(c) A chemist obtains a fresh sample of 84Rb and measures its radioactivity. She then determines that to do an experiment, the radioactivity cannot fall below 50.0% of the initial measured value. How long does she have to perform the experiment

Which process does not require the presence of a physical substance in order to transfer heat? air in the atmosphere is heated by the ground. this warm air then rises, and cooler air falls. this is an example of what type of process? how is conduction different from radiation?

What can be found in the ionosphere

Air can be considered a mixture. which statement does not explain why?