the density of an unknown liquid - · pdf file8th grade science essential labs page 2 of 23...

Name: ________________________ Date: ____/_____/________ 8th Grade Science Essential Labs Lab #1: The Density of an Unknown Liquid

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The Density of an Unknown Liquid

Scientific Question

“How do you determine the density and identity of an unknown liquid?”

Objectives

SWBAT:

Measure a liquid’s volume & mass using appropriate tools (graduated cylinder, scale)

Calculate density given the equation: Density = Mass/Volume (D = m/V)

Calculate density by reading a line of best fit on a graph of Mass/Volume

________________________

Background Information

Scientists study the properties of matter for many reasons. One reason is to help them identify unknown substances. Density is often used to identify an unknown substance. The density of the unknown is compared to a list of known substances to determine its identity. It is very unlikely for two substances to have the same density. In this experiment, you will use density to figure out the identity of an unknown liquid.

Safety Alerts

Wear goggles when using glassware.

Do not drink any substances in the lab including the unknown liquid that is provided.

Measurements

The unit of measurement for mass is grams (g)

The unit of measurement for volume is milliliters (mL)

Density’s unit of measurement is g/mL or g/cm^3

Materials

In a basket, you would place a 10mL graduated cylinder, a plastic pipette, a cup with

the unknown liquid. Each lab bench would also need an electronic balance.

Post-Lab Assignment

1. A liquid has a mass of 4.5 g and a volume of 5.5 mL. What is the density of the liquid?

_____

2. In the previous problem, the student was given the mass of a liquid. Describe how you would

find the mass of a liquid.

________

Lab Procedures:

1. Measure the mass of an empty 10mL graduated cylinder using an electronic balance. Record the mass of the graduated cylinder here _______ and in your data table. Record the volume of the empty graduated cylinder as 0 mL in (Data Point 1 under “volume”) since your graduated cylinder is empty.

2. Add 2 milliliters of the unknown liquid to your graduated cylinder using a pipette. Record the exact volume that you added in your data table here_____ and in your data table. (Data Point 2).

3. Measure the mass of the graduated cylinder with the liquid. Record this mass in your data table (Data Point 2).

4. Do not discard, which means do not throw away, the liquid or empty out your graduated cylinder!

5. Add another 2 milliliters of the unknown liquid to your graduated cylinder. Record the exact volume that you added here ____ and in your data table (Data Point 3).

6. Measure the mass of the graduated cylinder with the liquid. Record this mass here _____ and in your data table.

7. Repeat steps #4 - #7 three more times to fill in the remainder of your Data Table.

8th Grade Science Essential Labs of 23 Lab #1: The Density of an Unknown Liquid

Data

Record your data table in the chart below. Remember “no naked numbers!”-add units to ALL your

numbers!

Table 1: Data Collected from Experiment

Data

Point

Mass

Of Graduated

Cylinder ___g

Mass of

Graduated

Cylinder +

Liquid

__g

Volume

of liquid

added

each time

__mL

Volume of

Liquid in

Graduated

Cylinder

___mL

Mass of

Liquid in

Graduated

Cylinder

___g

Density

of

Liquid

g/mL

d=m/v

1

______g

0mL ___mL

2

3

4

5

6

7

Graph

Create a graph of your data of each density from all 7 data points.

Remember to put your independent variable on the x-axis, which is _____ and the dependent

variable on the y-axis, which is _________.

Create a scatterplot by plotting all your data points on the graph – no lines connecting them,

just points “scattered” on the graph

After you have plotted, draw a line of best fit – this is the one straight line that seems to “fit”

all the points you plotted in the scatterplot.

Determine the slope of that line. Show your calculations in the box below.

Calculation of Slope of the Line of Best Fit:

HINT!

Slope = rise/run

Slope = ΔY/ΔX


CFU Question #3:

What things are needed on a proper and accurate graph?

Analysis Questions

1. The table below contains different liquids and their densities.

Water 1.00 g/mL

Butane 0.60 g/mL

Citric Acid 1.66 g/mL

1. Which liquid is most likely the same as your unknown liquid? Justify your answer in complete

sentences. Support your answer by explaining your reasoning or showing calculations.

_________________________________________________________________

2. What is the average density of your unknown liquid?

3. Predict what would happen to the density of your liquid if you heated the liquid. Explain your

reasoning.


Conclusion

Using complete sentences, answer your scientific question on page 1. Use specific data from your lab

investigation as evidence.

1. A scientist found a bottle without a label in her lab’s chemical closet. The bottle contained a liquid that is cloudy in color and odorless. She measured the mass of an empty graduated cylinder. Then, she measured the mass and volume of the chemical and compared her findings with the data table below.

If the mass of the empty graduated cylinder is 45 g, what is the identity of the chemical? a. water b. vinegar

c. carbonic acid d. ethanol

e. isopropyl alcohol

2. Ricardo collected the

following data in his

experiment for the

density of an unknown

liquid.

Volume of Liquid 9 mL

Mass of Empty Graduated Cylinder

26 g

Mass of Graduated Cylinder + Liquid

39 g

Based on his data, what is the

mass of his liquid?

a. 9 mL

b. 3 g

c. 13 g

d. 26 g

e. 39 g

What is the volume of

Vivian’s rock?

a. 10 mL b. 20 mL c. 30 mL d. 40 mL e. 50 mL


Teacher Notes for the Density of an Unknown Liquid

General Description

In 6th grade, students were exposed to basic density calculations and focused on determining the density using

water displacement. For our 8th graders, we want to look at a different example, the density of the liquid. The

goal is for them to determine the density and ultimately its identity from a list of known liquids. The other

component is to begin to expose students to the idea that the slope of the line is often significant in science.

This is an important concept for physics and AP science courses.

Consumable Materials

Materials Amount Per Group Cost Vendor Item Number

food coloring (optional) 4-5 drops varies Wal-Mart 0005210016025

Scientific Equipment

Equipment Amount Per Group Cost Vendor Item Number

Graduated Cylinder, 10 mL 1 $15.20 Flinn Scientific GP2005

Electronic Balance 1 $95.95 Flinn Scientific OB2098


Student Completion Time

Pre-Lab Assignment: 15 minutes

o If you want to review the pre-lab assignment before starting the lab, you can assign the pre-lab

assignment as the homework assignment before the lab.

Data Collection: 15 minutes

o If you are worried about time, you could potentially use electronic balances to expedite this

process.

Calculations and Graphing: 20 minutes

o Before your students begin with the analysis and conclusion questions, it is recommended that

you go a quick check of their data, calculations and graph.

Analysis and Conclusion – 25 minutes

o Students may not be able to finish their conclusion in this time frame; however, they can finish

the rest for homework.

o Your other option would be to give students more time and have the students who finish early

work on density practice problems or an extension activity.

Common Mistakes or Misconceptions

Procedural Mistakes

Most 10mL graduated cylinders are marked every 0.2 mL. Students often assume that markings

represent 1 mL or 0.1 mL. One quick CFU would be to place a 10 mL graduated cylinder at each table

and fill it with a known amount of liquid. You could then let each table read the graduated cylinder at

their table. You could then get them to share out their reading and check against the reading that you

took. This will tell you who needs more help reading the volume from a graduated cylinder.

When a procedure asks students to add 2 milliliters of a liquid, they often assume that the amount of

liquid that they put in the graduated cylinder is 2 milliliters. Be sure that students are adding the liquid

and then using the graduated cylinder to measure the actual volume.

You may want to provide students with a plastic pipette, this may make data collection more efficient

because students will not fast time dumping excess water and then adding water back because they

dumped out too much water.

Students may dump the liquid out after measuring its mass, so they end up with a data points that all

measure the same (or very similar) volumes of the liquid. When walking around the room, check

student data to ensure that they are getting data for different volumes of liquid.

Content Based Mistakes

Students have been taught independent and dependent variables in 6th grade. However, you may want

to plan a 10 minute quick review of the concept prior to completing the lab.

Students will be creating a scatter plot. They have created graphs in math class, and we want to be

sure that we are holding them to same expectations. One common mistake is the scaling for the x- and

y-axes. You want to ensure that students have a consistent scaling.


Students will not have been introduced to the concept of a line of best fit. Plan to introduce this

concept to your students either before the lab or after data collection. (project or print out the visuals

to help explain these concepts)

Teacher Preparation Instructions

Preparation

To help prevent students from knowing the unknown is water. You can add a small amount of food

coloring, and it won’t significantly change the density.

Labs can be chaotic especially with 30-40 middle school students moving around one classroom.

Consider putting all of the materials at their lab benches. In a basket or small plastic tub, you would

place a 10mL graduated cylinder, a plastic pipette, a cup or beaker with the unknown liquid. Each lab

bench would also need a balance.

Ideas for Differentiation

For students who struggle, you can:

o Eliminate a data point. This will reduce the amount that they have to collect and the amount

they have to graph while still getting the full scope of the experiment.

o Provide students with one or both scales for the axes.

For students who need more of a challenge, you can:

o Have them support or refute their prediction about the heated liquid. You can have them

explore with warm water.

o Have them explore how the addition of salt affects the density of a liquid.

o Develop a reading about how density of the ocean affects ocean currents.

Disposal

Since the unknown liquid is water, it can be disposed of down the sink.

Answers to Pre-Lab Assignment

1. Would you classify density as a physical property or a chemical property? Justify your answer.

Density is a physical property. To determine the density of an object, you do not have to change its

composition or make-up.

2. A liquid has a mass of 4.5 g and a volume of 5.5 mL. What is the density of the liquid?

mL

g

mL

g

V

mD 82.0

5.5

5.4

3. In the previous problem, the student was given the mass of a liquid. List the steps you would take to

find the mass of a liquid.

1. Measure the mass of an empty container.

2. Record the mass of the empty container.

3. Add the liquid to the empty container.

4. Measure the mass of the empty container and the liquid.

5. Record the mass of the empty container and the liquid.

6. Subtract the mass of the empty container from the combined mass of the empty

container and the liquid.

4. Find the slope of a line in the graph below.

Comment [PI1]: I’d like to provide teachers more information than this about how to introduce this; however, I have no idea what should be taught nor how it should be taught.


23

6

21

06

run

riseSlope

Sample Data

Data Point Volume Mass

1 0.0 mL 27.0 g

2 2.0 mL 28.7 g

3 3.6 mL 30.2 g

4 5.4 mL 31.7 g

5 7.4 mL 33.9 g

6 9.6 mL 36.1 g

Sample Calculations

1. For your second data point, calculate the mass of the liquid. Show your work in the space below.

28.7 g – 27.0 g = 1.7 g

2. In the chart below, record your volume and the mass of the liquid. You will have to do the same

calculation that you performed in the previous question. However, you don’t have to show your work

for the other data points.

Table 2: Data from Your Calculations

Data Point Volume of Liquid Mass of Liquid

1 0.0 mL 0 g

2 2.0 mL 1.7 g

3 3.6 mL 3.2 g

4 5.4 mL 4.7 g

5 7.4 mL 6.9 g

6 9.6 mL 9.1 g


Sample Graph

Answers to Analysis Questions

1. What does the slope of your line represent?

The slope of the line represents the density of the liquid.

2. What is the density of your unknown liquid? Support your answer by explaining your reasoning or

showing calculations.

Answers will vary but should match experimental data. You also want students to use the slope of the

line by using two data points closest to their line of best fit.

mLmL

ggDensity

0.26.3

7.12.30.94 g/mL

3. The table below contains different liquids and their densities.

Water 1.00 g/mL

Butane 0.60 g/mL

Citric Acid 1.66 g/mL

Which liquid is most likely the same as your unknown liquid? Justify your answer in complete

sentences.

Answers will vary, but you want students to answer base on their answer to the previous question.

Here is an example.

The unknown liquid is water because the density of the unknown liquid is the closest to the value for

the density of water.

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10

Mas

s (g

)

Volume (mL)

Density of an Unknown Liquid


4. Your lab partner, Julian, spilled some of the unknown liquid on the balance. He never cleaned it up.

a. What part of your data would this affect: the mass or the volume? Explain.

It would affect the mass of the liquid since the liquid was on the balance but not in the graduated

cylinder.

b. How would this affect your experimental value for your density? Justify your reasoning.

The mass that I would read from the balance would be greater than what it should be. Since

the mass would be bigger, the calculated density would be bigger as well.

5. Predict what would happen to the density of your liquid if you heated the liquid. Explain your

reasoning.

The liquid’s density should decrease because as you heat the liquid, the particles will gradually spread

apart and occupy a bigger volume. Since the mass will be the same and the volume will be bigger, this

would make the value of the density decrease.

.


Visual Aids

Scatterplot – data points graphed with no lines

“Line of best fit” – line that is implied by the points

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10

Ma

ss (

g)

Volume (mL)

Density of an Unknown Liquid (blue)

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10

Mas

s (g

)

Volume (mL)

Density of an Unknown Liquid (blue)


slope of a line – “steepness” of a line

http://mreaton.com/wp-content/uploads/2011/11/race21.jpg

http://mreaton.com/wp-content/uploads/2011/11/race1.jpg


Practice Graphs


Name Date Period

Assessment on Lab #1: Density of an Unknown Liquid


Objective Summary # Correct

% Mastery

Mastered?

#1— I can calculate density given the equation: Density = Mass/Volume. /1

#2— I can measure an object’s volume & mass using appropriate tools.

/1

#3— I can calculate density to identify an unknown substance.

/1

#4— I can calculate density by reading a line of best fit on a graph of Mass/Volume.

/2

Directions: Do the best you can to answer the following questions. Take your time and read

carefully. You need to get 4 out of 5 on the entire quiz to earn an 80% or higher (mastery).

1. Ricardo collected the following data in his experiment for the density of an unknown liquid.


Mass of Empty Graduated Cylinder 26 g

Mass of Graduated Cylinder + Liquid 39 g

Based on his data, what is the mass of his liquid?

a. 9 mL

b. 3 g

c. 13 g

d. 26 g

e. 39 g

__ Objective #1 Question #s % Mastery Mastered?

RE

TA

KE

?

I can calculate density given the equation: Density = Mass/Volume.

1 0 correct 1 correct

50%..............100%

Over 80%?

Y/N

__ Objective #2 Question #s % Mastery Mastered?


2. Vivian finished her experiment early. With the teacher’s permission, she decided to conduct

her own experiment to determine the density of a rock. Below is a picture of part of her

experiment.

What is the volume of Vivian’s rock?


RE

TA

KE

?

I can measure an object’s volume & mass using appropriate tools.


50%..............100% Over 80%?

Y/N __ Objective #3 Question #s % Mastery Mastered?


3. A scientist found a bottle without a label in her lab’s chemical closet. The bottle contained a liquid that is cloudy in color and odorless. She measured the mass of an empty graduated cylinder. Then, she measured the mass and volume of the chemical and compared her findings with the data table below.

If the mass of the empty graduated cylinder is 45 g, what is the identity of the chemical? a. water b. vinegar c. carbonic acid d. ethanol e. isopropyl alcohol

4. The graph below shows the mass versus volume for three liquids of different densities. Which

of the following statement(s) is/are true?

RE

TA

KE

?

I can calculate density to identify an unknown substance.


50%..............100% Over 80%?

Y/N __ Objective #4 Question #s % Mastery Mastered?

RE

TA

KE

?

I can calculate density by reading a line of best fit on a graph of Mass/Volume.

4,5 0 correct 1 correct 2 correct

50%..............50%..............100%

Over 80%?

Y/N


I. The slope of the line represents the density of the liquids.

II. Graph I represents the liquid with the greatest density.

III. Graph III is impossible because as the volume of a liquid increases, the mass

should also increase.

a. I only

b. II only

c. I and II only

d. I and III only

e. I, II, and III

5. The graph represents the data collected from a student’s experiment to determine the density

of an unknown liquid.


a. Calculate the density of the liquid. Show all work in the space below.

b. Your lab partner wanted to use data point A. Would you agree or disagree with your lab

partner? Explain why.


Name Key Date Period

Assessment on Lab #1 (Density of an Unknown Liquid)

2. Ricardo collected the following data in his experiment for the density of an unknown liquid.


Mass of Empty Graduated Cylinder 26 g

Mass of Graduated Cylinder + Liquid 39 g

Based on his data, what is the mass of his liquid?

a. 9 mL

b. 3 g

c. 13 g

d. 26 g

e. 39 g

3. Vivian finished her experiment early. With the teacher’s permission, she decided to conduct

her own experiment to determine the density of a rock. Below is a picture of part of her

experiment.

What is the volume of Vivian’s rock?



4. A scientist found a bottle without a label in her lab’s chemical closet. The bottle contained a

liquid that is cloudy in color and odorless. She measured the mass of an empty graduated cylinder. Then, she measured the mass and volume of the chemical and compared her findings with the data table below.

If the mass of the empty graduated cylinder is 45 g, what is the identity of the chemical?

f. water g. vinegar h. carbonic acid i. ethanol j. isopropyl alcohol

5. The graph below shows the mass versus volume for three liquids of different densities. Which

of the following statement(s) is/are true?


I. The slope of the line represents the density of the liquids.

II. Graph I represents the liquid with the greatest density.

III. Graph III is impossible because as the volume of a liquid increases, the mass

should also increase.

a. I only

b. II only

c. I and II only

d. I and III only

e. I, II, and III

6. The graph represents the data collected from a student’s experiment to determine the density

of an unknown liquid.


a. Calculate the density of the liquid. Show all work in the space below.

mL

g

mLmL

ggDensity 2

39

618

Students, who calculate the density using the slope of the line, should be awarded full

credit, while students who use a single point should only be awarded partial credit even

though they will get similar values.

b. Your lab partner wanted to use data point A. Would you agree or disagree with your lab

partner? Explain why.

I would disagree because point A is one of the points farthest from the line of best fit.

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