Expanding Yeast

In Room 13 we are learning how scientists work by conducting science experiments and science investigations.

Curriculum Links:

Science - Nature of Science:

Understanding about Science: appreciate that scientists ask questions about our world that lead to investigations

Participating and Contributing: explore and act on issues and questions that link their science learning to their daily living

English - Speaking, Writing and Presenting

Purposes and audiences: Recognise how to shape texts for a purpose and an audience

We are learning how to think like scientists by observing our experiments and investigations and asking questions such as “I wonder what will happen if…..?”. We are also learning that when conducting investigations we need to have differences (variables). Variable means that a condition can be changed. We also learnt that you need to have at least one variable that is the same for each experiment. This will help us determine what changes to the experiment make the experiment work better or worse than other changes.

We have also learnt that sometimes scientists need to be patient.

Writing about our science observations, investigations and experiments have taught us that writing is used for many different purposes, not just stories.


discover what requirements or conditions yeast needs to grow best or, if some conditions slow or stop the yeast from growing.

Experiment: make yeast ‘balloons’ to determine what conditions or in what environment yeast grows the best.


We labelled four sandwich bags and then put 2¼  teaspoons of yeast into each bag.

Each bag needed to have different ingredients so we could investigate what will help the yeast to grow the best (or not).  We added to bag no:

  1. 120ml warm water and 2 teaspoons sugar

  2. 120ml cold water

  3. 120ml warm water, 2 teaspoons sugar and 1 tsp salt

  4. 120ml warm water


Scientists ask questions and make observations


Practising observation skills and patience


Observing our bean sprouts


Labelling is important

While we waited for something to happen we pondered on some questions:

  1. Which ingredient might help the yeast grow best?

  2. Is there an ingredient that makes the yeast not grow?

  3. Does yeast grow better in warm or cold water?

It took a long time for something to start to happen. What we noticed first was the yeast with warm water turned milky and the yeast in cold water did nothing. After lunch (a long time later when you are just 6) we observed that the yeast with the warm water and sugar had caused the bag to expand. The bag with the warm water, sugar and salt looked like honeycomb, the bag with just warm water was even more milky and the bag with cold water had done nothing.


At the end of the day


After a lunch this had happened

By hometime the bag with warm water and sugar had expanded so much it looked ready to pop! The science behind this is that yeast absorbs sugar which creates carbon dioxide.

Luckily, Miss Woodcock decided to take the bags home (and super lucky that she decided to put the bags into a tote tray) because the bag with warm water and sugar exploded just after she left school!

The following week we made bread to experience one-way yeast is used. Miss Woodcock has a recipe that you leave to rise overnight. We mixed flour, yeast, sugar, salt, and warm water to a dough in a glass bowl, covered it and set it in the sun to rise. Some curious minds wanted to investigate leaving the salt out of one of the dough mixtures to see if there would be a difference between using salt and not using salt. As the dough began to rise we were able to see that when the yeast had absorbed some sugar the carbon dioxide caused bubbles to form in the dough and the dough grew bigger in the bowl.


Making bread to see yeast in action

The next day, we observed that the two doughs looked the same as each other. After baking the bread we sampled it (all part of being a scientist…). Although both of the dough mixtures ended up looking much the same we noticed that the dough that did have salt added was a little bigger. This was interesting because in our ‘bag’ experiment the salt had stopped a lot of the carbon dioxide forming and the bag didn’t expand like the bag with no salt. Both bread doughs were left to rise in exactly the same conditions (on the table next to Miss Woodcock during parent interviews).


Scientists investigating the results




Bread dough just before baking

Our class conclusion was that the bread with salt tasted better and that given the same rising conditions there wasn’t much difference between the dough with salt and the dough without salt.


Scientists write their observations.jpg

Read about our Window Sprouts experiment here

2. practising observation skills and patience