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.
First make the classic slime recipe as follows:
Give students their materials: 4tspns of glue in a cup, an empty cup, a squeeze bottle of water and a tablespoon measure, borax powder in a cup and a 1/16tspn or equivalent measure (I use two scoops of a very small measure).
Instructions for students:
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.
Allow students time to play with their slime.
If slime is not stretch enough, gradually work in some more water.
Discuss how and draw how the borax molecules cross link (or bridge) the long glue molecules, binding them together so that the glue is more solid but can still flow.
Then make another slime recipe:
First tell students that they will add a lot more borax to this recipe and ask what more cross links of borax will do to the texture of the slime that they make [it will be more solid].
Do the experiment:
To an empty cup, students 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.
This recipe makes a much more solid ball, which can be bounced.
Review 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.
Other background 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/ch…
