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This article incorporates, in modified form, material from Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture.

 

In this session, we’ll use gravitational separation and the Tyndall Effect to test various samples to determine whether they are solutions, colloids, or suspensions.

 

 

 

 

Required Equipment and Supplies

  • goggles, gloves, and protective clothing
  • eye dropper or Beral pipette
  • beaker, 250 mL (1 or more)
  • watch glass
  • stirring rod
  • matches or lighter
  • laser pointer, red
  • incense or joss stick (1)
  • table salt (~ 1/4 teaspoon)
  • club soda (~ 200 mL)
  • homogenized milk (~ 20 drops)
  • vegetable oil (~ 20 drops)
  • starch water (~ 200 mL)
  • talcum powder (~ 1/4 teaspoon)

All of the specialty lab equipment and chemicals needed for this and other
lab sessions are available individually from Maker Shed or other laboratory
supplies vendors. Maker Shed also offers customized laboratory kits at special
prices, including the Basic Laboratory Equipment Kit, the Laboratory Hardware Kit, the Volumetric Glassware Kit, the Core Chemicals Kit, and the
Supplemental Chemicals Kit.

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CAUTION

Be careful with the laser pointer. Although standard 1 mW Class 2 laser pointers are reasonably safe to use, you should never look directly into the beam. (And be cautious about specular reflections, too. A beam accidentally reflected off something shiny can be as hazardous as direct exposure.) Although none of the chemicals used in this lab session are particularly hazardous, it’s always good practice to wear splash goggles, heavy-duty gloves, and protective clothing. Discard all food items when you’re finished; do not eat them!

Substitutions and Modifications

  • You may substitute any clear glass container of similar size for 250 mL beaker(s). It saves time to use multiple containers because you can make up samples and test the Tyndall Effect in one container while waiting for earlier samples to separate by gravity in other containers.
  • You may substitute a sheet of thin cardboard or plastic cling-wrap for the watch glass; anything that covers the beaker will do.
  • You may substitute any color of laser pointer for the red laser pointer, but be aware that green, blue, and white laser pointers are typically higher power and more hazardous to use.
  • You may substitute tobacco (cigar, cigarette, or pipe) smoke for the smoke produced by the incense or joss stick.
  • You may substitute any clear, colorless soft drink for the club soda.
  • You can produce starch water by boiling a small amount of macaroni or other pasta in 250 mL of water for several minutes. Decant the water into a clean beaker and allow it to cool before use. (Alternatively, you can just save the cooking water the next time you cook pasta; it will keep for at least a day or two if refrigerated.)
  • You may substitute any similar powder for the talcum powder, such as foot powder or body powder.

Procedure

  1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
  2. Light the incense or joss stick and blow it out. When it starts to produce smoke, place the 250 mL beaker, inverted over the incense, and allow the beaker to fill with smoke. Use the watch glass to cover the beaker.
  3. Direct the beam from the laser pointer into the beaker and note whether or not the Tyndall Effect is evident. Allow the beaker to sit undisturbed for at least a minute or two, and then note whether the smoke/air sample separates on standing. Based on your observations, decide whether the smoke/air sample is a solution, colloid, or mixture. Record your observations by circling the appropriate items on Line A of Table 18-3.
  4. Rinse the beaker thoroughly. Add about a quarter teaspoon of table salt to about 200 mL of water in the beaker and stir until the salt dissolves. Repeat the procedures in step 3 and record your observations on Line B of Table 18-3.
  5. Rinse the beaker thoroughly. Add about 200 mL of club soda to the beaker. Repeat the procedures in step 3 and record your observations on Line C of Table 18-3.
  6. Rinse the beaker thoroughly. Add about 200 mL of water to the beaker and then about 20 drops of homogenized milk. Stir until the contents of the beaker are thoroughly mixed. Repeat the procedures in step 3 and record your observations on Line D of Table 18-3.
  7. Rinse the beaker thoroughly. Add about 200 mL of water to the beaker and then about 20 drops of vegetable oil. Stir until the contents of the beaker are thoroughly mixed. Repeat the procedures in step 3 and record your observations on Line E of Table 18-3.
  8. Rinse the beaker thoroughly. Add about 200 mL of starch water to the beaker. Repeat the procedures in step 3 and record your observations on Line F of Table 18-3.
  9. Rinse the beaker thoroughly. Add about a quarter teaspoon of talcum powder to about 200 mL of water in the beaker and stir until the contents of the beaker are thoroughly mixed. Repeat the procedures in step 3 and record your observations on Line G of Table 18-3.

Table 18-3. Observe some properties of colloids and suspensions – observed data.

Sample Tyndall effect? Separates on standing? Classification
A. smoke in air
yes / no
yes / no
solution / colloid / suspension
B. salt in water
yes / no
yes / no
solution / colloid / suspension
C. carbon dioxide in water
yes / no
yes / no
solution / colloid / suspension
D. milk in water
yes / no
yes / no
solution / colloid / suspension
E. oil in water
yes / no
yes / no
solution / colloid / suspension
F. starch in water
yes / no
yes / no
solution / colloid / suspension
G. talcum powder in water
yes / no
yes / no
solution / colloid / suspension

Disposal

All waste solutions from this lab can be flushed down the drain with plenty of water.

Review Questions

Q1: What observable physical characteristic allows you to discriminate a colloid from a suspension?

Q2: What observable physical characteristic allows you to discriminate a solution from a colloid?

Q3: Lunar gravity is about one sixth Earth’s gravity. Might a sample that exhibits the characteristics of a suspension on the moon exhibit the characteristics of a colloid on Earth? Why?

Q4: Some samples are difficult to classify because their physical properties are intermediate or mixed between the characteristics of solutions, colloids, and suspensions listed in Table 18-2 (for example, they may separate under the force of gravity, but very, very slowly). Why do some samples display such intermediate/mixed properties?

Q5: Consider a mixture of a solid material in water which clearly exhibits the properties of a suspension. If you created a similar mixture, but using a different continuous medium (such as vegetable oil), might that mixture behave as a colloid? Why?

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