Entropy - IB Physics Definition
Reviewed by: Ann Howell
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In IB Physics, the entropy 
of a given system is a measure of the number of possible arrangements of the particles and their energies. In other words, it is a measure of how disordered a system is.
When a system becomes more disordered, its entropy will increase. The order of entropy for the different states of matter, from most disordered to least is:
gas > liquid > solid
The entropy of a substance changes during a change in state.
Entropy increases when a substance melts (solid → liquid) or boils (liquid → gas) . The particles in a gas can freely move around and are far apart from each other, and the entropy increases as the particles become more disordered.
Similarly, entropy decreases when a substance condenses (gas → liquid) or freezes (liquid → solid). The particles are brought together and become arranged more regularly, hence the entropy decreases.
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