Lopsided Liquid

Introduction

The liquid in the tube is unbalanced! How can this be possible? Amaze students with this simple liquid density demonstration.

Concepts

• Density
• Specific gravity
• Observation

Materials

Safety Precautions

Isopropyl alcohol is a flammable liquid and a dangerous fire risk—keep away from heat, sparks and flames. It is mildly toxic by ingestion or inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review 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. Waste liquids may be disposed of according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

1. Use a buret clamp to connect the U-tube to a ring stand. Place the clamp low on the U-tube so that the liquid levels will beeasily visible (see Figure 1).
   {13978_Preparation_Figure_1}
2. Add a few drops of blue food coloring to 13 mL of deionized or distilled water in agraduated cylinder.
3. Add a few drops of blue food coloring to 17 mL of isopropyl alcohol in a graduatedcylinder, matching the shade of blue to that of the water.
4. Pour the blue colored water solution into one side of the tube.
5. Carefully and slowly pour the blue isopropyl alcohol down the other side of the tubeto minimize mixing of the liquids. Once the isopropyl alcohol is completely added,the liquid levels will not be equal (see Figure 1).
6. Ask students to share their observations.

Student Worksheet PDF

13978_Student1.pdf

Teacher Tips

• Have the tube set up before students arrive, and ask them to help figure out why the levels are unequal.
• The amounts of water and isopropyl alcohol can be adjusted proportionately for a larger or smaller U-tube. A 3:4 ratio of water to isopropyl alcohol will compensate for the difference in densities.
• Slight bubbling will occur at the interface where the liquids meet, but this will last only a few seconds. The interface between the two liquids will be inconspicuous, and the tube will appear to contain a homogeneous liquid.
• Isopropyl alcohol is soluble in water. If the tube is shaken up, the levels will even out.
• To make the interface between the two liquids easier to view, leave both liquids clear or use food coloring in only one of the liquids. (The food coloring will mix slightly with the clear liquid right around the interface.)

Further Extensions

• The properties of the liquids may be further investigated by allowing the U-tube to sit undisturbed for a few weeks. Ultimately, the levels equal out as the isopropyl alcohol and water slowly diffuse into each other. Stopper the ends of the tube to reduce evaporation. This may be used to tie in the topics of diffusion and solubility.
• Add 10–12 mL of isopropyl alcohol to the tube followed by just enough water to raise the alcohol level slightly above the water level. Stopper the end of the U-tube on the alcohol side. Add additional water to the open end and observe as the water level equals out and then bypasses the alcohol level. Pressure exerted on the alcohol by the air trapped between the liquid and the stopper is greater than the pressure from the weight of the water on the other side, therefore preventing the alcohol from moving upward (see Figure 2).
  {13978_Extensions_Figure_2}

Answers to Questions

1. Draw a diagram of the setup for this demonstration. Label where the chemicals were in the apparatus.
   {13978_Answers_Figure_3}
2. Water and isopropyl alcohol have different densities. One substance has a density of 0.785 g/mL, and the other has a density of 1.00 g/mL. Which substance has which density? Explain your reasoning.

   Isopropyl alcohol must have the lighter density, 0.785 g/mL, because the level of the isopropyl alcohol was higher than the level of the water. Water must have a greater density to exert pressure on the isopropyl alcohol and cause it to rise.

3. What would happen to the levels in the U-tube if you were to shake it and mix the water and the isopropyl alcohol together?

   If the water and the isopropyl alcohol were to mix and diffuse together, the density of the resulting combined solution would be equal throughout the U-tube. Therefore, the levels in the U-tube would be the same.

Discussion

The U-shaped tube allows for free flow of a liquid, so it seems ironic that the heights of the liquid in each side should differ. The masses of the two liquids used in this demonstration are the same, although the volumes differ. Isopropyl alcohol has a density of 0.785 g/mL, and water has a density of 1.00 g/mL. Therefore 13 mL of water will have a mass of about 13 g. Seventeen mL of isopropyl alcohol will have a mass of approximately 13 g as well since 17 mL x 0.785 g/mL = 13.35. The two liquids also have similar index of refraction values which makes it difficult to identify the interface between the water and the isopropyl alcohol.

References

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers in Chemistry; University of Wisconsin: Madison; 1983; Vol. 1, p. 77.