Why does yeast make dough rise

Jr. scientist A often asks why we add different ingredients when we cook and one of the common ingredients we use (since we make a lot of pizzas) is yeast. I decided a while back to try to give a better answer than my standard, "yeast makes dough get bigger" response. I knew that there was carbon dioxide being released that formed little pockets in the dough, thus expanding it, so I thought we could do an experiment where we use yeast to blow up a balloon. Originally I put some yeast and water in a bottle and covered the mouth with a balloon. We checked on our experiment the next morning to find a still deflated balloon. I tabled the experiment for a while and was poking around online and found I was missing a key ingredient - sugar. We re-did the experiment with two bottles. The smaller bottle had yeast and water and the bigger one had yeast, water, and sugar. By the next morning the bottle with sugar had a nicely bulbous balloon sitting on top of it. Thanks to some laziness, we discovered that the bottle without sugar did eventually make enough carbon dioxide to partially inflate the balloon, but it took a few days to happen.

Hypothesis testing

Jr. scientist A came up with a hypothesis and we decided to test it. He had discovered that the pump for his stomp rocket had a little nozzle that could be removed. He hypothesized that the nozzle, with its smaller opening, would result in less air being pushed out of the pump and thus the rocket would fly a shorter distance (personally I thought the smaller nozzle would result in greater pressure and thus a longer distance flight). We tested the hypothesis by setting up the stomp rocket and firing off rockets either with or without the nozzle in place. We marked each place the rocket landed - blue for with the nozzle and white for without. To my surprise, all three white lines (no nozzle) were farther than all three blue lines (with nozzle). This was a moderately well received introduction to rigorous hypothesis testing, but clearly a tad bit advanced for Jr. scientist A.

Water-crayons

Jr. scientist A wanted to do an experiment with swishing water in a bottle. We started out trying to make a tornado in a bottle where you tape two bottle caps together facing opposite directions and poke a hole through them both. When you attach the caps to two bottles (one with water) and swirl the bottles, you're supposed to get what looks like a tornado. Unfortunately, our tape didn't hold and we were left with two separate bottles with holes in the cap. So what do you do with bottles with holes in the cap? Make water-crayons of course! When you turn the bottles upside-down and squeeze, you get a steady stream of water that you can use to "draw" with. We had a lot of fun drawing shapes, people, animals, or just scribbling (Jr. scientist A also had fun "drawing" on my feet every chance he got!). What a fun accidental experiment.

Mentos-powered fountain

I've been wanting to do this one for awhile. We added mentos to diet coke and got a big fountain of cola. We tried to add 4 mentos but couldn't get more than 3 in before the fountain started (I had also hoped to get far away before it went off, but oh well). Jr. scientist A really liked this one. Jr. scientist E also joined us for her first experiment, but I think it was too fast (a few seconds in total) for her to notice.

Vinegar and baking soda balloon, take 2

We decided to revisit an old favorite experiment - inflating a balloon using vinegar and baking soda. Since last time we had trouble getting the balloon on the bottle before all the gas escaped, we tried poking a hole in the bottle cap and attaching the balloon to the bottle cap. That let us quickly screw the cap on, balloon attached, before the gas escaped. We even tried filling the balloon with vinegar so that we could squeeze it out of the balloon and into the baking soda filled bottle after the cap was on. That let us control the reaction and capture the gas pretty well.

Unconventional bubble maker

Once again we used pressure created by warming air. We took a frozen empty bottle (my new motto is always keep an empty bottle in the freezer) and dipped the opening in bubble mix to form a soapy film over the opening. We put the bottle down and slowly a bubble started growing out of bottle. Squeezing the bottle, we could make the bubble grow quicker - o.k., it's just from the warmth from my hands, but it looks like I'm squeezing the bubble out of the bottle. Jr. scientist A had great fun popping each bubble as it formed.

Diving pen cap

We made a pen cap that dives on command. We took a pen cap, attached a string loop around it and added four paper clips to the loop as weights. We then filled a big soda bottle with water until it was almost completely full. We dropped the pen cap into the bottle and screwed the top on tight. When we squeezed the middle of the bottle, the pen cap sank because the air bubble inside it was compressed. When we let go, the air bubble expanded again and the cap floated back to the top. It took a little practice to get the weighting and water level right. When the cap capsized, causing the air bubble to escape and the cap to sink to the bottom, we used a magnet to drag the paper clips and cap to the top so we could reach in and pull it out without emptying the bottle.

Stomp rocket

Finally, a pressure-based rocket experiment that works! Jr. scientist A got a stomp rocket for his birthday. The kit has a Styrofoam rocket that gets launched by stomping on an air-filled container, sending air out a tube, creating enough pressure to launch the rocket sitting on the tube. We got the rocket to go high, we got the rocket to go far - a good time was had by all. We also played with the launcher itself, pushing on the air-filled container and having Jr. scientist A feel the air that came whooshing out so he could see how it all worked. Hmm, I wonder what else we can launch...

Baking soda and vinegar balloon

Having previously mixed baking soda and vinegar together to make a bubbly mess, we decided to try to harness that released carbon dioxide to inflate a balloon. We poured baking soda into a bottle, added vinegar, and then quickly put a balloon over the opening of the bottle. Very quickly the balloon started to inflate, much to Jr. scientist A's delight. A fun and repeatable experiment - just a little hard to get the balloon on in time.

Rocket balloon

This should work. Inflate a balloon, attach it to a straw with a tethered string running through it, and when you let the air go out of the balloon, the "rocket" should be propelled along the string. But despite various different rocket "designs," we haven't been able to get this to work. We've tried taping a big balloon directly to the straw with packing tape (the balloon may move a little along the rope but not very far - and re-inflating the balloon tends to make it pop because of the tape stuck to it), attaching a paper towel tube to the straw and sticking a long thin balloon into it (the rocket doesn't move at all), and taping a long skinny balloon to the bottom of the paper towel tube (again, basically no movement). Maybe it's not enough force, maybe the pressure is not being well directed. All I know is we can't seem to get this experiment to work. How frustrating!

Clanking coin

We tried an experiment from 365 More Simple Science Experiments with Everyday Materials. The idea is that if you put a bottle in the freezer, it fills with cold air. When you take the bottle out of the freezer, the air inside the bottle warms up and therefore expands. If you block the way out for the expanded air, it will cause pressure to build up behind the blockage.

We first tried with a plastic bottle, putting it in the freezer for 5 minutes. We took it out, put a wet quarter over the bottle opening, and waited. Unfortunately nothing happened. We tried a glass bottle with a smaller opening, thinking the quarter wasn't fully blocking the mouth (or that the sound of a quarter on plastic wasn't very noticeable). The quarter did lift up and clank back down as pressure built up and was released in the warming bottle . However, there was a long time between clanks. We tried cooling the bottle overnight to make sure the air inside was very condensed. This worked well. We got lots of clanking, one after the other. Jr. scientist A. got quite a kick out of it.

Baking soda and vinegar rocket

Materials
soda bottle
scissors
baking soda
vinegar

Results
Perplexing

Working theory
Combining baking soda and vinegar creates a large amount of carbon dioxide gas, causing the mixture to expand and bubble out of the container. If we can direct that pressure downward, we should be able to propel the container upwards.

Experiment
We took an empty soda bottle and used a scissors to poke a hole in the lid. We put a good amount of vinegar into the bottle and filled the cap with baking soda. After our safety goggles were on we put the cap on the bottle and shook it up (with a finger over the hole). We placed the bottle on the ground, cap side down and stood back. Although the bottle became very pressurized and a bubbly stream came shooting out of the hole, we didn't get any lift. We think that either pressure was going out of the edge of the cap, near the threads (aimed upwards) or the cap wasn't flush against the ground, causing the pressure to go out at an angle.