Which of the following is always valid? choose only one answer.
1. w = - p ∫ dv, for the calculation of work in a mechanically-reversible piston-cylinder arrangement
2. for any fluid undergoing a change in pressure, cp = dh/dt.
3. δh = δu + δ(pv)
4. δu = q + w
5. δh = δu + p.δv

4. ΔU = Q + W

Explanation:

Equation 4 corresponds to the definition of internal energy of a substance as part of a system. This equation is always valid and is fulfilled in all types of systems and refers to the known phrase:

"energy is neither created nor destroyed, transformed"First Law of Thermodynamics

the atomic mass is the bottom number on the element.

-steel jelly

the earth would become to overheated and everything alive on it will eventually die from overheating. plants will eventually die as well, water will evaporate, and it will be nothing but a hot planet without anything living on it.