In which of the following are crystalline structures observable? I. Gas II. Liquid III. Solid

Analyze the question: You're asked which states of matter can show a crystalline structure, meaning a regular, repeating arrangement of particles (a lattice).

Concept:

• Gas: Particles are far apart and move randomly, so there is no ordered arrangement.
• Liquid: Particles are close together but can slide past one another. They have only short-range order at most, with no long-range lattice.
• Solid: Particles are held in fixed positions. In crystalline solids (e.g., salt, quartz, metals) they form a regular, repeating 3D lattice. This is what X-ray diffraction reveals.

Step-by-step:

1. Gas (I): no fixed positions, so no crystal structure. ✘
2. Liquid (II): no long-range order, so no crystal structure. ✘
3. Solid (III): crystalline structures are observable. ✔

Only III is correct.

Answer: C. III only

Tip: "Crystal" means a fixed, orderly lattice, and only solids have particles locked in place. If a question asks about crystal structure, think solids only. (Note that liquid crystals are a special case, but they are not covered at this level.)