Investigating cross-linking | making slime

This experiment is great fun and always popular with students. It can be made as difficult or as straightforward as you wish, depending on the age and stage of the students. Making the slime and observing the changes in its properties may be enough. For those ready for more challenge, detailed measurements and observations are possible.

Equipment required

Per student, pair or group: 

• PVA solution approx 100 cm³ per group – see notes below 
• Borax solution – 0.8 g borax in 20 cm³ water will allow students to make about four batches of slime; scale up according to the number of groups (Minimal hazard)
• Food colouring or a water soluble dye such as fluorescein (optional) – can be added when mixing the solutions
• Approx 4 x 100 cm³ beakers
• 1 x 25 cm³ measuring cylinder
• 1 x 10 cm³ measuring cylinder
• Stirring rod.

Notes

PVA solution can be made from PVA glue (wood glue or white paper glue).

The composition of the glue may vary depending on its source so check that the mixture works and adjust the glue/water mix accordingly. Two parts glue to one part water generally works well.

Alternatively, it is possible to buy solid PVA. To make the PVA solution using this starting material, add about 4 g PVA to 100 cm³ water at 90 °C. Stir with a magnetic stirrer until the PVA dissolves.

Cool and add enough water to make the volume up to 100 cm³ (to replace any water lost by evaporation).

Health and safety

• Wear eye protection and/or clothing protection if desired.
• Read our standard health and safety guidance 
• Borax and PVA both represent a minimal hazard.
• As borax is a weak alkali, it can cause skin irritation in those with eczema, sensitive skin or cuts. Disposable gloves should be made available for these students. Most others will enjoy handling the slime as it is very tactile.
• Borax is toxic if large quantities are ingested so students should wash their hands at the end of this activity and certainly before any food or drink is consumed.

The best slime

Typically the ‘best’ slime can be made by using a 5:1 PVA to borax mixing ratio, but this can vary, especially if diluted wood or paper glue is used.

Basic slime

• Mix Put 20 cm³ PVA solution in a beaker.
• Add between 1 and 10 cm³ borax solution (make sure you know how much you have added).
• Stir vigorously for several minutes to ensure the PVA and borax are thoroughly mixed.
• Leave the mixture for a few minutes for all the cross-links to form and then remove it from the beaker and roll it into a ball.

Testing for viscosity

• There are several ways you could do this – a couple of suggestions are given here, but there are many other ways too.
• Put the slime in a small beaker and leave it to settle. Place a coin on the surface and time how long it takes to sink.
• Draw a circle on a piece of paper about the size of a small beaker. Draw another circle outside the first one, about 1 cm away. Put an overhead projector slide on top or put the paper into a plastic wallet.
• Roll your slime into a ball and put it in the middle of the circles.
• Start timing when the slime reaches the first circle and stop when it reaches the second.