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 it’s top. When the balloon was secure, we tipped in the salt.

Right 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 thousands 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 originating from natural reactions deep within the Earth.

Science is all around us. McMahon has a 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

Why is the sky blue? –Lanie, 10, St. Louis, MO

Dear Lanie,

Just the other day I looked up and wondered the very same thing. The sky is certainly blue, I thought. But on second thought, it isn’t always blue. Sunsets burst in pink and orange. The night sky is black.

That’s when it hit me. If our sky gets dark when the Sun is out of sight, maybe the answer to your question had something to do with light.

I decided to visit my friend Cigdem Capan, a physicist at Washington State University.

“When we look at the sky during daytime, the sky does not emit the light,” she said. “It receives it from the Sun and spreads it around. Only some of the rays will reach the surface of Earth, or our eyes.”

I wanted to know more about how it all worked. Capan explained that sunlight is actually white. It’s a mixture of all the colors of the rainbow.

If you’ve ever seen light zipping through a prism, you know it can break into different colors—red, orange, yellow, green, blue, and violet.

But as you’ve observed, the sky is blue, not rainbow. Something must bring out blue, I thought. I wondered what that something might be.

It was actually a 19th century physicist, Lord Rayleigh, who suggested that the air itself is what might help make this happen. He thought the particles in the air made the sky blue.

Light from the Sun travels to our Earth in about 8 minutes. It moves in a straight line and really fast. That is, unless something gets in the way—something like the air in our atmosphere.

The molecules that make up air, like nitrogen or oxygen, get really energized when they meet up with the incoming light.

The molecules scatter the light, and in all kinds of directions. It’s a bit like when a bunch of Ping-Pong balls get hit by a bunch of rackets at the same time, Capan said. The molecules such as oxygen and nitrogen are like the rackets and the light would be like the Ping-Pong balls.

Rayleigh discovered that these little oxygen and nitrogen molecules could scatter blue rays much more than red rays. There are significantly more blue rays than red rays reaching us from the air, Capan said. It is the reason we see the sky as blue when the Sun is high in the sky.

Light can scatter to bring color into our world. But our brains and eyes help us translate these different rays of light into color, too. In fact, our eyes have millions of itty-bitty parts called cones and rods that help us see color. We cats don’t just see black and white. We can see colors, too. My eyes not only help me see during the day, but also in the dark of night.

As you can see—literally—when light from the Sun scatters, it makes our sky appear blue. But your question got me wondering about something else: If the sky is blue, why is a sunset orange and pink? Write me sometime and let me know what you think.

Sincerely,

Dr. Universe

Ask Dr. Universe is a science-education project from Washington State University. Send your own question to Dr.Universe@wsu.edu or vote in this week’s reader poll at askDrUniverse.wsu.edu.