Explain in terms of particle volume, why a sample of helium can not be compressed by a piston to a volume of zero. please can someone explain

Apeak with a retention time of 407 s has a width at half-height (w1/2) of 7.6 s. a neighboring peak is eluted 17 s later with a w1/2 of 9.4 s. a compound that is known not to be retained was eluted in 2.5 s. the peaks are not baseline resolved. how many theoretical plates would be needed to achieve a resolution of 1.5?

What layer of the atmosphere is directly above the troposphere?

What are the similarities of physical and chemical change ?