In the reaction $2Br^- + X_2 \to Br_2 + 2X^-$,$X_2$ is .......

What is the correct order comparing the acid strengths of $HF$,$HCl$,$HBr$,and $HI$?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R.$
Assertion $A :$ Metallic character decreases and non-metallic character increases on moving from left to right in a period.
Reason $R :$ It is due to increase in ionisation enthalpy and decrease in electron gain enthalpy,when one moves from left to right in a period.
In the light of the above statements,choose the most appropriate answer from the options given below :

Select the correct order.

Which of the following gases can displace all other gases from a mixture?

Which one of the following reactions does not occur?