How Do Trays in a Fractionating Column Create Temperature Differences?

[Godwin Kessy]

Liquid mixture boils at the same temperature, and on the fractionating column are series of trays with bubble caps and overflow pipes, that circulates the liquid mixture. Then how comes the column contains trays at diferent temperatures?

 

[Borek]

Mixture composition is not contant, it changes when you go up/down. The higher you are, the higher the fraction of the substance with lower boiling point.