The diagram below shows students set-up for the preparation and collection of oxygen gas X
Oxygen gas
Sodium
peroxide
(a) Name substance X used
(b) Write an equation to show the reaction of sodium peroxide with the substance named in 1

the answer is c. nacl

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.