If two objects have the same mass, we can know they are made of the same substance. If two objects have the same volume, we can know they are made of the same substance. If two objects have the same density, we can know they are made of the same substance.

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They can measure  the amount of energy gained or lost

Qrepresents joules in the formula

Radon-222 or rn-222 has a half-life of 3.82 days. to determine the fraction of an original sample of rn-222 that remains unchanged after 7.646 days, we calculate the amount of rn-222 remaining after 7.646 days and divide it with the original amount. using 1 gram as a basis and using half-life calculation methods, we can determine that  0.249 or 0.25  grams of rn-222 will remain, and thus the fraction remaining is 0.25 or 1/4.