Q.Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.

Substances with a long-range ordered arrangement (sharp melting point) are crystalline; polymers/glassy materials with only short-range order are amorphous.

Step 1 – Criterion.

A crystalline solid has a long-range ordered arrangement of particles, a sharp melting point and definite geometry. An amorphous solid has only short-range order (like a super-cooled liquid), softens over a range and lacks definite geometry. Most polymers and glasses are amorphous.

Step 2 – Classify each.

• Polyurethane — polymer $\Rightarrow$ amorphous
• Naphthalene — molecular solid $\Rightarrow$ crystalline
• Benzoic acid — molecular solid $\Rightarrow$ crystalline
• Teflon — polymer $\Rightarrow$ amorphous
• Potassium nitrate ($KNO_3$) — ionic $\Rightarrow$ crystalline
• Cellophane — polymer $\Rightarrow$ amorphous
• Polyvinyl chloride (PVC) — polymer $\Rightarrow$ amorphous
• Fibre glass — glassy $\Rightarrow$ amorphous
• Copper — metal $\Rightarrow$ crystalline

Sort by presence of long-range order: molecular solids, ionic salts and metals are crystalline; polymers and glasses (short-range order only, soften over a range) are amorphous.

• Calling polymers like teflon/PVC crystalline — most common polymers are amorphous.
• Forgetting that metals (copper) and simple salts ($KNO_3$) are crystalline.
• Mislabelling fibre glass as crystalline; glass is the archetypal amorphous solid.