If you've ever watched a kid (or even an adult on occasion) chew gum, you may have been unlucky enough to witness him stretch it from his teeth on out. It's not the most appetizing sight or the most polite gesture, but it demonstrates the very nature of the substance itself. It's no mystery why gum is stretchable--just simple science.
Features
Basically, the ingredient that makes gum stretch is the same ingredient that makes it chewy--gum base, a "non-nutritive, non-digestible, water-insoluble" compound that is made from a mixture of a resin (either paraffin or wax), an emulsifier (glycerol, otherwise known as a sugar alcohol, or lecithin, a fatty substance derived from plants), an elastomer like artificial rubber or natural latex and other ingredients.
Function
It isn't just the gum base alone that makes the gum stretch. It is the process of chewing that essentially brings the product into its more pliable form. When you begin to chew a stick of gum, your teeth push your saliva into the very makeup of the gum. With the introduction of your saliva, the molecular structure changes, so to speak, and the molecules that were once enmeshed can now sort of slide by one another, making the gum much more elastic and malleable. Without your teeth, the gum wouldn't stretch.
Effects
If you were to put a name to this property, it would be viscoelasticity which means that gum basically has two distinct characteristics--viscosity and elasticity--that are present at the same time. For all intents and purposes, viscosity is a measure of a substance resistance to flow (i.e. thickness). Corn syrup has a very high viscosity, whereas milk would have a much lower viscosity. This couples with gum's elasticity, or its pliable nature. Both characteristics can change based on the stress or strain put on them by an external force, otherwise known as the chewing that you do with your teeth.
Significance
All of these features (its main ingredient, its characteristics, the external action and the introduction of moisture) play a part in making gum stretch. Once all of the forces are removed, meaning it is no longer subject to stress and wetness, the gum will go back to its original, much more firm form. Of course, when you reintroduce your teeth and saliva, it will again become stretchable.
Types
As with any product on the market today, each brand of gum is made of a different percentage of each ingredient, though they are all fairly similar in the basic recipe. If you've ever chewed a piece of bubble gum and then a piece of stick gum, you can tell by the way each stretches that their general makeup is different. This shows that a gum's viscoelasticity is based on the amount of each of its components.
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