Your students can discover the irreversible change of adding yeast when making dough with this fun science investigation.   Two batches of pizza dough will be prepared. One will include yeast and the other will not. Your students will observe the differences between the doughs and measure their height during the rising time.   The investigation includes an easy-to-follow instructions page with all ingredients and a set of worksheets designed to guide your students through the science inquiry process. Note that the worksheets include a duplicate page with a data table template and a duplicate page with graphing axes and labels to help with differentiation.   When you have decided which version of the worksheets to give to which students, you can print the number of copies required, gather the ingredients and some bowls and begin the investigation.   As an optional extra, your students could separate each batch of dough into eight balls and roll out the pizza bases. They could add toppings of their choice and cook the pizzas in a preheated over at 220^oC for 10-15 minutes. Your students could then compare the taste and texture of the dough with and without yeast.   Adding yeast to the dough causes an irreversible change. The yeast feeds on the sugars in the flour and makes carbon dioxide and ethanol. This process cannot be reversed to get the original ingredients back. The carbon dioxide gets trapped in the dough which causes it to expand and rise.   The recipe used in this investigation includes the use of hot appliances and food ingredients. It is the responsibility of the educator using this recipe to check for food allergies and to provide suitable health and safety measures for the activity.   This resource is part of our Chemical Sciences Year 6 UNIT PLAN.