All Resources

Polymers, Polarity and Predicting Reactions

In this demonstration, students will observe interactions between polystyrene, water and acetone and explain the results in terms of their molecular structure

Chemists categorize molecules in a variety of different ways.

Molecules with evenly distributed electrons are nonpolar and molecules that have an uneven electron distribution are polar.

A common saying in chemistry is “Like dissolves like.” In other words, polar molecules can dissolve polar molecules, and nonpolar molecules can dissolve nonpolar molecules. A mixture of polar and nonpolar molecules will remain separate (immiscible) unless special molecules with qualities of both are present to help them mix together.

Organic molecules always contain carbon and hydrogen and may contain other elements such as nitrogen, oxygen, phosphorous and sulphur. Inorganic molecules are composed of other elements than carbon. Organic molecules are often found in living systems, but could also be synthesized in a laboratory. Some organic molecules are very large. Knowing that a molecule is organic does not help chemists to predict how it will react, but does influence the way these molecules are named and studied.

Some of the largest organic molecules are polymers. Polymers are long chains or webs made of smaller groups of atoms called monomers, joined together like beads on a string. Common examples of polymers include DNA, proteins, nylon and Silly Putty. Chemical reactions can attach monomers together into polymers and can also break them down again.

Objectives

  • Predict, observe and explain interactions between water, polystyrene and acetone.

  • Identify changes as being physical or chemical.

  • Categorize molecules as polar or nonpolar, organic or inorganic, and identify polymers and their monomers.

Materials

  • Per Student:
    Background Research and Molecular Interactions sheet

  • Per Class:
    goggles, gloves, fume hood
    polystyrene (Styrofoam® cups or packing peanuts with a #6 recycling code will work best)
    200ml acetone (Note: pure acetone is available at most hardware stores. Most acetone-based nail polish removers do not contain a high enough concentration of acetone to yield good results)
    3 x 250ml glass beakers
    food colouring
    glass stirring rod
    tweezers
    large watch glass

Key Questions

  • What are some of the ways of categorizing molecules?
  • How can we use our knowledge of molecular structure to predict how molecules will interact?
  • Were any of the molecules difficult to categorize in any way? Explain.
  • Describe the appearance of the polystyrene before and after it is mixed with acetone.

What To Do

Part 1:
Research and Predict

  1. Have students look at the images of water, acetone and polystyrene on the Molecular Structure sheet.
  2. Ask students to copy and fill in the Venn diagram to describe the similarities and differences between these three molecules. The aim is for them to use as many of the words in the vocabulary list as they can.
  3. Have students predict how these chemicals will react by having them fill in the first 2 columns of the Molecular Interactions sheet. Have them record their observations of Reaction as you demonstrate the interactions during Part 2 and then have students fill in their Explanation following the demonstrations.

Part 2:
Molecular interaction (Teacher Demonstration)

Teacher SAFETY Tip: Acetone is a volatile, flammable substance with some toxic effects. Familiarise yourself with the MSDS information for acetone. Always demonstrate acetone under a fume hood, away from sparks, and follow safe disposal methods.

Preparation

  1. Fill up 2 beakers with 100ml of acetone each.
  2. Fill the third beaker with 100ml of water and add a drop of food colouring so it is easy to identify.

Demonstration

  1. Place a piece of polystyrene into the beaker of water. Use a stirring rod to submerge the plastic into the water. Place as much polystyrene into the beaker as you can safely fit. Ask students to observe and record what happens.
  2. Use tweezers to remove the polystyrene from the water.
  3. Pour 100ml of water into a beaker containing 100ml of acetone. Ask students to observe and record what happens.
  4. Finally, place some polystyrene into the remaining beaker containing 100ml of acetone. Place as much polystyrene into the beaker as you can safely fit. Ask students to observe and record what happens.
  5. Remove the polystyrene from the beaker with your tweezers and place it on a watch glass. Allow the acetone to evaporate overnight in the fume hood.

Extensions

  • Identify examples of physical changes and examples of chemical changes observed during the demonstrations. If any of the observations were difficult to categorize as being physical or chemical changes, explain what else you would want to know to help you reach a conclusion.
  • Repeat steps 1, 2, 4 and 5 of the demonstration with different types of plastics or biodegradable packing peanuts. Have students look at the chemical structures first and then make predictions about what will happen.

Other Resources

Science World | Background Research and Molecular Interactions sheet