Solutions, Colloids and Suspensions

Introduction

Solutions, colloids and suspensions are mixtures in which one substance appears to be more or less uniformly dispersed throughout another substance. The size of the dispersed particles influences the properties of a mixture and determines whether the mixture is a solution, colloid or suspension. Are the particles large enough that they will settle upon standing or be trapped by a filter? Are the particles small enough that they will pass through a semipermeable membrane? Test student understanding of the properties of mixtures using this four-part demonstration.

Concepts

• Solution
• Colloid
• Light scattering
• Semipermeable membrane

Materials

Safety Precautions

Ammonium hydroxide solution is a corrosive liquid and is extremely irritating to the eyes and respiratory tract. It is toxic by ingestion and inhalation. Work with ammonium hydroxide in the hood or in a well-ventilated lab only. Hydrochloric acid is moderately toxic by ingestion and inhalation and is corrosive to eyes and skin. Copper(II) sulfate solution is a body tissue irritant and is slightly toxic by ingestion. Sodium thiosulfate solution is a body tissue irritant. Iodine–potassium iodide solution is a skin and eye irritant. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Any remaining copper sulfate solution and starch solution may be disposed of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b. Excess iodine–potassium iodide solution may be reduced with 50% sodium thiosulfate solution and disposed of according to Flinn Suggested Disposal Method #12a. The sulfur produced in the light scattering test may be separated by filtration and disposed of in a landfill according to Flinn Suggested Disposal Method #26a.

Prelab Preparation

1. Make copies of the student data table master and distribute one to each student.
2. The starch suspension is made by mixing 1 g of soluble starch with 100 mL of distilled or deionized water at room temperature.

Procedure

1. Obtain 100 mL of copper(II) sulfate solution, colloidal starch, and starch suspension in three separate 125-mL Erlenmeyer flasks. Label each flask.

Part A. Are the particles large enough to settle upon standing?

2. Stopper the Erlenmeyer flasks and shake each mixture vigorously for 15 seconds. Allow the mixtures to stand for a few minutes and have students record their observations in Part A of the data table. (Upon standing, a white solid will slowly settle out of the starch suspension. The copper sulfate solution and colloidal starch do not settle upon standing.)

Part B. Are the particles small enough to pass through a filter?

3. Set up three filter funnels with qualitative filter paper and place the funnels in a funnel support clamp.
4. Briefly shake each mixture from step 2 and pour half of each mixture through a separate funnel. Collect the filtrates in clean, 125-mL Erlenmeyer flasks or large test tubes.
5. Have students observe and record in Part B of the data table whether any solid remains behind on the filter paper in each case. (The copper sulfate solution and colloidal starch will pass unchanged through the filter paper—the filtrates appear identical to the starting solutions. The starch suspension will separate to give a white solid, which remains behind on the filter paper, and a clear and colorless filtrate.)
6. Add 10 drops of 6 M ammonium hydroxide testing solution to the original copper sulfate solution and to the filtrate. Did the composition of the solution change? Have students record their observations in Part B of the data table. (No change—both the original solution and the filtrate turn a deep, royal blue color due to the formation of a copper–ammonia complex ion.)
7. Add 10 drops of iodine testing solution to the original colloidal starch and to the filtrate. Did the composition of the mixture change? Have students record their observations in Part B of the Data Table. (No change—both the original mixture and the filtrate turn dark blue due to the formation of the familiar iodine–starch complex.)
8. Add 1 drop of iodine testing solution to the original starch suspension and to the filtrate. Did the composition of the suspension change? Have students record their observations in Part B of the data table. (The original starch suspension turns dark blue. The filtrate may turn pale, greenish-brown or dark blue if a small amount of starch particles manage to pass through.)

Part C. Are the particles small enough to pass through a semipermeable membrane?

9. Obtain a six-inch piece of pre-soaked dialysis tubing and clamp one end with a tubing clamp. Note: See the Tips section for directions for soaking the dialysis tubing.
10. Pour about 30 mL of colloidal starch into the dialysis tubing. Clamp the other end of the tubing so that the tube is securely sealed and will not leak.
11. Add about 300 mL of distilled or deionized water, followed by 10 mL of iodine test solution, to a 600-mL beaker.
12. Place the dialysis tubing in the beaker. Observe the solutions in the beaker and in the dialysis tubing and have students record their observations in Part C of the data table. (The colloidal starch solution will turn blue within a few minutes. No color change will be observed in the iodine solution in the beaker. Iodine molecules are small enough to pass through the semipermeable dialysis membrane. Starch molecules are large and will not pass through the membrane.)

Part D. Are the particles large enough to scatter light?

13. Obtain 40 mL of 2 M sodium thiosulfate solution in a large, 600-mL beaker. Add distilled water to the 400-mL mark of the beaker. Stir.
14. Shine a flashlight through the solution in the beaker and have students record their observations in Part D of the data table. (The beam of light passes through the solution and can be projected on a screen or wall. The path of light in the solution itself is not visible when viewed through the side of the beaker.)
15. Add about 25 mL of 1 M hydrochloric acid to the sodium thiosulfate solution and shine a flashlight through the resulting mixture.
16. Observe any changes in the path of the beam of light and have students record their observations in Part D of the data table. (In a few minutes the mixture will become turbid due to the formation of colloidal sulfur. As the turbidity increases, the beam of light becomes visible as it passes through the mixture. The beam of light becomes visible due to light scattering by the relatively large colloidal sulfur particles. Because blue light is scattered more than red light, the reflected light in the mixture itself appears blue while the transmitted light that emerges from the mixture appears dark orange.)

Student Worksheet PDF

13965_Student1.pdf

Teacher Tips

• This kit contains enough material to perform the demonstration as written seven times.
• Dialysis tubing is a semipermeable membrane made of cellulose. Small molecules pass through the membrane, while larger molecules do not. The tubing must be soaked in water before use—rinse the tubing with distilled or deionized water and allow it to soak for 5–10 minutes in distilled or deionized water prior to use. Once wet, the tubing should not be allowed to dry out again.
• Part C demonstrates that solute particles (I₂ and KI) pass through the membrane, but that colloidal particles (starch) do not. If desired, a second dialysis demonstration may be done by placing the CuSO₄ solution in the dialysis tubing and distilled water in the beaker. Copper and sulfate ions will diffuse through the membrane and the water in the beaker will turn blue.
• In Part B, the initial drops of ammonium hydroxide form a cloudy blue precipitate of copper hydroxide. As more ammonium hydroxide is added, the solution clears as the copper hydroxide is converted to the copper–ammonia complex ion.
• Colloidal starch is prepared by mixing/dissolving starch in boiling water. Starch mixed with room temperature water produces a suspension.

Sample Data

Properties of Mixtures

Data Table—Part A Data Table—Part B Data Table—Part C Data Table—Part D

Answers to Questions

1. Using the Properties of Mixtures and data from Parts A, B and C, what can you conclude about the nature of each mixture?

   The mixture of starch and cold water is a suspension. The starch settles after the mixture stands. The particles do not pass through a filter. The copper sulfate–water mixture is either a solution or colloid. It is not changed by either standing or filtration. The colloidal starch mixture is not changed by either standing or filtration, but its particles do not pass through the membrane of dialysis tubing.

2. What type of mixture is created in Part D? Explain.

   The mixture is a colloid. It does not settle upon standing and the particles scatter light, producing a Tyndall effect.

Discussion

Solutions, colloids and suspensions differ from one another in the size of the particles that are dispersed throughout a continuous phase. They are defined and distinguished from one another primarily in terms of their properties. Colloids, for example, may be defined as mixtures in which the dispersed particles are small enough to pass through a filter but too large to pass through a semipermeable membrane. Although colloids, like solutions, may appear uniform throughout, only solutions are considered truly homogeneous mixtures. The particles in a colloid are large enough that they will reflect or scatter light in all directions. In a true solution the dispersed particles are too small to scatter visible light. Suspensions are defined as mixtures in which the particles are large enough that they will settle upon standing (due to the effect of gravity) and will not pass through a filter. The following table summarizes the properties of solutions, colloids and suspensions. Notice that the particle size for each type of mixture is a range and not an absolute or fixed value. There is thus a continuum of properties for solutions, colloids, and suspensions.

The scattering of light by particles in a mixture is called the Tyndall effect and makes it possible to view a beam of light as it passes through a colloid or a suspension. Different wavelengths of visible light are scattered to different degrees by the dispersed particles and so the light may appear different colors when viewed from the side or after it has passed through the mixture. In Part D, colloidal sulfur is produced by the reaction of sodium thiosulfate with hydrochloric acid (Equation 1). Elemental (solid) sulfur forms rapidly but remains dispersed throughout the aqueous phase. The dispersed sulfur particles are relatively large and will thus scatter or reflect light that is passed through the mixture.

References

This demonstration has been adapted from Flinn ChemTopic™ Labs, Volume 12, Solubility and Solutions; Cesa, I., Ed., Flinn Scientific: Batavia, IL, 2003.