Some rocks contain carbonate minerals, and the acid test can be used to help identify them.
The mass in a chemical reaction remains (mostly) the same.
(except for radiation/nuclear fission, in which mass gets converted into energy)
Dimensional analysis is a convenient technique to determine if the solution you're following would lead you to the answer that you're looking for. It is the manipulation of units. Like units are cancelled whenever they appear on the numerator and the denominator side. For example, if the speed is 5 m/s and the time is 2 s, the distance would be:
(m/s)*(s) = m
So, you would know that the solution would be 5*2 because it yields the unit for distance.
Hence, dimensional analysis could prevent crash by calculating the right velocities or distances of the two vessels to prevent collision.
About 4. the weight of He is about 4 amus and the weight of O is about 16 amus, so you would need 4 He atoms to get close to the same mass as an O atom. i hope this helps
