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Specific Gravity: The Relative Density of Liquids

Grades: 5-8
Author: Joyce Brumberger
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Abstract

Module Description

As a result of the professional development provider-conducted module, participants will learn through the creation of their handmade hydrometers, how a hydrometer functions and what it measures. Using their hydrometers, they will determine the relative differences in specific gravity of liquids and compare these values to those obtained from a commercially manufactured hydrometer. Based on their findings, participants will create their own liquid density column. Participants will design a lesson they can implement in their own classroom.

Objectives

Participants will learn the term specific gravity
Participants will learn the function of a hydrometer
Participants will learn the specific gravity of different substances
Participants will be able to construct a hydrometer
Participants will be able to determine the relative specific gravity of different liquids
Participants will be able to create a liquid density column
Participants will produce a lesson plan to guide their own students in the design and implementation of an experiment

Materials

Engagement

Short Clip Videos as described in Engagement Phase

Exploration and Elaboration

20 oz water bottles with tops cut off, 1 for each liquid used, 7 bottles/ set - 1 set/group
2 2-L bottle
3 oz. plastic drinking cups
Distilled water
Kosher salt
Sugar
Corn syrup
Isopropyl alcohol (purchased at pharmacy)
Vinegar
Select one oil: Corn oil, Olive oil, Baby oil

Materials for hydrometer: To be made by each participant

Drinking straws
Permanent markers - 2 different colors
Small nails to fit inside straw
Ruler
Plasticine or blu-tac
Scissors

25 mL graduated cylinder
Paper towels
3 x 5 index cards
Universal hydrometers, 1 for each group (Nasco Science, 1-800- 558-9595 - item #SB16439M /$7.40 each) (hydrometer can be shared between groups if necessary)
Glass or plastic container for disposal of liquids in density columns

Explanation

Hydrometers of varying types for:
- Salt water tanks - purchase at pet store or online science catalog
- Car batteries - auto parts store, Wal-Mart, K-Mart
- Antifreeze in car radiators - auto parts store, Wal-Mart, K-Mart
Tap water
Rainwater

Procedures

Engagement

Show a video of someone trying to start the car, but the battery is dead or relay a story of what that is like.

Ask participants, "Have you ever had a similar experience?" Most will say yes. Ask participants, "Do you know how to check your car's battery to insure it is in good working order?" Answers will vary.

Play the video clip that will show how to use a device to check the fluid levels in a car battery. http://video.aol.com/video-detail/auto-and-truck-battery-reconditioning-step-5/1607610001

Assessment: Assessment is on going with verbal responses from participants.

Exploration

Instruct participants that they are going to make a device to determine relative density of different liquids.
Divide participants into groups of three and provide the materials necessary for each participant to make a hydrometer. Give each group a cut off water bottle nearly filled to the top with distilled water.
Instruct the participants to measure and cut the straw to a length of 10 cm.
Instruct participants to make a small ball with the Plasticine and seal one end of the straw so that it is water tight.
Instruct participants to place two nails, point side down into the straw.
Instruct participants to place the straw into the water to check if it floats straight up and down. If not, instruct them to add one nail at a time until in does. If the straw is hitting the bottom of the bottle, either add more water or trim the length of the straw.
Instruct participants to use a permanent marker and mark where the surface of the water touches the side of the straw.
Instruct participants to gently wipe the straw dry with a paper towel and make a distinct thin line for the water mark.
Instruct participants to use a different colored marker and using the sharp tip of the marker make marks every 2 mm on either side of the water mark line for a total of 7 markings on each side of the water line.
Provide each group with a set of sample liquids to be tested and instruct participants that they are each to use their own instrument to test the various liquids.
Tell participants that the water line value is 1.0 and that each mark has a value of .2. If the liquid surface line is between lines when the hydrometer is floating in the bottle, tell them to estimate to the best of their ability.
Tell participants that the measurements will reflect only slight differences, but to do their best and to record their findings.
Tell participants to gently wipe dry the straw between liquids.
Instruct participants of each group to share their data within the group after everyone has completed measuring.
Based on the data collected, tell participants to illustrate and label a density column on their worksheet. The densest substance is indicated on the bottom, least dense at the top. If there is more than one liquid of the same density, list them together.
Tell each group that you are going to provide them with an instrument that was manufactured and to use it in the same way as their straw device.
Instruct them to read and record the values from the instrument for each of the liquids. Remind them to gentle wipe it dry between liquids.
Tell participants to illustrate and label a second density column on their worksheet based on the manufactured instrument. The densest substance is indicated on the bottom, least dense at the top. If there is more than one liquid of the same density, list them together.

Assessment: Assessment is ongoing as participants work together, record data, and discuss findings with each other.

Explanation

Ask participants the following questions:

What was your experience when working with the instrument you made?
- Responses will vary, but most will find that it was difficult to get an accurate measurement. Some may discuss difficulties they had with the instrument staying upright or other faulty behavior.

Compare and contrast the experience of working with the manufactured instrument.
- Responses will vary, but most will say that the manufactured device provided more accurate measurements and was easier to use. The two instruments were similar because they both floated up or down depending on the liquid substance used.

What is this instrument called and what does it accurately measure?
- The instrument is a hydrometer and its function is to determine the specific gravity of a substance.

What is specific gravity?
- Specific gravity is similar to the density of a substance, but it has no units of measure. This is because specific gravity is a comparison of the density of a substance to the density of water. The specific gravity of water is given the value of 1.000 when the temperature of the water is 60° F.

Did you find the value of distilled water to be 1.000? If not, why?
- The water was at room temperature and the value of 1.000 is given when the water is 60° F. Slight differences may have been seen.

Do you think the specific gravity of tap water or rainwater would be the same as distilled water? Why or why not?
- Tap water has minerals in it, unlike distilled water. Demonstrate this by placing the hydrometer in a sample of each separately and have a volunteer come up to read the values.

Do you think the specific gravity of milk and skim milk will be the same?
- The specific gravity is not the same because of the fat content in the milk. However, there are many components of milk and the fat content has to be determined with other methods.

Did the density column you created come out the same as your illustration?
- Responses will vary, but a common problem is that some substances mixed together such as saltwater and sugar water or alcohol. This is a good example that can later lead into discussion of solutions.

Have you seen or can you think of places where hydrometers are used?
- Having seen the videos, participants will respond that hydrometers are used to check car batteries. Hydrometers are also used for checking antifreeze level in car radiators, salt water fish tanks, wine and beer making, testing milk, and for specific needs in many other industries. When they get a urine test at the doctor's office, the urine strips are providing a specific gravity value. The normal range is between 1.020 -1.030. Values higher or lower than that may indicate that the body is not functioning normally and the doctor can begin to do other tests to make a proper diagnosis.
- Show samples of other hydrometers that were collected for this lesson.

Salt water fish are very sensitive to the changes in salinity. How can you monitor the water to insure the proper amount of ocean salt is in the tank?
- A hydrometer floating in the tank will provide a specific gravity value. You then have to use a special chart that compares the temperature of the water and the specific gravity value to find the salinity level.

Place an object that sinks into a water glass. What would you do to make this object float?
- Add any of the substances that increased the specific gravity of water such as sugar or salt.

Assessment: Assessment is ongoing with oral responses and discussion of questions.

Elaboration

Tell participants that their group is going to create a liquid density column based on the measured specific gravity of the substances with which they previously used.
Instruct participants to select 4 liquids they wish to use to create the column, other than distilled water, and illustrate what they think the density column will look like.
Tell participants to share their data and proposed density column with another group and explain their reasons for listing the liquids in the order they illustrated supporting it with data.
Tell participants to use the 25 mL graduated cylinder to create their density column.
Instruct participants to calibrate a small plastic drinking cup for 10 mL by pouring 10 mL of water into the cup and marking the level on the outside of the cup with a permanent marking pen.
Instruct participants to use the liquids in the sampling bottles used previously to create their density columns.
Tell participants to use the plastic measuring cup they created to measure 10 mL of each liquid to be used in the density column.
Instruct participants to slightly tilt the cylinder and to slowly pour each liquid down the side of the cylinder in the order they illustrated, starting with the most dense.
Tell participants to illustrate on a 3 x 5 card their density column, as they did on their worksheet, and display it next to their constructed column.
When everyone is completed, tell participants to walk around and view density columns of all the other groups being sure not to disturb the graduated cylinders.
Instruct participants to empty contents of graduated cylinder in the designated container and to wash the graduated cylinders with soap and warm water.

Assessment: The density columns will reflect student understanding and oral discussion will reveal things that were correct or that could be improved in the future.

Rationale

Density is a concept that is very conceptual and many hands-on examples and experiences help to foster better understanding. A common misconception is that density applies only to solids when, in fact, it applies to liquids and gases as well. The density of a liquid when compared to water is called specific gravity. Specific gravity of liquids is a measure constantly used in industry, but it is also used very commonly in our everyday lives from checking proper levels of car battery and radiator fluids to urine tests at the doctor's office.

Science Standards

NSES CONTENT STANDARD A: Science as Inquiry

As a result of activities in grades 5-8, all students should

Develop the use of tools and techniques, including mathematics, which will be guided by the question asked and the investigations students design. The use of computers for the collection, summary, and display of evidence is part of this standard. Students should be able to access, gather, store, retrieve, and organize data, using hardware and software designed for these purposes.
Base their explanation on what they observed, and as they develop cognitive skills, they should be able to differentiate explanation from description--providing causes for effects and establishing relationships based on evidence and logical argument. This standard requires a subject matter knowledge base so the students can effectively conduct investigations, because developing explanations establishes connections between the content of science and the contexts within which students develop new knowledge.
Think critically about evidence including deciding what evidence should be used and accounting for anomalous data. Specifically, students should be able to review data from a simple experiment, summarize the data, and form a logical argument about the cause-and-effect relationships in the experiment. Students should begin to state some explanations in terms of the relationship between two or more variables.

NSES CONTENT STANDARD B: Physical Science

As a result of their activities in grades 5-8, all students should develop an understanding that

A substance has characteristic properties, such as density, a boiling point, and solubility, all of which are independent of the amount of the sample. A mixture of substances often can be separated into the original substances using one or more of the characteristic properties.

NSES PROFESSIONAL DEVELOPMENT STANDARD A

Professional development for teachers of science requires learning essential science content through the perspectives and methods of inquiry. Science learning experiences for teachers must

Involve teachers in actively investigating phenomena that can be studied scientifically, interpreting results, and making sense of findings consistent with currently accepted scientific understanding.
Address issues, events, problems, or topics significant in science and of interest to participants.
Introduce teachers to scientific literature, media, and technological resources that expand their science knowledge and their ability to access further knowledge.
Build on the teacher's current science understanding, ability, and attitudes.
Incorporate ongoing reflection on the process and outcomes of understanding science through inquiry.
Encourage and support teachers in efforts to collaborate.

NSES PROFESSIONAL DEVELOPMENT STANDARD B

Professional development for teachers of science requires integrating knowledge of science, learning, pedagogy, and students; it also requires applying that knowledge to science teaching. Learning experiences for teachers of science must

Connect and integrate all pertinent aspects of science and science education.

Best Teaching Practices

Learning Cycle
Inquiry
Science Process Skills

Time Frame

1 1/2 hours

Preparation

Create a salt water solution by using 35 grams of kosher salt to 1 L of water. Combine in the 2L bottle and shake vigorously until the salt is dissolved completely.
Create a sugar solution as described for the salt water.
Cut the tops off the 20 oz. water bottles to create a container that is 10 cm high from the bottom.
Create a set of liquids for each group by filling the cut-off water bottles 1 cm from the top. Label each bottle appropriately with a permanent marker. The set should include 1 bottle of each of the following:
1. distilled water
2. salt water
3. sugar water
4. corn syrup
5. isopropyl alcohol
6. oil
7. vinegar

Safety

Density columns should be emptied in a designated container. Container can be washed and rinse in the sink with soap and warm water.

Assessment

N/A

Explanation of Science

Hydrometers are calibrated, cylindrical glass tubes that are weighted with lead shot or mercury and are used to find the specific gravity of a liquid. Specific gravity is the ratio of the density of a liquid compared to the density of water at 60 degrees F. There are no units of measure associated with it because it is a comparison. For example, the density of water at 60 degrees F is 1.000 g/cm3. Its specific gravity is 1.000. The density of ammonia is .8974 g/cm3. Its specific gravity compared to water is .8974, which means that it is lighter than water.

The hydrometer is lowered into a tall column of the liquid to be measured and allowed to float upright. A reading is taken off the hydrometer at the point at which the surface of the liquid touches the glass stem. Hydrometers function on the basis of Archimedes' Principle. When the hydrometer is placed in a tall column of liquid, the liquid is buoyed up by a force proportional to the weight of the displaced liquid. Hydrometers float lower in low density liquids and higher in high density liquids.

There are universal hydrometers, but in various industries the range of specific gravity for liquids they use is very small. Therefore, hydrometers are designed for large values or small values.

Handouts

Liquid Measurement Data Table and Worksheet

Extensions

Activities in Hands-on-Plastics Kit 2 from the American Chemical Council

Bakeries measure the specific gravity of their batter to insure quality and consistency. Research other industries to learn how specific gravity is important in what they do.

Investigate how a car radiator works and the importance of antifreeze in the radiator. Learn how to find the specific gravity of the fluid so that the radiator functions properly.

Lesson Implementation Template

Download Lesson Implementation Template: Word Document or PDF File

Equity

Try to insure that all participants have participated and expressed their ideas either verbally or through written comments. When working in pairs or groups try to make the groups as heterogeneous as possible being sensitive to specific needs of individuals.