Summary

In this demonstration, students will make predictions about various household materials and whether or not each will sink or float when placed in water. Data will be collected and then used to calculate the density value of each item. Finally students will analyze the relationship between the density value and the observed outcome, and use their knowledge to identify unknown materials.

Grade Level

Middle and Elementary School

NGSS Standards

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

• 5-PS1-3: Make observation and measurements to identify materials based on their properties.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data

Objectives

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

• Measure mass using a balance scale.
• Calculate the density of a substance.
• Analyze density values to determine whether an object will sink or float in another given substance.

Chemistry Topics

This demonstration supports students’ understanding of

• Density
• Mass
• Volume
• Measurement
• Identifying an Unknown

Time

Teacher Preparation: 30 minutes

Materials

• 8 disposable water bottles (500 ml)
• Each of the following (Enough to fill 500ml bottle with as little airspace as possible)
  • Cotton balls
  • Salt water
  • Cooking oil
  • Rubbing alcohol
  • Laundry soap
  • Gravel
  • Oil & water mixture
  • Water
• Electronic Balance Scale
• Calculators
• Clear tub or fish tank with water

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

• This resource is designed as a demonstration, but could also be completed as a small-group or station-based laboratory activity if enough materials are available for student use.
• Additionally, this demonstration can be modified to use alternate materials in place of the listed suggestions.
• Teacher should fill 8 empty (clean/dry) 500 ml water bottles with the substances above leaving little to no air space in each and label with the substances. Alternate sized water bottles can be used, but the teacher will need to update the accompanying student worksheet based on the new volume.
• Students will make predictions as to whether each of the substances will sink or float when placed in water.
• Instruction should take place regarding the density of water = 1 g/cm³ (1 cm³ = 1 ml).
• Allow students to choose the substance to test first. Then gather student predictions (verbally or visually) and place the bottle containing that substance in the tub/tank of water. Students will record the outcome (sink or float).
• Next, using the balance scale measure the mass of the substance in the bottle. It may help/be a good opportunity to allow students to practice using the scale as well.
• Note that the mass of the empty water bottle can be collected, and subtracted from the total mass before it is used in the density calculation. However, the mass of the bottle should be minimal and not contribute significantly to the outcome of the calculations.
• Students will calculate density values using the mass collected for each substance and the volume of the water bottle used (500ml suggested). If the substance sinks, the calculation will be greater than 1. If the substance floats it will be less than one.
• Substances with a very similar density value to water may be observed as not fully sinking of completely floating—these substances will likely submerge in the tub, but not completely sink/rest on the bottom.
• The teacher can alternate testing the object in water first or calculating its density first to enhance the density concept.
• Some results will cause surprise and discussion. Others will be easily predicted.
• This demonstration can also provide an opportunity to discuss errors/discrepancy in density value with students (ex: mass of the bottle, substance not occupying entire volume of bottle, etc.)
• Use the table of densities below as reference when students are calculating densities during the demonstration.