why do the heating, cooling and weighing process need to be repeated until a constant mass is obtained in copper (ii) oxide experiment​

The correct answer would be b) volume versus temperature, because they are also directly proportional.

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.

It starts to rust i think

The answer is d) light rays pass through mirrors at an angle.