Name and state the law governing the expansion of gases when they are heated or cooled at constant pressure.

Explanation

**Step-by-Step Solution:**

1. **Identify the Conditions**: The question specifies that the expansion of gases is being considered under conditions of constant pressure while the temperature changes (either heating or cooling).

2. **Understand the Relationship**: Under constant pressure, the behavior of gases is governed by a specific law that relates volume and temperature.

3. **State Charles's Law**: Charles's Law states that at constant pressure, the volume of a gas is directly proportional to its absolute temperature (measured in Kelvin). This can be mathematically expressed as:
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