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We are sorry you had problems getting the yeast respiration lab to work in your classroom.  Simply mixing yeast, sugar, and warm water (80-115 F) into a homogeneous solution will cause the yeast to start respiring.  If yeast do not respire (release gas and make bubbles) when mixed with warm water and sugar there is either something wrong with the 1) yeast or 2) water. 

Problems with the yeast could be that it is too old or was exposed to extreme heat or cold.  Problems with water could be that it was too cold (below 80 F) or too hot (above 120 F) or has too many impurities (this would be very rare and would probably only be the reason if you used non-potable water).  If you start with water that is too hot and it cools to warm water, the yeast may still have been killed while the water was too hot. 

The speed at which the yeast respire will depend on how fast the warm water cools off in your classroom (how warm or cold your classroom is), however as your yeast mixtures didn't produce any bubbles I suspect that was something off with your yeast or water.