Summary

In this demonstration, students will observe the relationship between the density of a solution and its impact on an object’s ability to float or sink. Uncooked eggs will be placed in a tap water sample and in a salt water solution so that students can make observations and compare the results in order to make a determination regarding the relative density for each item.

Grade Level

Elementary School

NGSS Alignment

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• 2-PS1-1: Plan and conduct an investigation to describe and classify different kinds of materials by their observable properties.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

By the end of this demonstration, students should be able to:

• Explain the meaning of density as it relates to sinking and floating behavior.
• Interpret an observation.
• Understand that different solutions can have different densities.
• Indicate that salt water is denser than tap water

Chemistry Topics

This demonstration supports students’ understanding of:

• Density
• Solutions
• Observations

Time

Teacher Preparation: 5 minutes
Lesson: 25 minutes

Materials

• 2 uncooked eggs
• Tap water
• ½ cup measuring cup
• ½ cup of table salt
• 2 large transparent containers or cups (deep glass bowls will also work; be sure it can hold 2-3 cups)
• Marker
• Masking/labeling tape
• Mixing spoon
• Student handout (per student)

Safety

• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Teacher Notes

• Before class begins, set up a table with all of the materials in the front of the classroom. Make sure that every student will be able to observe the demonstration from where they are sitting. Distribute goggle and aprons and the student handout for students to record their observations.
• Have students individually complete the first question on the handout – listing two objects that they have seen sink in water and two objects that they have seen float – and then share answers to create a class list.
• Explain the meaning of density to your students as it relates to whether one object will sink or float in another – objects that are more dense than the liquid they are placed in will sink, and objects that are less dense will float. Relate it to students’ experiences – floating in a swimming pool, or floating toys in a bathtub, for example.
  • Note that students do not need to understand the mathematical definition of density or the relationship between density, mass, and volume (D = m/V), to complete this activity, but this could be added in if used with older students.

Demonstration Procedures:

1. Fill both transparent containers with tap water (approximately 3 cups in each).
2. Add ½ cup of salt to one container and stir to dissolve and create a salt water solution.
3. Place one uncooked egg into the container containing only tap water.
4. Record observations.
5. Place the other uncooked egg into the container containing salt water.
6. Record observations.
7. Compare the behavior of the eggs in each of the containers to rank the densities of the three materials (egg, salt water, and tap water).

• Results: The egg will float in salt water, and the egg will sink in tap water. This indicates that the egg is less dense than the salt water but more dense than the tap water

• Extension ideas:
  • Create different concentrations of salt water solution (use less or more salt) and see if this affects the egg’s floating behavior.
  • Compare and contrast the observations of the uncooked eggs in salt vs. tap water to boiled eggs in salt vs. tap water and discuss the differences. Make sure to label uncooked eggs and boiled eggs.
  • Add something that floats on both tap and salt water (ex: ping pong ball, pencil, woodchip, sponge, etc.) and something that sinks in both tap and salt water (ex: pebble, marble, coin, eraser, dry pasta, etc.). Have students add these items to their density ranking (including egg, salt water, and tap water) justify their order with their observations.
  • Consider creating a simple density column with liquids of different densities (ex: from most to least dense: corn syrup, water, oil) to show the relative densities of other liquids as well.
  • Ask students if they think they would have different results if they used other liquids or solutions (ex: oil, rubbing alcohol, sugar water, etc.). Test them out, if time allows.
  • Older students could be introduced to density calculations and then tasked with designing procedures to help them collect the data they need to calculate the density of each material in the demonstration.
  • See list at the end of teacher notes for more AACT density resources for elementary students.
• Post-demo real world connection: This could be related to oil spills. Like ships, oil floats because it is less dense than water, but unlike ships, oil requires no special engineering design in order to float. Oil is naturally less dense than water, which is why even oil and vinegar salad dressing separates, with the oil floating on the water-based vinegar. Though oil spills are detrimental to the environment, the ability of oil to float aids cleanup.
• Classroom resources from the AACT Library that may be used to further teach this topic include:
  • Animation: Density
  • Lab: Under the Sea: A Density Aquarium!
  • Lab: Comparing Density of Liquids & Solids
  • Activity: Sink or Float
  • Lesson Plan: What is Density?
  • Demonstration: Sinking Soda
  • Demonstration: Household Densities