Explain the term residual entropy.

Explanation

**Step-by-Step Solution:**

1. **Definition of Residual Entropy**: Residual entropy refers to the entropy that a substance possesses at absolute zero temperature (0 Kelvin). It is the measure of the disorder or randomness that remains in a system even when it is cooled to this theoretical limit.

2. **Understanding Entropy at Absolute Zero**: According to the third law of thermodynamics, the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero. However, for some substances, even at absolute zero, there can be multiple configurations or arrangements of the particles that contribute to a non-zero entropy value.

3. **Positive Change in Entropy**: It is observed that for certain substances, the change in entropy (ΔS) remains positive at absolute zero. This means that these substances have residual entropy due to their inherent structural disorder, which does not completely vanish even at the lowest temperature.

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