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.
Middle and Elementary School
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
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.
This demonstration supports students’ understanding of
- Identifying an Unknown
Teacher Preparation: 30 minutesLesson: 35-45 minutes
- 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
- Oil & water mixture
- Electronic Balance Scale
- Clear tub or fish tank with water
- 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.
- 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.
|Substance||Density (approximation)||Observation (in water)|
|Cotton Balls||<0.5 g/ml||Float|
|Salt Water||1.029 g/ml||Sink|
|Cooking Oil||0.93 g/ml||Float|
|Laundry Soap||1.03 g/ml||Sink|
|Oil & Water Mix||
(depending on mix)
|Sink/Float (see note above)|
|Water||1 g/ml||Sink/Float (see note above)|
For the Student
Density is a property of matter that describes how closely molecules are compacted and it can be used to identify a substance (often indicated on the Periodic Table of Elements). Density is calculated as mass ÷ volume:
- The density value of water is 1 g/cm³ or 1 g/ml. 1 cm3 = 1 ml
- A substance with a density less than 1 g/cm³ will float and a substance with a density greater than 1 g/cm³ will sink.
What determines if an object will sink or float?
Can an object’s density help us determine if it will sink or float?
- Make predictions as to whether the substances listed on the table will sink or float when placed in water.
- Observe and record results as each bottle of household substances is placed in the water.
- Measure and record the mass of the bottle of each substance using the scale.
- Calculate its density using the equation and record your result in the data table.
- Record any additional observations
Sink or Float?
Sink or Float?
(cm³ or ml)
Density = mass/volume
|Oil & Water||500|
- Compare the density values that you determined for each substance with the actual known density values provided by your teacher. Were there any noticeable differences? If so, what are some reasons for these differences?
- Density can be used to identify an unknown material. It can also be used to predict how it will behave with other substances.
- Calculate the density values for the unknown substances given in the data table.
- Compare each density value with known values of the household substances from the demonstration to try to determine its identity.
- Indicate whether the unknown substance will sink or float in water.
|Substance||Mass||Volume||Density||Possible Substance||Sink or Float in Water?|
|Unknown Substance 1||456 g||643 ml|
|Unknown Substance 2||1600 g||1500 ml|
|Unknown Substance 3||653 g||610 ml|
|Unknown Substance 4||437 g||441 ml|
|Unknown Substance 5||125 g||450 ml|
Explain how and why knowing the density value of a substance can be useful. Give an example!