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Tangled chains straightening out

Why Do Rubber Bands Snap Back?

Rubber is long tangled molecules. Stretching straightens them and they pull back into a tangle.

Relaxed Long molecules tangledupStretched Chains pulled straight— they want tore-tangle
Elasticity is a tangle wanting to be a tangle again.

  1. Step 1 of 5

    Rubber is made of very long molecules.

    A rubber band Long chains, coiled andtangled

    Each one is a chain thousands of units long, coiled and tangled with its neighbours like a bowl of noodles.

  2. Step 2 of 5

    Stretching pulls the tangle straight.

    1 Coiled andrandom2 Pulled straight3 Chains aligned

    The chains do not get longer — they uncoil and line up. That is why rubber stretches so far compared to metal.

  3. Step 3 of 5

    Straight is a very unlikely arrangement.

    Ways to be tangledan enormousnumberWays to be straightvery few

    There are countless ways to be tangled and very few to be straight. Left alone, random jiggling puts it back into a tangle.

  4. Step 4 of 5

    Cross-links stop it flowing apart.

    Uncross-linked rubber Chains slide apartpermanentlyCross-linked Chains return to place

    Sulphur bridges tie the chains together at points, so they slide back to where they started instead of sliding past for good.

  5. Step 5 of 5

    It warms when stretched.

    Stretched quickly Gives off heat

    Straightening the chains releases a little heat, and letting go absorbs it again. Stretched rubber against your lip feels noticeably warm.

    Rubber is the only common material that pulls tighter when heated. Warm a stretched band and it contracts.

The short version

Rubber snaps back because stretching straightens its tangled molecule chains, and tangled is overwhelmingly more likely.

Try it yourself

Stretch a thick band quickly and touch it to your lip. It is warm. Let it relax and touch again — cool.

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