Question

action of conc.hcl on manganese dioxide​

Answer 1

Explanation:

Black coloured manganese dioxide on treating with conc. HCl gives light brown coloured MnCl2 with the evolution of greenish yellow chlorine gas                                                      

Answer 2

Speed

The physical quantity which remains constant in uniform circular motion is Speed. Uniform circular motion means that the body is moving along a circular path with constant speed.