Why do different isotopes of an element generally have the same physical and chemical properties?

Describe in detail the melting point behavior of the 80: 20 benzoic acid-mandelic acid mixture

Agroup of students is studying convection currents. they fill two identical balloons with the same amount of helium. one balloon is placed in a freezer and the other in an area with warm air. after 10 minutes, the balloons are released from a height of 1 meter. which of the following do the students most likely observe? a. the balloons both rise. the cold balloon is larger than the warm balloon. b. the balloons rise at the same rate. both balloons are the same size. c. the warm balloon expands and rises. the cold balloon shrinks and sinks. d. the cold balloon expands and rises. the warm balloon shrinks and sinks.

Stoichiometry- i need with 14 and 15! an explanation would be appreciated!