Why does soda fizz? -Emily, 9, Florida

Dear Emily,

If you’ve ever had flat soda, you know a sip isn’t the same without some fizzy bubbles. We can hear them pop and feel them burst on our tongue.

I wondered if there was some secret ingredient that made soda bubbly. My friend Kenny McMahon, who researches food science here at Washington State University, and I decided to investigate.

We grabbed a balloon, a bottle of soda, and salt. We filled the balloon with about a teaspoon of salt. Then, we twisted the cap off the bottle and stretched the balloon over the top. When the balloon was secure, we tipped in the salt.

Screen Shot 2016-05-06 at 4.08.12 PMRight before our eyes, the balloon started to inflate. I was tempted to pop it with my claws, but resisted.

The salt caused the soda to produce carbon dioxide gas. This was no surprise to McMahon. His research is all about the bubbles and fizziness made from carbon dioxide gas.

You may be familiar with this gas, too. It’s the one we all breathe out and plants use to make food. It’s also what makes the bubbles in soda—and makes us burp when we drink it.

At soda bottling plants, carbon dioxide from tanks is pumped into the liquid, where it dissolves and later forms bubbles.

Liquids like soda can change under different temperatures and pressures. Liquid at a higher temperature can hold more sugar in a solution, for example. And liquid at a higher pressure can hold more gas in a solution.

A whole bunch of gas gets crammed into a pretty small space and creates a lot of pressure inside a soda can.

There probably wouldn’t be a whole lot of bubbles in the can since the gas is in equilibrium—a balance between gas dissolved in the liquid and the gas in the space at the top of the can.

But when you open the can, the pressure lowers and the gas escapes. You can actually hear this happening as the soda fizzes. Bubbles quickly form in the liquid, rise to the surface, and pop to release carbon dioxide into the air.

The carbon dioxide can escape in all different directions. And of course some of it lands on the tongue’s taste bud receptors when you sip your soda. Your brain translates this into “fizziness” and it just might make your face twinge.

Soda isn’t the only place we find carbonation, though. We can also find it in nature.

While researching your question, I stumbled upon a group of carbonated springs in Idaho. They are fittingly named, Soda Springs. Just like in a can of soda, there’s a lot of pressure in the ground beneath the springs. The carbonation originates from natural reactions deep within the Earth.

Science is all around us. McMahon has few things for you to keep in mind as you continue to explore.

“Remain observant,” he said. “Keep asking questions and don’t let anyone burst your bubble.”

Sincerely,

Dr. Universe