Yeast Experiment - Science Class

I find it hard getting my pupils interested in the little biology things that I find so enjoyable. Clearly you have to be a certain type of a person to enjoy these things and most of my pupils are not THAT type. And even if they were, at their age, they would probably be quite afraid to show it openly. 

I have fought with it, but let's face it - you are thirteen, you come home after your spring break and one of the few lessons you are forced to sit through is on algae and yeast and other unimportant gross things. How the hell are you supposed to enjoy it? 

Well, this is a really simple experiment that can be explained, performed and talked about within one lesson (45 minutes). It shows how yeast works, what it does and what conditions it needs to do the work properly.

You will need :

• PLASTIC BOTTLES (I tried 1.5l and 0.5l, the smaller the bottle, the sooner you see the result, but it is doable with both sizes)
• BALOONS (doesn't matter what size or colour, although the kids seem to be excited to be able to pick their own colour)
  
  
  Note: make sure the baloon can be put on the top of the bottle comfortably. Some plastic bottles have wider mouths and that might cause problems.
• YEAST (for baking) - here it comes in little 42g packs that look like this. I used half a pack for each bottle.
• HOT WATER BATH - I used IKEA box, but it can be a bowl, a pot or an aquarium - anything waterproof able to withstand 40°C.
• SUGAR - a few teaspoons for every bottle.
• WARM WATER - approximately 40°C.
  
  Note: Check, if the classroom you will be doing this in has a hot water tub. Ours do not. I usually boil water in a kettle and then carry a small flask to the lesson with me - we mix it with cold water and we get our 40 degrees. You need enough warm water to fill the water bath and put a bit into the bottles with yeast, too.

Instructions:

1) Take half a pack of baking yeast and crumble it into the plastic bottle.
2) Add two or three teaspoons of sugar.
3) Pour warm water into the mixture and seal the bottle with a baloon.
4) Put the bottles in a water bath and observe.

What should happen:

The yeast cells feed on sugar and produce carbon dioxide. This process works best in warm environment. The carbon dioxide fills the bottle and as its amount increases, it produces enough pressure to fill the baloon as well.

My pupils had great fun watching the baloon, especially during the short period of time, when it "stands up". Knowing them, they did have something pervy in mind while watching, but I ignored it and was glad they liked the experiment.

Note: You definitely do not need to spend the whole lesson sitting down and staring at baloons. Put the bottles on one desk in the front so that you can watch them all the time and explain the topic or write notes. If something interesting happens, you will be able to see it without wasting precious teaching time.

Bonus:

If you really want to, it is very easy to put some yeast cells under a microscope (just mix yeast and water, if you add some sugar as well, you might see some yeast cells budding). My kids are incredibly clumsy and slow when working with microscopes, but you can set up one yourself and make everyone have a look.

Yeast cells under a microscope. Unfortunately, I do not remember how many times this was magnified, but I do not think any of my pupils care.

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Have I forgotten anything?
Please leave a comment and I will fix it!

Do you have more ideas or tips? I will be glad to hear them as well!!