First Law of Thermodynamics
The relationship ∆U = Q + W is applicable for all substances and all processes.This is known as the First Law of Thermodynamics.Let us consider the transition of a system from an initial to a final state.A change in the internal energy of a system occurs during a process.It is due to the amount of heat Q provided to the system and work W done on the system.The first law of thermodynamics states that the energy of the system can neither be created nor destroyed.It can only change in magnitude as a result of exchange of heat and work with the surroundings.
Entropy Change
The particles in a solid are very closely and orderly packed together.A solid has very low entropy.The particles in a liquid are fairly close together and they are not orderly as they are in the solid.The entropy of liquid is higher than solids, but it is less than when compared to gas.The particles of gas are very far apart and they are moving randomly.They have very little order.A gas has very high entropy.