Specific Gravity - The Relative Density of Liquids
Author: Joyce Brumberger
Students will learn, through the creation of their handmade hydrometer, how a hydrometer functions and what it measures. Using their hydrometer, 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, students will create their own liquid density column.
What should students know as a result of this lesson?
- Students will learn the term specific gravity
- Students will learn the function of a hydrometer
- Students will learn the specific gravity of different substances
What should the students be able to do as a result of this lesson?
- Students will be able to construct a hydrometer
- Students will be able to determine the relative specific gravity of different liquids
- Students will be able to use a commercially manufactured hydrometer
- Students will be able to create a liquid density column
- 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 bottles
- 3 oz. plastic drinking cups
- Distilled water
- Kosher salt
- Corn syrup
- Isopropyl alcohol (purchased at pharmacy)
- Select one oil:
- Corn Oil
- Olive Oil
- Baby Oil
- Materials for hydrometer: To be made by each student
- Drinking straws
- Permanent markers- 2 different colors
- Small nails to fit inside straw
- Plasticine or blu-tac
- 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
- 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
- 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:
- distilled water
- salt water
- sugar water
- corn syrup
- isopropyl alcohol
Show a video of someone trying to start the car, but the battery is dead.
Ask students, "Have you or someone you know had a similar experience?" Most will say yes. Ask students, "Do you know that a car battery has fluid in it that can be checked to insure it is in good working order?" Answers will vary.
Play a video clip that will show how to use a device to check the fluid levels in a car battery.
Assessment: Assessment is ongoing with verbal responses from students
- Instruct students that they are going to make a device to determine relative density of different liquids.
- Divide students into groups of three and provide the materials necessary for each student to make a hydrometer. Give each group a cut off water bottle nearly filled to the top with distilled water.
- Provide students with the following instructions:
- Measure and cut the straw to a length of 10 cm.
- Make a small ball with the Plasticine and seal one end of the straw so that it is water tight.
- Place two nails, point side down into the straw.
- Place the straw into the water to check if it floats straight up and down. If it doesn't, 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.
- Use a permanent marker and mark where the surface of the water touches the side of the straw.
- Gently wipe the straw dry with a paper towel and make a distinct thin line for the water mark.
- 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.
- Get a set of sample liquids to be tested from the teacher. Each member of the team will use their instrument to measure all the liquids.
- The water line value is 1.0 and each mark you made has a value of 0.2. If the liquid surface line is between lines, estimate to the best of your ability.
- Measurements will only have slight differences, so do your best and record your findings.
- Gently wipe dry the straw between liquids.
- Share your data within the group after everyone has completed measuring.
- Based on the data collected, illustrate and label a density column on your 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.
- Use the commercially manufactured instrument given to you by the teacher to repeat the process.
- Take turns in your group reading and recording the values from the instrument for each of the liquids. Remember to gentle wipe it dry between liquids.
- Illustrate and label a second density column on your worksheet based on the data collected with 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 students work together, record data, and discuss findings with each other.
Ask students 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 behaviour.
- 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 degrees 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 degrees 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, students 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.
- Tell students 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 students 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 students 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 students to use the 25 mL graduated cylinder to create their density column.
- Instruct students 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 students to use the liquids in the sampling bottles used previously to create their density columns.
- Tell students to use the plastic measuring cup they created to measure 10 mL of each liquid to be used in the density column.
- Instruct students 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 highest density.
- Tell students 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 students to walk around and view density columns of all the other groups being sure not to disturb the graduated cylinders.
- Instruct students 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.
Understand the concept of density. Lesson "Float Your Boat" - optional
Best Teaching Practices
- Learning cycle
- Science Process Skills
Alignment with Standards
- MS-PS1-3 Gather and make sense of information to describe that synthetic materials come from natural resources and impact society.
- MS-PS2-2 Plan an investigation to provide evidence that the change in an object's motion depends on the sum of the forces on the object and the mass of the object.
- MS-ESS2-1 Develop a model to describe the cycling of Earth's materials and the flow of energy that drives this process.
Common Core Standards:
- RST.6-8.1 Cite specific textual evidence to support analysis of science and technical texts.
- RST.6-8.3 Follow preciesly a multistep procedure when carrying our experiments, taking measurements, or performing technical tasks.
- WHST.6-8.2 Write informative/explanatory texts, including the narration of historical events, scientific procedures/experiments, or technical processes.
- Grades 5-8 Content Standard A
- Grades 5-8 Content Standard B
- Grades 5-8 Scientific Ways of Knowing Benchmark A and C
- Grades 5-8 Scientific Inquiry Benchmark A
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.
Density columns should be emptied in a designated container. Container can be washed and rinsed in the sink with soap and warm water.
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.
Grouping Suggestions: Try to insure that all students 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.
Pacing/Suggested Time: Engagement: 10 minutes; Exploration: 25 minutes; Explanation: 20 minutes; Elaboration: 30 minutes
Printable PDF Worksheets
Liquid Measurement Data Table and Worksheet