Quantum Tunneling: How Water Molecules Break Bonds at Low Temperatures

March 22, 2016 | Joanne Kennell

water droplet
Photo credit: pixabay.com

Scientists discover a strange behavior of water that can only be explained by quantum mechanics.

Today (March 22) is World Water Day — the annual celebration started as part of a United Nations campaign to raise awareness about water scarcity and safety issues around the world. And even though water, one of the most common substances on Earth, covers almost three-quarters of the planet’s surface, there is still a lot to learn about it.

In fact, just last week, a team of chemists discovered something very bizarre about the substance we often take for granted, but require to survive.

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In their liquid state, water molecules cling to one another through what is known as hydrogen bonding. However, they are also constantly making and breaking bonds as they jiggle about. In order to study these bonds, researchers performed low-temperature experiments on a cluster containing only six water molecules — the smallest droplet that can form a three-dimensional shape.

Surprisingly, the researchers found that those molecules could not only rearrange themselves just one at a time, but they can actually break two bonds at once, as shown in the video posted below. Two molecules simultaneously broke their hydrogen bonds with their neighbors and rotated off one another like gears in a watch.

Why does this happen? It comes down to the strange world of quantum mechanics.

The low temperatures used in the experiment did not allow the molecules to jiggle with enough energy to break even one hydrogen bond — the energy barrier was just too high to scale. However, thanks to quantum uncertainty, there was still a chance that the cluster would simply “pop” through the energy barrier, which would be the equivalent of you walking through a wall.

This weird effect is known as quantum tunneling and that’s exactly what happened, according to the chemists who reported their findings in the journal Science.

This simultaneous bond-breaking could play a role in how water behaves in cells and on mineral interfaces, which is now a popular area of research. Clearly, there is still a lot we have to learn.

Plain old water isn’t so plain afterall!


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