The triple point of water is a standard fixed point in mordern thermometry. Why?

Explanation

**Step-by-Step Solution:**

1. **Understanding the Triple Point**: The triple point of a substance is the specific condition at which all three phases (solid, liquid, and gas) coexist in thermodynamic equilibrium. For water, this occurs at a specific temperature and pressure.

2. **Defining the Conditions**: For water, the triple point occurs at a temperature of 0.01°C (273.16 K) and a pressure of 611.657 pascals. These conditions are unique and consistent, making them ideal for calibration.

3. **Graphical Representation**: In a pressure-temperature graph, the triple point is represented as a specific point where the lines of phase transitions (solid to liquid, liquid to gas, and solid to gas) intersect. This intersection indicates that at this point, the three phases are in equilibrium.

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