Which of the following phases are present for a sample of water at its triple point?

• A. Only liquid
• B. Only gas
• C. Only solid and liquid
• D. Solid, liquid, and gas

Answer: (D)

Verified step by step guidance

1. Understand the definition of the triple point: it is the unique set of conditions (specific temperature and pressure) at which all three phases of a substance—solid, liquid, and gas—coexist in equilibrium.
2. Recall that for water, the triple point occurs at approximately 0.01°C and 611.657 pascals, where ice, liquid water, and water vapor are all stable simultaneously.
3. Recognize that at the triple point, the system is at equilibrium among the solid, liquid, and gas phases, meaning none of these phases is absent.
4. Therefore, the phases present at the triple point of water are solid, liquid, and gas all together.
5. Conclude that the correct answer is the presence of solid, liquid, and gas phases simultaneously at the triple point.