# Density Lab (Volume by Formula)

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## Objective

Students will be able to calculate the density (D=m/V) of multiple cubes with the volume formula (V=lwh).

#### Big Idea

Learning to calculate density is done best with experience.

## NGSS Background

This lesson is based on California's Middle School Integrated Model of NGSS.

MS-PS1: Matter and Its Interactions

PE: MS-PS1-2 - Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred. This lesson is specifically designed to assess the property of density of a substance.

DCI: PS1.A: Structure and Properties of Matter - Each pure substance has characteristic physical and chemical properties.

SP5: Using mathematical and computational think & SP7 Engaging in Arguments from evidence - the students will have to use the formula for volume (V=lwh) and the density formula (D=m/V) to calculate the density of different materials and make predictions on what what density should be assigned to what material. Students often need to argue their point of view based on the evidence they collect to determine the different densities.

CCC: Patterns - Density is a property of matter that can be used to identify a substance. Density is a predictable pattern in science based on the interaction of the molecules contained within any given substance.

With this lab student will find the density of ten cubes of equal size. The true identify of the density cubes is disguised. The students  compare their density results to a chart of known densities to determine what material each cube is.

The density cubes are wrapped in electrical tape to disguise them and numbered with a permanent marker. This allows easy identification at the end of the lab. The electrical tape does add mass and does affect overall density. I address this and provide reasoning in the 'Reflection - Why hide the identity of the density cubes'.

This activity is designed to complement the Density Lab (Volume by Displacement)

## Prelab (Set-up)

25 minutes

I demonstrate to the students how to measure and calculate volume and density without giving away any results. The students must be familiar with how to weigh objects, how to measure the length of objects, and how to use formulas to solve equations.

Materials

1. 10 density cubes
2. Triple beam balance scale
3. Metric ruler
4. Calculator

Instructions

1. Determine the mass of each sample (1-10). Place the sample on the scale and weigh it. NOTE: It is important to subtract the weight of the tape (3.7g) for samples 1,2,3,4,5,7,10 in order to determine the correct mass.
2. Determine the volume of each sample (1-10). Use the ruler to measure the length, width, and height of the cube. Multiple the length, width, and height to determine volume. (V=lwh). NOTE: The volume should be the same for each sample.
3. Divide mass by volume to determine density for each sample (D=m/V).
4. Using the ‘Approximate Densities Table’ determine the material of each sample.

Data

As you can see in the following two images, it would be very easy for students to cheat and simply look at the exposed density cube and guess at what material they are made of. Wrapping the density cubes in electrical tape preserves their identity.

Density Cubes

Density Cubes with electrical tape to mask their identity

## Lab (Student Activity)

50 minutes

Pass out a copy of Density Lab - Volume by Formula to each student.

NOTE: The density cubes have been wrapped in electrical tape to conceal their identities (see my Reflection - Why hide the identity of the density cubes).

Students measuring mass and volume

TIP: As the students are calculating density they have a natural tendency to try to identify each cube as it is calculated. The problem arrises when they find a cube that has a similar calculated density as a previously calculated cube and the students are not sure which material to assign to that cube.

I tell my students to calculate ALL the cube FIRST, then they can use the 'Approximate Density Table' as a guide to make approximate assignments. I remind them that there are often differences (standard error of the mean) between theoretical results (Approximate Density Table) and experimental results (their data) and differences are to be expected.

Student Work Sample

Student Work Sample Questions

1. Describe density in your own words.
2. Would the sample of pine (wood) have the same density as a pine tree? Explain.
3. Why does the sample with the greatest mass have the greatest density?
4. Will dense objects always weigh more than less dense objects? Explain

At the conclusion of the lab I reveal the actual identify of each density cube.

1. Aluminum
2. Steel
3. Brass
4. Copper
5. Acrylic
6. Oak
7. Nylon
8. Pine
9. Poplar
10. PVC

## Extension

45 minutes

I created a Powerpoint presentation Density is mass per unit volume to teach how to calculate density.

I teach the density formula as D stands for divorce and divorce leads to a broken heart. I draw it on the board and ask the kids if they see the density formula of D=m/V.