- Age:8+
- Time:10 min
- Difficulty:Medium - adult runs the microwave
- Mess level:Low
- Supervision:Yes
🧒 In one sentence
Heat makes a crisp packet shrink into a tiny version of itself as the stretched plastic pulls back together.
What are the polymers?
The bag of chips is made out of polymers, long chains of molecules. Polymers are large molecules built out of the smaller repeating units which are chemically linked.
The word polymer comes from the Greek poly which means many and mer which means parts - so as the name says they are large molecules (macromolecules) composed of smaller units (monomers).
Monomers, which are smaller molecules, can chain together and create polymers.
All living things build proteins (a specific type of polymer) from monomers called amino acids. Plants and animals use only 20 of the 500 different types of amino acids that have been discovered to construct proteins.
Polymers can be both natural, for example, DNA, and already mentioned proteins. They can also be produced synthetically, the most common example is plastic and plastic products.
In an artificial polymer, each link in the chain will be the same as its neighbors. But the links in the chain of natural polymers can vary from one neighbor to another.
Natural polymer-based materials such as cotton, wool, and silk have been used for a long time. Cellulose is a natural polymer and the main component of wood and paper. Other examples include the starch molecules that we can find in plants.
Polyethylene terephthalate (PET) is used for packaging because of its properties: it is lightweight, chemically unreactive, strong, and easy to mold in any shape.
The natural state of the polymers is to be bunched up close together. However, when manufacturing a bag of chips, to get its expected shape, polymers are stretched out.
What are Microwaves and how do they create heat?
Microwaves are one type of electromagnetic radiation, just like X-rays, ultraviolet radiation, gamma-rays, and radio waves. They are used for many purposes, like communication, in radars, and are probably best known - in cooking.
A microwave oven creates microwave radiation that, when passing through food, creates dielectric heating. Simply put, dielectric heating is when an electromagnetic field grabs hold of molecules and makes them flip back and forth. As they twist and jostle, they bump into their neighbours, and all that motion is heat. In food, the molecule being grabbed is almost always water, because water is polar - it has a positive end and a negative end for the field to pull on. This is why a microwave heats your soup beautifully and does very little to a dry ceramic plate.
So why does a dry, empty bag heat up at all?
Good question, and it is worth asking, because our chip bag has no water in it. Something else is going on here.
A chip bag is not just plastic. It has an extremely thin layer of aluminium on the inside, which is what makes it shiny and keeps the crisps fresh. Metal does not politely let microwaves pass through it. Instead, the microwaves push the metal's free electrons around, and in a thin, ragged piece of foil that current has nowhere good to go. It piles up at the edges and crinkles, and that is what produces the crackling and the little sparks you will see. This is also exactly why we are told not to put foil in the microwave, and why this experiment takes five seconds and not fifty.
That concentrated energy heats the metal layer fast, and the metal passes its heat to the plastic pressed against it. The plastic never needed to absorb the microwaves itself. It just needed to get hot.
Microwaves have a longer wavelength than visible light and we can't see them.
It is said that Percy L. Spencer discovered microwave heating properties by accident. One day while he was standing next to a radar set, he noticed that the candy bar in his pocket started to melt. That led him to research the phenomena more and eventually to build the first microwave oven.
Materials Needed For Shrinking Bag Experiment
What we need is a Microwave oven, potato chips bag, kitchen tongs, and possibly scissors.
- Empty bag of chips. Buy any bag of chips and enjoy its content. After you have finished eating, make sure the bag is empty. Shake out all the crumbs from the bag to make it ready for the experiment.
- Microwave oven. Any working microwave oven will do the trick. Hopefully, you have one in your kitchen. Just remember to supervise children if they are not allowed to use the microwave oven alone. Or ask your caregivers for assistance if you are the child who is trying to conduct the experiment.
- Kitchen tongs. We will use the kitchen tongs to get out the shrunk bag from the microwave oven since it will be hot.
- Scissors. They are needed only if the chips bag is too big and doesn’t fit the microwave oven. Then we can use scissors to cut the bag and make it smaller.
Instructions for Shrinking Bag Experiment
Watch the video at the beginning of the article for a video guide, or continue reading for step-by-step instructions on making the shrinking bag experiment.
👨👧 Adult supervision needed
An adult runs this one, and read this part first.
A chip bag has a thin metal layer, and metal in a microwave sparks. Over about 5 seconds that is the effect we came for and it is usually harmless. Left longer, those sparks can scorch the inside of the oven, set the bag alight, and damage the microwave itself - repeated arcing is hard on the magnetron, the part that makes the microwaves. This is not a good experiment to run in a microwave you cannot afford to harm, and it is a bad idea in a school or shared kitchen microwave without asking first.
So: 5 seconds, at full power, and do not walk away. Stop it immediately if the bag catches fire or the sparking becomes more than a few brief flashes. The bag comes out very hot - lift it out with tongs and let it cool before handing it to a child.
If you would rather not risk your microwave, Shrinky Dinks demonstrate exactly the same polymer science in a normal oven, with no metal and no sparks.
What will happen to the chip bag in the microwave?
Make your prediction, then tap an answer to check!
- Empty the bag of chips and flatten it as much as you can. If the bag is too big, cut the top part of it so it fits in the microwave oven.
- Put the bag inside the microwave oven. Set the heat to maximum temperature and heat the bag for 5 seconds. You will notice cracking noise and sparkles coming out of the bag and poof, the bag will shrink.
- Carefully, take the bag out of the microwave oven with the help of the tongs. It’s hot!
- That’s it! You have a miniature bag of chips! Talk with your child about what could be the cause of this “magical” transformation and explore the chemical properties of the polymers together.
The Science behind Shrinking Bag Experiment
To get the bag of chips we all know and love, it needs to be processed in a factory using polymers and other materials.
Polymers that are used to produce chips bags have a tendency to be disorganized and clumped together. But they can be heated and stretched to make the needed shape for the bags we all know today.
And that process of manufacturing bags makes polymers stretched out and locks them in that position. Now they retain the shape of the bag and don’t clump together anymore. This is done to make the bag lighter and easier for transportation and general use.
After we expose polymers to heat, they will clump together and make the material shrink.
But if the material is heated, as we are doing when putting it in the microwave, it releases polymers from their stretched state. Now they are again trying to return to their normal state, close together, so the bag is shrinking.
But how come we get the miniature bag and not just a ball of polymers? The bag maintains its shape because polymers are still bound to each other and surrounded by layers of other materials such as aluminum and paint. These materials don’t change shape so they make sure we get a miniature bag.
🔬 Make it a real experiment
Compare different packets, since not all plastics behave the same. With an adult running the microwave for the same few seconds each time, shrink a crisp packet, a chocolate wrapper, and a different crisp brand, and measure each one before and after with a ruler. Rank which shrank the most. Never push the timing past a few seconds, whatever the packet.
What will you develop and learn by doing the Shrinking Bag Experiment?
- What are the polymers, what are their properties, and what are they used for
- What are microwaves and how do they function
- Effects of heat on the molecular composition
- The scientific method, observation, and conducting experiments
Key takeaways
- A chip bag is made of polymers - long chains of repeating molecules (monomers) linked together.
- During manufacturing the polymers are stretched out and locked into the flat bag shape.
- In the microwave, heat releases the polymers so they snap back to their natural, bunched-up state, and the bag shrinks.
- Layers of aluminium and paint keep the bag's shape, so you get a miniature bag rather than a shapeless lump.
- Microwaves normally heat food by making its water molecules flip back and forth (dielectric heating). A dry bag has no water, so here it is the thin aluminium layer that soaks up the energy, sparks, and passes the heat to the plastic.
Frequently Asked Questions
Why does a chip bag shrink in the microwave?
The bag is made of polymers that were stretched out and locked in place when it was manufactured. Heat from the microwave releases them, so they pull back to their natural, bunched-up state and the whole bag shrinks. The aluminium and paint layers keep it looking like a tiny bag.
Is it safe to microwave a chip bag?
For about 5 seconds with an adult running it and watching, yes. Be aware of what you're accepting, though: the bag's metal layer sparks, and sparking metal can scorch the oven's interior and, over a long enough exposure, damage the magnetron that produces the microwaves. Five seconds is fine; fifty is not. Don't do it in a microwave you can't afford to harm, don't leave it unattended, stop it if anything catches fire, and lift the very hot bag out with tongs.
Why does the chip bag spark in the microwave?
Chip bags have a very thin metallic (aluminium) coating. Microwaves push electric currents through thin metal, and at the crinkled edges that energy jumps as a spark. It's the same reason foil doesn't belong in a microwave. Over the few seconds of this experiment it's usually harmless - and it's what heats the bag - but it's exactly why you keep the time short, watch closely, and accept a small risk to the oven.
What are polymers?
Polymers are very large molecules (macromolecules) built from many smaller repeating units called monomers, chemically linked into long chains. They can be natural - like DNA, proteins, and cellulose - or synthetic, like the plastics used to make chip bags.
Does the bag shrink evenly or does it get thicker?
As it shrinks, the bag gets smaller in area but the material becomes thicker and stiffer, because the same amount of plastic is now packed into a smaller space. That's why the mini bag feels sturdier than the original.
Can I use an oven instead of a microwave?
This activity is designed for a microwave, where the bag's metal layer heats up within seconds. A conventional oven heats differently and can easily melt or burn the bag and release fumes, so we don't recommend it.
We hope you enjoyed this “magical” science experiment as much as we did. And if you’re still searching for fun experiments, we have some great recommendations for you:
- After shrinking the chips bag, you can try and crush the soda can using just water in the how to demonstrate air pressure with can crush experiment.
- And if you’re interested in which materials conduct heat the best, check out this great heat conduction experiment.
- And for more shrinking and expanding, you can check out the gummy bear osmosis experiment and learn about osmosis.
- For more polymer activity examples, check out 5 amazing balloon experiments and learn even more about polymers!
- And last but not least, we recommend this amazing kitchen activity of creating your own plastic from milk and vinegar. A ton of fun is guaranteed even after the activity is done.
And until next time, happy shrinking!
