Expt 042 -- Hydrometer

A plastic pipet is modified and calibrated to make a hydrometer. Densities of several solutions are measured. This experiment may be paired with Experiment 041 to test the solutions prepared by students.

Objectives

Determine how the density of solutions change with the concentration of a solute.

Safety

Use ordinary laboratory safety procedures.

Procedure

1. Fill a thin stem plastic pipet to about 1/3 with Zn dust. Squeeze and pull in the dust the same as you would water.

   !!!Click here to See Movie.

2. Put about 25 mL of water in a 25 mL graduated cylinder. Float the pipet (bulb-end down) in the cylinder. The pipet may stick to the sides. Tap the bulb a bit to even the Zn dust, and straighten the tip if necessary. Float the pipet again to test it.

   !!!Click here to See Movie.

3. The pipet should float with 5-8 cm of stem extended above the water. It it floats too high, add a bit more Zn and try again. If it floats too low, squeeze out some Zn. When it does float within the desired range, then remove and hot glue the stem closed.

   !!!Click here to See Movie.

4. Calibrate the sealed pipet by marking the level with a solution of known density. Use permanent markers. With the pipet floating freely, mark the outside of the cylinder at the water level. Grasp the pipet, aligning the level of your finger with the top of the cylinder. Remove the pipet from the water, and hold it next to your mark, aligning the level of your finger with the top of the cylinder again. Dry the pipet tip with your other hand. Mark the plastic pipet to align with the mark on the cylinder.

   !!!Click here to See Movie.

5. Calibrate with salt or sugar solutions of different concentrations.
6. Measure the concentration of a solution by measuring the density.

   !!!Click here to See Movie.

Questions

1. How does density change with increasing concentration of sugar or salt?
2. Radiator fluid in automobiles is checked with a slightly different device for measuring the density of the solution. Why would the density of this solution be measured?
3. Two solutions are prepared:

   5 g of NaCl dissolved in 20 mL of water;

   5 g of sugar in 20 mL of water.

   Are the densities of these solutions necessarily the same? Explain.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 042 Hydrometer

Watch the movies and answer the questions.

Curriculum-

Use this activity when introducing density of liquids or solutions.

Activity-

• Experiment - Laboratory Only
• Use this experiment as a hands-on laboratory activity. It works well as a follow up to BBExperiment 041 where student prepare their own solutions in calibrated flasks.
• If you are short on time, you may test the solutions that students prepared in BBExperiment 041 with a previously calibrated hydrometer.
• See Coop Learn below for variations.

Safety-

Select solutes which are easy to handle and dispose. If alcohols are used, keep all flames out of the laboratory, and provide adequate ventilation.

Time-

Teacher Preparation: 5 minutes

Class Time: 20 minutes

Materials-

(Per Group)

• 5 g Zn dust
• 50 g sucrose (or table sugar)
• 10 g NaCl
• Unknown solution for students:
  • 30 mL 12% sucrose (Dissolve 68.2 g of C₁₂H₂₂O₁₁, sucrose in 500 mL of water. )
    • and/or
  • 30 mL 1.5 M NaCl (Dissolve 43.8 g of NaCl, sodium chloride in 500 mL with water.)
• 1 thin stem pipet
• 25 mL graduated cylinder
• hot glue gun and glue

Optional:

• 20 mL methanol

Disposal-

The solids used in the experiment may be disposed of with ordinary solid trash. The liquids may be discarded at the sink.

Lab Hints-

• As a complete student laboratory, assign different groups different concentrations of sucrose (5%, 10%, 15%, and 30%) or NaCl (0.5 M, 1.0 M, 2.0 M, and 5.0 M) to prepare. Provide a solution for students to use for the concentration determination. (12% sucrose and/or 1.5 M NaCl)
• Ethanol or Methanol may be used to illustrate the differences in density of pure liquids. The solvent is flammable so remove all flames.

Answers-

Q1. How does density change with increasing concentration of sugar or salt?

A1. As the sugar or salt concentration increases, the density of the solution increases.

Q2. Radiator fluid in automobiles is checked with a slightly different device for measuring the density of the solution. Why would the density of this solution be measured?

A2. The radiator fluid contains ethylene glycol to depress the freezing point of water. As the amount of glycol increases the density increases. The density is used to measure the concentration of ethylene glycol. The density is a good measure of concentration because the system contains only water and ethylene glycol.

Q3. Two solutions are prepared:

5 g of NaCl dissolved in 20 mL of water;

5 g of sugar in 20 mL of water.

Are the densities of these solutions necessarily the same? Explain.

A3. No. The masses of the samples are the same, but the volumes of the two solutions are different because NaCl and sugar interact differently with water. Density is a measure of mass/unit volume.

CoopLearn-

Different solutes (NaCl, sugar, or an alcohol) may be assigned to different groups of students to measure. Data may be collected and displayed for a discussion of the differences.

Key Words 1-

density, solubility, solute, solvent, separation, calibration