell the students that they are going to use bowls of water, salt, and several objects to measure an object’s density. When the object is in water, you will need to make the water more dense than the object to get the object to float. This is accomplished by adding salt to the water. When enough salt is added, the object will float.


  1. Table salt
  2. Bowls
  3. Water
  4. Measuring cup
  5. Measuring spoon
  6. Objects to measure
  7. Notebooks
  8. Pencils


  1. Decide how many objects you have time to test. Ideally you should plan to test between five to ten objects. Each object should be different. You will need to use one bowl for each object that you wish to test.
  2. Have a student volunteer to measure and pour two cups of water into each of the bowls.
  3. Place one object in each bowl. Label the bowls and have the students create a chart in their notebooks. The chart should say bowl #1: Title of the Object. Then have a space for how much salt had to be added to the water to make the object float and a space for notes.
  4. Start with bowl number one. Pour one tsp. of salt into the bowl at a time and mix it. Keep doing this until the object floats. For example, an egg should float after the 8th teaspoon of salt. Stop and record the data (number of teaspoons of salt).
  5. Stop after fifteen teaspoons of salt. If the object does not float after fifteen teaspoons, then it will not. The water is as dense as it can be. Use a paper-clip to demonstrate this.
  6. Check to see if the object floats without adding any salt first. An apple is a good example of this.
  7. Continue this process until all items have been checked and the information is recorded in the notebook.

Review: Talk to the students about which objects floated without adding salt. Ask them why they think that object didn’t need salt. Talk with them about the objects that didn’t float at all. Discuss the meaning of density by reading the definition and applying the definition to the project they just performed.

Homework: Have the children repeat this experiment at home, but have them first draw up a hypothesis as to whether they think the object they are measuring will float on its own, float after so many spoonfuls of salt, or not float at all. Have them tell why. They must then record the experiment process as well as the outcomes. Have them draw up a conclusion as to whether their hypothesis was correct or incorrect.