Three solid substance `x, y and z` are heated, the number of free surfaces of substances `x and y` decreases to zero and one, respectively. The number of free surfaces of `z` does not changes. The substances associated with an increase in PE is/are ________.

Explanation

To solve the problem, we need to analyze the changes in the states of the substances X, Y, and Z when they are heated, and determine which substances are associated with an increase in potential energy (PE).

### Step-by-Step Solution:

1. **Understanding Free Surfaces**:
- The problem states that the number of free surfaces of substance X decreases to 0 when heated. This indicates that substance X has likely changed from a solid to a gas, as gases have no free surfaces.
- For substance Y, the number of free surfaces decreases to 1. This suggests that substance Y has changed from a solid to a liquid, as liquids typically have one free surface (the surface exposed to air).

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