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

gas

explanation:

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since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.