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LAB in Density, Graphing. Last updated September 16, 2022.

### Summary

In this lab, students will collect and plot both volume and mass data in order to better understand density as a constant by using the line of best fit. They will then model and analyze a perplexing situation involving density to consider the application of density in everyday life.

High School

### NGSS Alignment

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

• HS-PS1-3 Matter and Its Interactions: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
• Scientific and Engineering Practices:
• Using Mathematics and Computational Thinking
• Developing and Using Models
• Analyzing and Interpreting Data
• Planning and Carrying Out Investigations
• Engaging in Argument from Evidence

### Objectives

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

• Demonstrate a clear understanding of density including the directly proportional relationship between mass and volume in the density of a substance.
• Calculate the density of a substance given data of mass and volume.
• Develop models to observe chemical phenomena related to the density of a substance.
• Utilize models to explain chemical phenomena related to the density of a substance.
• Graph independent and dependent variable data points and interpret the slope and y-intercept of a graph as real-world properties of a substance.

### Chemistry Topics

This lab supports students’ understanding of:

• Density
• Mass
• Volume
• Graphing
• Linear or Directly Proportional Relationships
• Intensive Properties

### Time

Teacher Preparation: 30 minutes
Lesson: 90 minutes

### Materials (per group)

• Ethyl Alcohol (95%) in wash bottle
• Sodium Thiosulfate Pentahydrate (Na2S2O3 • 5 H2O), 50 grams
• Plastic weigh boat
• 100 mL Beaker
• Distilled Water
• Black Sharpie
• Medicine Cup
• 6 Pennies
• Balance (0.1-gram precision)

Disposal:

• Large rectangular aluminum pan
• 1000 mL Waste Beaker

### Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students will be using 95% Ethyl Alcohol which is an extremely flammable substance. There are to be no flames or heat sources around this laboratory at all.
• Students are to use latex gloves and eye protection when working with the ethyl alcohol as well as the sodium thiosulfate.
• Be sure to adhere to disposal procedure outlined in the Teacher Notes section below. Do not place any sodium thiosulfate down the sink.
• Students should wash hands thoroughly with soap and water before leaving the lab.
• Avoid all contact of chemicals with eyes and skin.
• Follow the provided instructions for cleanup of materials and disposal of chemicals.
• Notify teacher and clean up all spills immediately.
• Refer to the SDS information for all chemicals used in this activity.

### Teacher Notes

Background:

• In my experience, many students have broad misconceptions about density since our minds more easily understand concepts such as mass and volume. Students will often have a misconception that heavy substances have high densities and therefore always sink while light substances have low densities and always float. They will also incorrectly conclude that if mass can be added and volumes can be added, then density can be added as well. Since the concept of density deals with the relationship between two variables - mass per unit volume, my students have often shown difficulty fully understanding it.
• This is a good experimental starter with many classes directing them towards a more scientifically sound derivation of a constant such as density. Therefore, they cannot simply plug numbers into an equation but must use the line of best fit data (just like with Beer's Law). This helps understand experimental error and the relationship between a scientific constant and the linear slope of data.
• An Answer Key document that includes sample data, expected results and photographs for teacher reference is available to download.

Lab Preparation and Understanding:

• For this lab activity, I suggest that students work in groups of two.
• Each group should be given the following for Part A and B: a sealed container with approximately 50 grams of sodium thiosulfate pentahydrate, a wash bottle containing 95% ethyl alcohol, a 100 mL graduated cylinder and a weighing boat for measuring.
• Electronic balances measuring 0.1 grams can be shared between groups.
• Students should use the background information provided on the student handout, and complete the pre-lab activity as a guide for completing the lab portions.
• Helpful Notes for Lab Part B:
• It’s important to ensure that student groups are only measuring and adding 5.0 grams of sodium thiosulfate to the solution in each trial in Part B. The total masses should be compiled in the data table.
• The ethyl alcohol helps account for the volume in between the sodium thiosulfate particles and helps us accurately measure the volume displacement of the 5.0 g mass increments.
• In this part of the lab, students are using the volume displaced by the sodium thiosulfate divided by the displacement of the mass of sodium thiosulfate. They cannot simply plug numbers into an equation but must use the best fit line of data. This again helps understand experimental error and the relationship between a scientific constant and the linear slope of data.
• Helpful Notes for Part C:
• I suggest that teachers circulate and try to only ask questions to stimulate discussion during the Predict phase and Investigate phases of the laboratory.
• It’s helpful for classroom management to have a specified area where students can pick up and return their materials for Part C.
• Approximately 6 pennies yield a significant change in the water level and does not sink the boat (medicine cup).
• An additional suggestion for Part C is to have students whiteboard or sketch out the two models (the cinder block in the boat and the cinder block at the bottom of the lake) to better explain their prediction and observational understanding.
• Helpful photographs and explanations for Part C can be found in the Answer Key Document.
• Disposal: Adhere to the disposal instructions below and you will be able to use these same materials year after year with very little waste.
• Ethyl alcohol and sodium thiosulfate should not be disposed of down the drain. Have a central location where a 1000 mL beaker can be used to collect waste. Direct students to discard all ethyl alcohol and sodium thiosulfate in this beaker.
• Decant a large majority of the ethyl alcohol back into the ethyl alcohol container. Sodium thiosulfate is insoluble in ethyl alcohol so you can save almost all of the ethyl alcohol you will use in this laboratory.
• Take the left-over sodium thiosulfate and a small amount of ethyl alcohol and spread them out on a large rectangular aluminum pan.  You can lay paper towels down on the aluminum pan as well.  Leave overnight and the ethyl alcohol will evaporate leaving behind dry sodium thiosulfate. You can transfer this sodium thiosulfate back into the container and use it for this laboratory again the following school year.
• There is almost no waste in this laboratory as the ethyl alcohol and sodium thiosulfate can be collected and used again in the future.

### Background

The story of Archimedes and his discovery of density is a well-known tale. As the story goes, the famous Greek scientist, Archimedes of Syracuse, was given the task of determining whether King Hiero’s goldsmith was stealing the precious metal by replacing a golden wreath with a cheaper alloy. Archimedes was not able to find the volume of the wreath since it was an irregular object and the king did not allow him to crush it down into a cube in order to compare its mass to a similar cube of gold.

The story goes on to state that Archimedes took a bath in order to calm his mind and contemplate about how to solve the problem. As he dipped his body in the bathtub, the water level rose, and he instantly concluded that water displacement could determine a substance's volume. Upon this epiphany, Archimedes jumped out of his bathtub and ran naked through the streets shouting, “Eureka, eureka, I have found it!”

Each substance, whether solid, liquid, gas, or solution, has a specific density at a given temperature (and pressure for a gaseous substance). Density is an intensive property which means that it does not depend upon the amount of material or size of the object.  As a comparison, density is like temperature in that both values are intensive properties.  If water is at a room temperature of 20°C, that temperature does not change whether you have 50mL or 100mL. The density of that water would not change as well but would be a constant at that temperature.

### Equation for Density

• Density often has units of grams per mL or grams per cubic centimeter (cm3) or kilograms per cubic meter (m3).
• The equation for density can be rearranged into slope-intercept format (y = mx + b) as:

Therefore, density cannot be directly perceived or measured, it must be inferred or derived from the relationship between mass and volume. This laboratory will challenge you to acquire a deeper understanding of density through the measurements of mass and volume.

### Experiment Overview

The purpose of this inquiry laboratory is to measure the volume and mass of two specific substances (a solid and an aqueous solution) to determine the density of these chemical species through experimentation. Then a perplexing situation involving density will be modeled and experimented to conclude the effects of density in everyday life.

### Materials

• Ethyl Alcohol (95%)
• Sodium Thiosulfate Pentahydrate (Na2S2O3 • 5 H2O)
• Balance (0.1-gram precision)
• Plastic weigh boat
• 100 mL Beaker
• 1000 mL Waste Beaker
• Distilled Water
• Black Sharpie
• Medicine Cup
• 6 Pennies
• Large rectangular aluminum pan

### Pre-lab Activity

A student is given the task to determine the density of distilled water using a balance that reads to 0.1 grams, a 100 mL graduated cylinder, and a bottle of distilled water. The student begins by placing approximately 10 mL of distilled water in the graduated cylinder. The student correctly measured the precise volume of the distilled water and records it in a data table. The student then measures the mass of the distilled water and graduated cylinder on the balance. The student continues to place approximately 10 mL of distilled water increments in the graduated cylinder recording the measured volume and mass each time. The student’s data table is shown below.

 Volume (mL) Mass (grams) 9.85 162.3 20.03 172.4 31.15 183.5 39.95 192.4 50.18 202.6

### Pre-lab Questions

1. What is the independent variable in this experiment?
1. What is the dependent variable in this experiment?
1. Plot the student’s data below:
1. Draw a best-fit line on the data in the graph above.
1. Calculate the slope of the best-fit line (be sure to include units in your answer).
1. What does the slope or constant represent for the distilled water?
1. Continue your best fit-line to determine the y-intercept of your data. What is the y-intercept?
1. What does this y-intercept represent in the experiment above?

### Safety Precautions

• Always wear safety goggles when handling chemicals in the lab.
• You will be using 95% Ethyl Alcohol which is an extremely flammable substance. There are to be absolutely no heat sources or flames around this laboratory at all.
• Wear latex gloves when handling ethyl alcohol and sodium thiosulfate.  Be sure to adhere to disposal procedure below.
• Avoid all contact of chemicals with eyes and skin.
• Wash your hands thoroughly with soap and water before leaving the lab.
• Follow the provided instructions for cleanup of materials and disposal of chemicals.
• Do not place any sodium thiosulfate down the sink.
• Notify teacher and clean up all spills immediately.

### Part A: Density of Ethyl Alcohol

1. Using the procedure modeled in the Pre-lab Activity, you are to determine the density of 95% ethyl alcohol using a 100 mL graduated cylinder and a balance.
2. Record all data in the table below.
3. Be sure to use at least 5 measurements of volume and mass of ethyl alcohol.
4. Do not dispose of the ethyl alcohol as it will be used in Part B.

 Data: Density of 95% Ethyl Alcohol Volume (mL) Mass (grams)

### Part A: Analysis

1. Plot the data in the space provided.
1. Label the x-axis and y-axis scales on the graph.
1. Draw a best-fit line and calculate the slope.
1. What is the density of 95% Ethyl Alcohol?
1. What does the y-intercept represent in your graph below?

### Part B: Density of Sodium Thiosulfate

1. Measure approximately 5.0 grams of sodium thiosulfate in a weigh boat and record the exact mass in the data table below.
2. Carefully transfer the 5 grams of sodium thiosulfate into the graduated cylinder containing the 50.0mL of ethyl alcohol from Part A.
3. Determine the volume of sodium thiosulfate and ethyl alcohol and record it in the data table below.
4. Continue transferring 5 gram measurements of sodium thiosulfate into the graduated cylinder while recording the total volume.
5. In the data table, be sure to record the total mass of sodium thiosulfate each time. (For example, the first mass would be ~5.0 grams of sodium thiosulfate, but for the second mass—you added 5.0 grams more for a total of 10.0 grams of sodium thiosulfate. Be sure to do at least five 5.0-gram measurements during the experiment.

 Data: Density of Sodium Thiosulfate Volume (mL) Mass (grams)

### Part B: Analysis

1. Plot the data in the space provided.
1. Label the x-axis and y-axis scales on the graph.
1. Draw a best-fit line and calculate the slope.
1. What is the density of Sodium Thiosulfate?

• Pour the ethyl alcohol and sodium thiosulfate into the labeled waste beaker (1000 mL beaker).
• DO NOT POUR the ethyl alcohol or the sodium thiosulfate down the drain!

### Predict

Suppose you are in a boat in the middle of a calm lake with a large, heavy cinder block inside the boat with you. While the cinder block is inside the boat with you, the height of the lake is measured to be exactly 10 feet deep. You then take the cinder block, lift it on the outside of your boat, carefully place the cinder block in the lake, and let it sink all the way to the bottom of the lake.

1. Did the height of the lake water become greater than 10 feet deep, less than 10 feet deep, or stay exactly 10 feet deep after you placed the cinder block in the lake water? (Circle Your Answer)

### Investigate

Now, you will test this cinder block and boat scenario to see if you were correct. You will be given the materials shown in the table below. Before you conduct this experiment, write what each of the materials will model or represent from the cinder block/boat scenario above (the first answer is given for you).

 Materials What does each material represent? Sharpie To record the “10 feet” mark on the beaker Beaker Distilled Water Medicine Cup 6 Pennies

Now that you have your “cinder block” in the “boat” in the “lake”, mark the “10 foot” mark on the beaker using the sharpie. Then place “cinder block” at the bottom of the “lake” and place the “boat” back in.

1. Did the height of the “lake” water become greater than 10 feet deep, less than 10 feet deep, or stay exactly 10 feet deep after you placed the cinder block in the lake water? (Circle Your Answer Using the Evidence Provided in the Model)
2. Draw two sketches to represent this model: The first sketch should represent the cinder block in the boat, and the second sketch should represent the cinder block at the bottom of the lake. Make sure to include all elements of the model (cinder block, boat, lake and water) in your sketches.

### Part C: Analysis

1. Did the density of the cinder block change when it was in the boat as opposed to in the water? Justify your answer with your understanding of density.
1. What attribute of the cinder block was playing a greater role in lake water rising while the cinder block was inside the boat (the mass of the block or the volume of the block?)
1. What attribute of the cinder block was playing a greater role in lake water rising while the cinder block was submerged on the bottom of the lake (the mass of the block or the volume of the block?)
1. Explain the relationship between the mass of a solid substance per the volume of a solid substance in determining the density of a solid substance.
1. How would a balloon filled with a gas in this boat/lake scenario differ in comparison to the cinder block? How does mass per unit volume relationship of a gas change this scenario?