Short Answer 
Phase diagrams are essential tools for understanding the behavior of substances across different states (solid, liquid, gas) at various temperature and pressure conditions. Key points include the existence of multiple solid phases (polymorphs), the presence of a unique triple point for each substance, and the importance of recognizing that not all phases may be in equilibrium simultaneously.
Step 1: Understand Phase Diagrams
Phase diagrams are crucial for visualizing the relationships between temperature, pressure, and the various states of matter: solids, liquids, and gases. These diagrams help in determining how substances behave under different conditions. It’s important to note that each phase diagram has unique characteristics that define its phases, including their points of equilibrium.
Step 2: Identify the True Statements
Several statements regarding states of matter can be assessed for their truthfulness. Key facts include:
- Many substances exhibit more than one solid phase, known as *polymorphs*, which can vary with temperature and pressure.
- Every phase diagram does indeed feature exactly one *triple point*, where solid, liquid, and gas can coexist in equilibrium.
- It’s incorrect to assert that a substance with multiple solid phases will have multiple *critical points*; there is only one critical point per substance.
Step 3: Clarify Equilibrium Conditions
Not all phases are in equilibrium simultaneously. While two phases can be in equilibrium at specific temperature and pressure conditions, this is not the case universally for solids, liquids, and gases. Understanding these equilibrium conditions is essential to accurately interpret phase diagrams and the behavior of substances under various situations.