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what is an endothermic and exothermic reaction? ?

All chemical reactions is known to involve the transfer of energy.

Endothermic processes require an input of energy to proceed and are signified by a positive change in enthalpy, while exothermic processes release energy upon completion, and are signified by a negative change in enthalpy.

Explanation:

the major difference between endothermic and exothermic reactions is seen  in the words themselves. "Thermic" refers to heat, just as in the word "thermometer." "Exo" means "outside" and "endo" means "inside." Thus, an endothermic reaction pulls heat into an object or area, while an exothermic reaction expels heat

the second type of galaxy is called a barred spiral, which has a thing called a bar straddling the nucleus

the generally accepted parts of modern cell theory include: all known living things are made up of one or more cells. all living cells arise from pre-existing cells by division. the cell is the fundamental unit of structure and function in all living organisms.