Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Prepare 20% w/w sodium chloride solution by dissolving 100 g of sodium chloride in 400 mL of distilled water.
2. Prepare 15% w/w glucose solution by dissolving 75 g of dextrose (glucose) in 425 mL of distilled water.
3. Set up five activity stations throughout the laboratory. Each station will have three graduated cylinders and a 600-mL beaker with approximately 400 mL of the specified solution. Three groups may be at each activity station at the same time. Sample activity stations are as follows:

   Station 1: Distilled water
   Station 2: Sodium chloride, 20% w/w
   Station 3: Isopropyl alcohol, 70%
   Station 4: Unknown 1 (15% glucose solution)
   Station 5: Unknown 2 (Mineral oil)

Safety Precautions

Isopropyl alcohol is a flammable liquid and a moderate fire risk. It is slightly toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Sodium chloride solutions and dextrose solutions may be disposed of according to Flinn Suggested Disposal Method #26b. The mineral oil may be saved for future labs or disposed of according to Flinn Suggested Disposal Method #18b. The isopropyl alcohol may be disposed of according to Flinn Suggested Disposal Method #18a.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. There are five activity stations and three groups can use a station at a time. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment should be completed before coming to lab, and the data compilation and calculations can be completed the day after the lab.
• If students get liquid in the hydrometer, instruct them to make another.
• Other liquids or solutions may be included to vary the standards and unknowns from class to class.
• The hydrometer should read between 9 and 11 in distilled water. If desired, all groups can start with water before moving on to the other stations.
• The mineral oil is messy and must be removed from the test tube using acetone. If desired, have all groups visit this station last.

Answers to Prelab Questions

Determining the Identity of an Unknown Using a Calibration Curve

1. Plot the values of the hydrometer reading and the actual density for each of the three known calibration liquids in the following Calibration Curve graph.
2. Use a ruler to draw a best-fit straight line through the data points. Extend the line in each direction.
   {13543_Data_Figure_4}
   Identity
   Unknown solution 1 ___Ethylene glycol___
   Unknown solution 2 ___30% Ethyl alcohol___

Sample Data

Prelab Activity

Data Table 1

1. Plot the values of the hydrometer reading and the actual density for the three calibration liquids.
2. Use a ruler to draw a best-fit straight line through the three data points. Extend the line in each direction.
   {13543_Data_Figure_5}
3. Locate the unknown solution 1 hydrometer reading on the y-axis. Locate this reading on the best-fit line, then determine the corresponding density reading for this point on the x-axis. Record this value in Data Table 2.
4. Repeat step 3 for unknown solution 2.
5. Use the Densities of Various Liquids Table to determine the identity of each unknown solution.

Answers to Questions

1. Use the Densities of Various Liquids Table to determine the identity of each unknown solution. Select the closest match for each unknown solution density. Record the identity of each unknown solution.

   Unknown solution 1 ___15% Glucose solution___
   Unknown solution 2 ___Mineral oil___

2. In which beaker would the solution contain the most glucose?

   Beaker 3. The hydrometer would sit highest in the densest solution.

3. In which beaker would the solution contain the least glucose?

   Beaker 1. The hydrometer would sit lowest in the least dense solution.

4. Explain your reasoning for the answers to Questions 2 and 3.

   Student answers will vary.

Introduction

Build a hydrometer and use it to determine the density of several unknown liquids.

Concepts

• Density
• Buoyancy

Background

All elements and compounds have unique physical and chemical properties. One of these physical properties is density, the mass of a substance contained in a given volume. For mixtures such as liquid solutions, the density of the solution varies with changes in solute concentration.

A hydrometer is an instrument used to determine the density of liquids. It usually consists of a glass cylinder with a calibrated paper scale inside and weights on the bottom. When placed in a liquid, the hydrometer floats at the level where the mass of the hydrometer equals the mass of the displaced volume of the liquid. As the density of the liquid decreases, a larger volume of displaced liquid is needed to equal the mass of the hydrometer, and therefore the hydrometer floats at a lower level. The paper scale inside is calibrated using liquids of known densities (see Figure 1).

Experiment Overview

The purpose of this experiment is to construct a hydrometer using a test tube, paper scale and weights. The scale will be calibrated using liquids of known densities. The hydrometer will then be used to determine the densities of two unknown liquids.

Materials

Prelab Questions

Read the entire Procedure and then complete the following exercise.

The identities of two unknown liquids are to be determined by measuring their density values using a hydrometer. The hydrometer was calibrated using three liquids of known densities. The hydrometer reading for each known liquid was entered into the data table below, along with the liquid’s actual density.

The hydrometer reading for each unknown liquid was also measured and entered into Data Table 1.

Data Table 1.

1. Plot the values of the hydrometer reading and the actual density for each of the three known calibration liquids in the following calibration curve.
2. Use a ruler to draw a best-fit straight line through the data points. Extend the line in each direction.
   {13543_PreLab_Figure_2}
3. Use the calibration curve to estimate the density of unknown solution 1. To do this, locate the value of the hydrometer reading on the y-axis. Follow this value across horizontally to where it meets the best-fit line, then determine the corresponding density for this point on the x-axis. Record this value in Data Table 1.
4. Repeat step 3 to estimate the density of unknown solution 2.
5. Use the Densities of Various Liquids Table on the next page to determine the identity of the unknown liquids. Select the closest match for each unknown solution density. Record your answers in the Identity section.

Identity
Unknown solution 1 __________________
Unknown solution 2 __________________

Safety Precautions

Isopropyl alcohol is a flammable liquid and a moderate fire risk. It is slightly toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Calibrating the Hydrometer and Measuring Unknowns

1. Obtain a note card and cut out a 12 x 120 mm strip.
2. Use a pencil and a ruler to mark the strip every half-centimeter starting at the top. Number the strip as shown in Figure 2. Note: The first half-centimeter mark is labeled with the number 2.
   {13543_Procedure_Figure_2}
3. Place the strip inside of the test tube (see Figure 3).
   {13543_Procedure_Figure_3}
4. Add 10 paper clips to the test tube.
5. Report to the workstation assigned to your group by the instructor.
6. Make sure that 100 mL of the specified solution is in the graduated cylinder. If not, add or remove solution so that the cylinder contains 100 mL.
7. Place the test tube in the solution. Allow the tube to settle and float and read the number on the strip that corresponds to the level of solution in the graduated cylinder. Record this number in Data Table 2 to the nearest tenth.
8. Carefully remove the test tube from the cylinder. If necessary, rinse the outside of the tube and dry it completely. Note: When measuring unknown solution 2 it is helpful to rinse the outside of the tube with acetone and then dry it before moving on to another station.
9. Rotate through the four remaining stations, measuring the hydrometer reading in each solution. Record each value in Data Table 2 (see worksheet). Caution: Do not allow any liquid to get inside the tube.

   Densities of Various Liquids

   {13543_Procedure_Table_2}

Student Worksheet PDF

13543_Student1.pdf