What is yeast and why is it used for making bread?

yeast is a micro organism

It is a fungi

it is used in making breads as it helps bread to become spongy and soft to eat.

this process is known as fermentation

your spelling is a little off : ]

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.