1 介词+which可以把介词挪到从句末尾，此时which 和that均可用，或省略。
the change in which a new substance is formed is called s chemical change.
怎样把这句话的IN改到末尾？
the change a new substance is formed is called s chemical change in 对吗？
2 关系副词 when可以代替in/on which引导的时间定语从句。
依据上面的描述。
he arrived in beijing the day when i left.能不能改成 he arrived in beijing the day in which（还是on which ？) i left。

this is usually the time when the place is full of people. 能不能改成this is usually the time in which(还是on which ？)the place is full of people.
3 关系副词where可以代替in/on which引导的地点定语从句。
依据上述
let us meet at the gate where you can see the pandan 能不能改成let us meet at the gate in which(还是on which ？)you can see the pandan。
she is teaching at the university where her mother once studied能不能改成she is teaching at the university in which (还是on which ?) her mother once studied

请详细回答，谢谢

No.1
the change a new substance is formed in is called s chemical change .
in 放在 formed 的后面。“新物质在这个变化中形成的过程叫做化学变化 。”
No.2
he arrived in beijing the day when I left.只能改成 he arrived in beijing the day on which I left。
因为在特指的某一天一定要用介词on。
No.3
let us meet at the gate where you can see the pandan
能改成let us meet at the gate in which you can see the pandan。
因为这个介词in 只能和the gate 搭配，in which 在定语从句中代指in the gate，表示在“这个门口”你能看见熊猫。


同理：
she is teaching at the university where her mother once studied
改成she is teaching at the university in which her mother once studied 。
in which在定于从句中代指 in the university ，表示她的妈妈曾经“在那个学校”学习过。

OK