The only thing more natural than sleeping late on a Saturday is having fun with your food. And today, that’s exactly what Mary and Peter do…
As was normal for a Saturday morning, the Secret Science Society was in the backyard, making a mess. And the mess was everything that they had hoped it would be. Taking turns, Mary, Peter, and their friend Daniel would each open up a bottle of soda and then drop several small candies into it and quickly jump back to avoid the geyser of foam spewing out of the bottle.
“This is great!” Peter enthused. “I wish we had more soda!”
“That would be fun, but I wish we knew why it worked,” Daniel said.
“Me, too,” Mary replied. “Why does adding candy to soda make it spurt out like that? A scientist would know how to figure it out!”
At that moment, a voice from behind them called out “Then it is a good thing that you are all scientists, isn’t it?”
“Hi, Mom!” Peter said. “Taking a break from the cosmos?”
“Yes; I’ve found enough new planets for this week.” Peter’s mother studied planets around other stars and did most of her work at home. “I decided to come out and see what all the squealing was about.”
“We’ve been making soda fountains,” Mary said. “But we can’t figure out why it happens.”
“Well, let’s think about this,” Peter’s mother replied. “What goes into the reaction?”
“Carbonated soda and candy,” Daniel said.
“And we get foam and a lot of carbon dioxide out,” Peter added.
“OK, so we have to decide what it is about the candy that makes the carbon dioxide come out so quickly. What is the candy made up of?”
“It is mostly sugar with some mint flavor,” Mary said.
“And is the candy smooth or is it rough?”
Daniel peered closely at one of the candies in his hand. “It is sort of rough on the outside; there are lots of little bumps and holes on it.”
“Then we’ve got three possibilities,” Peter’s mother said. “First, it could be that the mint oil makes the reaction happen. Second, it could be that the sugar makes it happen. Third, it could be that the candy’s rough outside makes it happen. How can we find out the answer?”
“We could put a little oil into a bottle of soda,” Peter said. “If it makes the soda fountain out, then we’ve found the answer.”
“And we could try adding sugar to soda,” Daniel added. “If it makes the soda fountain out, then we’ve found the answer.”
“And we could add something that isn’t sugar but looks like it to the soda,” Mary concluded. “If the soda boils out then we know that it isn’t sugar that makes it go. But what has rough edges like sugar?”
“Salt does,” Peter said. “Let’s try it and see what makes the soda go!”
Eagerly, the three ran into the kitchen to gather up the supplies that they’d need. Daniel grabbed a bowl of sugar. Mary picked up a salt shaker. And Peter rummaged in the pantry until he found the oil. The friends then went back outside to run their experiment.
What do you think will happen? Do the experiment!
“Me first!” Peter said. He grabbed a soda bottle and took off its cap before setting it back on the ground. He carefully poured a little oil into the bottle and moved back.
“Nothing’s happening!” Daniel said. “It must not be the oil in the candy. Let’s try the sugar.” He opened a second bottle of soda and set it on the ground. He poured in some sugar and jumped back to avoid the rush of foam. “Aha! It’s the sugar!”
“Don’t jump to conclusions,” Mary said. “Let’s see what happens with the salt.” Mary took her turn opening a bottle of soda and then added salt to it. Again the soda fountained out of the bottle.
“So it isn’t sugar that makes it work,” Peter said. “I guess we should have known that because soda with sugar doesn’t spray out of the bottle.”
“Not unless you shake it up,” his mother said. “What happened is that both salt and sugar have a lot of rough edges; you can see them in a magnifying glass if you look. Those edges give the carbon dioxide a place to come out of solution.”
“Neat!” Mary said. “So anything with rough edges will make it work?”
“That’s right,” Peter’s mother replied. “If you look carefully at a glass with soda in it, you will see that there is often a stream of bubbles coming form a place on the glass. That’s where the glass has a small crack or a bit of something stuck on it. Scientists call those nucleation points. The more nucleation points there are, the more gas that can come out of solution.”
“But why do the bubbles come out at the edges?” Daniel asked.
“The exact reasons aren’t known yet,” she replied. “We know that part of the reason is because water molecules like to stick together; we call that surface tension. At a nucleation point, the water sticks to itself and not the glass or sugar or whatever. But the gas doesn’t stick together, and fills the gap. That pushes the water back a little, which lets more gas into the area. The reaction feeds on itself and you get a bubble that is too big to stay in place so it floats up and a new one starts. Do it fast enough by having lots of nucleation points and you get…”
“A soda fountain!” Mary exclaimed.
“OK,” Daniel said. “That makes sense. But why does diet soda work better?”
“That’s because of another effect,” Peter’s mother explained. “The sweetener in diet soda makes the water molecules stickier so that they make strong bubbles. That lets the foam hold together, which makes it go higher. But you could do the same thing by adding some glycerine and soap to a regular soda.”
“Yuck! I sure wouldn’t want to drink that!” Mary exclaimed.
“Me neither!” Peter’s mother replied. “But I would like to have some fun.”
Grabbing the candy, she turned to the soda to make her own fountain.