The " triple point " of a substance is that point for which the temperature and pressure are such that :

Options

A. Only solid and liquid are in equilibrium.
B. Only liquid and vapor are in equilibrium.
C. Only solid and vapor are in equilibrium.
D. Solid, liquid and vapor are all in equilibrium .

Answer

To solve the question regarding the "triple point" of a substance, we need to understand the definition and characteristics of the triple point.

### Step-by-Step Solution:

1. **Understanding the Concept of Triple Point**:
- The triple point of a substance is a specific condition where three phases (solid, liquid, and gas) coexist in thermodynamic equilibrium. This means that at this particular temperature and pressure, all three states of matter can exist simultaneously without any one phase dominating.

2. **Identifying the Conditions**:
- At the triple point, both temperature and pressure are precisely defined. For example, for water, the triple point occurs at a temperature of 273.16 K and a specific pressure (approximately 611.657 pascals).

3. **Analyzing the Options**:
- The question provides multiple options regarding the states of matter at the triple point:
- A) Solid and liquid are in equilibrium.
- B) Only liquid and vapor are in equilibrium.
- C) Only solid and vapor are in equilibrium.
- D) All three states (solid, liquid, and vapor) are in equilibrium.

4. **Selecting the Correct Answer**:
- Since the definition of the triple point states that all three phases must be in equilibrium, the correct answer is option D: "All three states can be in equilibrium at that point."

### Final Answer:
The triple point of a substance is that point for which the temperature and pressure are such that all three states (solid, liquid, and vapor) can coexist in equilibrium.

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