Why are radioactive isotopes used in studies of metabolic pathways?

2. how is heavy water different from normal water

If i initially have a gas at a pressure of 12 atm, a volume of 23 liters, and a temperature of 200 k, and then i raise the pressure to 14 atm and increase the temperature to 300 k, what is the new volume of the gas?

Why does a plastic bottle that is sealed at a high altitude change it’s shape when taken to lower altitude