Cross Linking STEM Activity Slime

This is another one of my students favorite Chemistry classes, making slime.


Cross Linking, The Science Behind  Slime 

What’s the science behind the slime? The borate ions in the starch (borax powder or boric acid) mixes with the PVA (polyvinyl-acetate) glue and forms this cool stretchy substance. 

This is called cross linking.

The glue is a polymer and is made up of long, repeating, and identical strands or molecules. These molecules will flow past one another keeping the glue in a liquid state.

The addition of water is important to this process. Think about when you leave a gob of glue out, and you find it hard and rubbery the next day.

When you add the borate ions to the mixture, it starts to connect these long strands together. They begin to tangle and mix until the substance is less like the liquid you started with and is thicker and rubberier like slime.

This is my favorite slime recipe. The saline solution makes a nice smooth slime and if kids accidentally get some in their eye it wont hurt them.

How to make Saline Solution Slime

½ Cup White Glue

1 Tablespoon Saline Solution {contact solution}

½ Cup Water

½ teaspoons Baking Soda

Food Coloring


Bowl, Measuring Cups, Spoon

Sandwich ziplock bags or 5oz portion cups with lids


 Measure 1/2 cup of glue and pour into your bowl. 

To that, add 1/2 cup of water and mix the two ingredients together.

Once the two are well mixed, it’s time to add color and glitter. Make sure to stir well.

The next two steps are important to the forming of the slime itself.

Add a 1/2 teaspoon of baking soda to the mixture and mix it in. This helps the slime firm up a bit more and create a solid.

Then you will add one tablespoon of saline solution to the whole mixture. Make sure that your saline solution contains both boric acid and sodium borate for the best slime activation.

Mix well.  You need to stir fast like you are trying to really whip up something. If it still feels loose, you can squirt a bit of solution into the bowl.

The slime will begin to form immediately thanks to the mixture of boric acid and sodium borate that is present in the saline solution. These two ingredients are cross linking agents that create your slime.

The final step is to knead your slime. Once the slime looks like it is pulling away from the sides and bottom of the bowl, it’s time to remove it from the bowl.

This is where you get to play with the slime. Have fun

Slime Fun

I hope you find this activity fun, my students definitely did.

As always, follow me on Instagram @beinglaurakmarshall for the latest .

Until next time.

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