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Activity

Slime and Silly putty recipes

Summary: 
Make slime/silly putty and understand the underlying chemistry. Make two different consistencies and understand their different properties in terms of the molecular structure in each.
Science content (2016 curriculum): 
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: Atoms, Molecules (3-7)
Chemistry: Chemical Changes (2, 7)
Materials: 

For the classic "slime" recipe:

  • Tablespoon measure
  • Tablespoons of white glue same as number of students, plus a few
  • equal number of Tbspns water as white glue
  • 2 teaspoons borax mixed into 2 cups water
  • small (Dixie) cups, two per student
  • one coffee stirrer per student
  • 2 baggies per student

To make two different slime/silly putty recipes and compare consistencies:

  • 2 empty Dixie cups for each student
  • marker pen to label cups for each pair of students
  • 3 Tbspns water for each student, plus a Tbspn measure for each pair of students
  • borax in a cup for each pair - about 2 Tbspns, with 1/2 tspn and 1/16 tspn measure
  • stir sticks, 3 per student e.g. half coffee stirrers
  • 4 tspns white glue in each of two cups for each student
  • tray for each student
  • cornstarch on a plate
  • 2 baggies per student
  • towels for cleanup
Procedure: 

To make classic slime.
Make 50% white glue in water. Give students 2 Tablespoons each in a cup.
Mix 1 tspn borax into 1 cup water. Give students 1 Tablespoon each in a cup.
Students mix the cups together and stir quickly until combined.
(For measuring everything individually, use recipe 2 below.)

Play with the slime: it can flow between the fingers, but when it is suddenly pulled it breaks.
Chemical explanation of how slime is made:
The glue contains long molecules (called polymers). When borax is added it makes permanent (covalent) bonds between the glue molecules, called cross-links. These cross links form a branching web of glue polymer molecules, giving the slime its thick texture.
The following activity compares two different kinds of slime/silly putty, with different amounts of cross linking.
Chemical explanation of how silly putty behaves:
There are other bonds between the slime molecules (called hydrogen bonds), which are weaker bonds and can easily break and reform. When the slime is pulled slowly, a few of the hydrogen molecules break, but then reform with another adjacent polymer. As these hydrogen bonds continually break and reform, the slime stays in one piece but can flow and change shape. However, when slime is pulled on suddenly, many hydrogen bonds are broken at once, so it breaks apart.

To make two different slime/silly putty recipes, with different consistencies
Ask students to work on the tray, as this activity is messy.
Write up the recipes below for students to refer to.
Ask students to label the empty cups 1 and 2. These will be used for the borax component of recipe 1 and recipe 2.
Ask students to label the cups of glue (4 tspns each) 1 and 2. These will be used for the glue component of recipe 1 and 2.

If necessary, step students through the recipes - there are a lot of cups to keep track of.

Recipe 1
To an empty cup, add 1 Tbspn water and ½ tspn borax. Mix well.
Pour the borax mixture into the cup of 4 tspns white glue. Mix well.
Lift the blob out of the cup, rest for 5 minutes.
Mould with hands into a ball.
Roll in cornstarch to make less sticky.

Recipe 2
To an empty cup, add 1 Tbspn water and 1⁄16 tspn borax. Mix well.
To 4 tspns glue in a cup, add 1 Tbspn water. Mix well.
Pour the borax mixture into the white glue mixture. Mix well.
Lift the blob out of the cup, and into a small baggie.
Mould with hands through the bag to mix completely.
Pull the blob out of the bag, leaving any liquid behind.
Mould with hands until smooth.

Students can make recipe 1, and while it is resting, make recipe 2. Then finish up recipe 1.

Compare the two recipes - recipe 1 makes silly putty that is bouncy, hard, like a ball. Recipe 2 makes slime that is gunky and slimy.
The difference in texture is because of the different amounts of borax and water added to each recipe.
When we add more water, and less borax, the product is more liquidy.

Discuss the consistencies in terms of the molecules that make silly putty (see last photo):
In a chemical reaction, the borax molecules bridge (or "cross-link") the glue molecules together, turning the two liquids into something more solid.
Recipe 1 has glue molecules closer together (as no water was added to the glue), and also more borax molecules (as more borax is added). So there are more cross links which makes a harder, more solid silly putty.
Recipe 2 has glue molecules further apart (as water is added to the glue), and fewer borax molecules (as less borax is added). So there are fewer cross links between the glue molecules, so this slime is softer and more liquid.

Other chemistry: White glue is a polymer, which is a long chain of repeating units. Other polymers are nylon and plastics, as well as naturally occurring rubber. Polymers can be cross-linked at any of the repeating units along their chain, so the amount of cross-linking determines how solid they become.

Slime is a non-Newtonian fluid - its viscosity changes depending on how much force is applied. (Standard Newtonian fluids only change viscosity with changes in temperature.) When a sharp force is applied to slime it becomes rapidly more viscous and behaves more like a solid.
More info on slime: http://www.acs.org/content/dam/acsorg/education/resources/highschool/che...

Notes: 

Notes to self: see other silly putty recipes collected in Science Club folder.

Grades taught: 
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 7
Teacher: 
Ingrid
Sonia Ko
Sonja Watson
Teaching site: 
Eton Arrowsmith Camp
General Gordon Elementary Science Club
ingridscience afterschool
Laurier Elementary
After School Program at Elementary schools in New York City
Activity originally developed and delivered: 

School at Columbia After School programs for basic recipe.
Gordon Elementary Science Club for comparison of silly putty recipes.