STEM Feature of the Month: The Chemistry Behind Slime

Breaking news: slime is officially having a major revival on social media. My feed is filled with videos of teens making slime on any surface imaginable, in a bowl, on a desk, oreven on the floor. I saw this revival in slime as the perfect time to make some myself and help de-stress from the pressures of AP week. Now that I’m older and have developed a new appreciation for science and chemistry, making slime sparked the question of why a few seemingly random household goods can come together to form this addictively fun and soothing substance.
 If you’ve never made slime before or never seen one of the iconic ASMR (autonomous sensory meridian response) slime-making videos, here’s a brief introduction into how slime is made. Glue (something like your favorite Elmer’s Glue from elementary school) and a borax solution get mixed together, along with fun add-ins like glitter, beads, clay, and food coloring. The substance formed is unique in its textur: thick, slippery, and viscous.
 The peculiar texture of slime makes it difficult to determine whether it is a solid or liquid. Slime doesn’t flow the same way as a typical liquid, like water, does, but it clearly isn’t as rigid and uniform as a solid is. It turns out that slime actually is a liquid, specifically a type called a non-Newtonian liquid. A Newtonian liquid, such as water, does not change viscosity when force is applied, while non-Newtonian liquids do. This classification means that the more you squeeze slime, the more it resists being squeezed. Additionally, since some liquids can be so viscous that they don’t flow, slime is often mistaken for a solid.
 Now onto the main question: how does slime really work? The glue you use to create slime is primarily made up of a polymer called polyvinyl alcohol (PVA). Polymers are large molecules made up of long chains of up to thousands of repeating smaller molecules (monomers). Examples of polymers in nature include DNA, which is made of nucleotides, and cellulose in plant stems, which is made of glucose molecules. PVA is a unique polymer that has a long carbon-chain with hydroxyl (-OH) groups on some of the carbons. These hydroxyl groups can form hydrogen bonds with water (a special type of force between molecules), making PVA soluble in water. This means that glue holds a lot of water, giving the slime a wet, liquid-like consistency.
 However, the hydroxyl groups on PVA can also form hydrogen bonds with other ions like those in borax. Borax, or sodium tetraborate, is often used in household cleaning supplies like laundry detergents. Borax is very soluble in water, and dissolves into sodium ions and tetraborate ions, which then further break down into borate ions in water. The borate ions can form hydrogen bonds with the hydroxyl groups on PVA, making the long polymer strands of PVA twist and turn, and begin to link together. The new substance that is formed, the slime, is called a cross-linked polymer, and the more connections and crosses in the polymer, the more tough and stiff the slime. This is why when too much borax solution is added to glue, the resulting slime will be rubbery and tough like a bouncy ball. Conversely, if too little borax solution is mixed with glue, the resulting slime will be liquidy and runny. You can alter the ratio of glue to borax accordingly to achieve a variety of textures in slime-making.
 The chemistry behind slime is tricky, but that doesn’t mean you can’t enjoy slime and all the fun it brings. Now that school is ending, I highly encourage you to try a little bit of chemistry: go out and make some slime with your friends! Experiment by making some fun colors and textures of slime to celebrate yet another successful year of high school.