Slime
Summary
Mixing PVA glue with borax turns a runny liquid into a stretchy, bouncy polymer1 network by cross-linking its long chains. You’ll be able to explain what a polymer is, what the borate links do, and why slime flows when pulled slowly but snaps when pulled fast — the hallmark of a non-Newtonian fluid.
History
The slime we know today was accidentally invented in 1976 by Mattel employee Marvin Belmore while trying to create a rubber substitute. The product, sold as “Slime” in a small plastic trash can, became a cultural phenomenon.
The chemistry, however, was understood earlier. Polyvinyl alcohol (PVA) was first prepared in 1924, and its cross-linking with borate ions was studied in the 1940s. The resulting material is a classic example of a non-Newtonian fluid - it behaves like both a liquid and a solid depending on how it’s handled.
Slime became a staple of chemistry education because it demonstrates polymer chemistry in a tactile, memorable way. The “gross” factor makes it especially appealing to children, who learn about cross-linking while playing.
Hazards & preparation
PPE: none needed, but keep it out of mouths and eyes.
- Borax is a mild irritant and should not be eaten; it’s not for very young children who put things in their mouths.
- Wash hands after playing, and don’t handle slime with broken skin.
Disposal: trash finished slime (don’t pour it down the drain — it can clog). Leftover borax solution can go down the drain with water. See the Safety page.
Materials
- PVA glue (white school glue) - 120mL (about 4 oz)
- Borax - 1 teaspoon (5g) dissolved in 120mL warm water
- Water - 120mL
- Food coloring - 3-4 drops (optional)
- Two containers (bowls or cups)
Procedure
- Mix 120mL glue with 120mL water in the first container and add food coloring.
- In the second container, dissolve 5g borax in 120mL warm water until clear.
- Slowly pour the borax solution into the glue mixture while stirring — it should start to gel at once.
- Knead for 2-3 minutes; it goes from stringy and sticky to smooth and stretchy.
- Store in a sealed container.
What you should see
The moment the borax hits the glue, the mix thickens into clumpy strings. After a couple of minutes of kneading it pulls together into one smooth, cohesive mass that stretches slowly, snaps when yanked, and bounces if you roll it into a ball and drop it.
| Symptom | Likely cause | Fix |
|---|---|---|
| Stays sticky and stringy | Not enough borax, or under-kneaded | Add borax solution a teaspoon at a time and keep kneading |
| Turns stiff and rubbery, tears | Too much borax (over-cross-linked) | Add a little more glue/water mix to soften |
| Won’t come together at all | Wrong glue (must be PVA/PVOH) | Check the label; clear “gel” glues behave differently |
The reactions
Not a bond-breaking reaction but a physical network forming (simplified):
long PVA chains + borate ions → chains loosely tied together into a stretchy gel
The borate bridges continually break and re-form, which is what makes slime both flow and hold together.
The Science
Borate ions cross-link the long PVA polymer chains, creating a viscoelastic material that’s both liquid and solid:
- PVA glue contains long polymer chains that can slide past each other (liquid behavior)
- Borate ions form temporary bridges between chains (cross-linking)
- The bridges constantly break and reform, giving slime its unique properties
This creates a non-Newtonian fluid:
- Pull slowly → flows like liquid
- Pull quickly → snaps like solid
- Roll into ball → bounces
- Let sit → flows flat
Questions to Explore
What is a polymer and why does it behave so differently from small molecules? PVA glue contains long chains of thousands of repeating vinyl alcohol units. How does the length of these chains — rather than what the chains are made of — produce the characteristic behavior of polymers like elasticity and slow flow?
Hint / answer
Very long chains tangle and grip each other over their whole length, so they can’t slide freely the way small molecules do. That entanglement is what gives polymers their stretch and their slow, syrupy flow — a property of length, not of the particular repeating unit.
What are the borate cross-links actually doing? Borate ions form reversible hydrogen-bond-like connections between nearby PVA chains. Why does connecting chains to each other change the material from a liquid that flows to a solid-like network — and why is the connection being reversible so important?
Hint / answer
Tying the chains together turns a bowl of separate strands into one giant connected web, so it holds a shape instead of pouring away. Because the links keep breaking and re-forming, the web can still slowly rearrange — that’s why slime flows over time instead of being a hard solid.
Why does slime snap when pulled fast but flow when pulled slowly? This non-Newtonian behavior comes from the cross-links having a characteristic relaxation time. When stressed slowly, chains have time to rearrange; when stressed fast, they cannot. What does this tell you about the timescale of breaking and re-forming the borate cross-links?
Hint / answer
Pull slowly and the links have time to let go and re-form elsewhere, so the chains slide and it flows. Pull faster than the links can release and they hold like a solid, so it tears. The switch-over speed tells you roughly how fast the borate bridges break and re-form.
How is the slime cross-linking similar to rubber vulcanization? Natural rubber is a polymer of isoprene. Vulcanization creates permanent sulfur cross-links between chains. How is this similar to borax cross-linking PVA — and why do permanent covalent cross-links produce a harder, less reversible material than the reversible borate links in slime?
Hint / answer
Both stiffen a floppy polymer by tying its chains together. Vulcanization uses permanent sulfur bonds, so the chains can never fully slide again — you get durable, springy rubber. Slime’s borate links are temporary, so it stays soft and can flow and be reshaped.
What would happen if you added more borax? Adding more cross-links should make the network stiffer and more solid-like. If you doubled the borax, what behavior would you predict — and is there a limit beyond which adding more borax stops changing the properties?
Hint / answer
More borax means more links, so the slime gets firmer and more rubbery and less stretchy. Eventually every chain that can be linked already is, so extra borax just stays in solution and stops making a difference — and too much makes it crumbly.
Going further
- Dial the stiffness. Make two batches, one with half the borax and one with double, and compare stretch, bounce, and how fast each flows flat. Find the point where more borax stops mattering.
- Variations to try: shaving cream before the borax for fluffy slime, iron oxide powder for magnetic slime, or foam beads for crunchy slime.
Footnotes
Polymorphism — The ability of a solid to exist in more than one crystal structure.↩︎