The chemical and physical changes that occur in gypsum wallboard surfaces during a fire.

The chemical and physical changes that occur in gypsum wallboard surfaces during a fire.

• A. Calcination
• B. Oxidation
• C. Saponification
• D. Combustion

Answer: (A)

Calcination is what gypsum wallboard undergoes when exposed to fire. Gypsum is calcium sulfate dihydrate (CaSO4·2H2O); when heated, it loses its water of crystallization. First it converts to calcium sulfate hemihydrate (CaSO4·0.5H2O, plaster of Paris) and with further heating to higher temperatures becomes anhydrous calcium sulfate. This dehydration is a chemical change that changes both the substance’s composition and its crystal structure, and it is endothermic, absorbing heat in the process. The physical effects include loss of mass, formation of a more porous or brittle layer, and potential cracking or spalling of the board as the core dries out. Other concepts like oxidation or combustion describe burning of material or organic matter, which gypsum itself does not undergo in the same way, and saponification is a reaction of fats with bases, so those do not fit the scenario.

Calcination is what gypsum wallboard undergoes when exposed to fire. Gypsum is calcium sulfate dihydrate (CaSO4·2H2O); when heated, it loses its water of crystallization. First it converts to calcium sulfate hemihydrate (CaSO4·0.5H2O, plaster of Paris) and with further heating to higher temperatures becomes anhydrous calcium sulfate. This dehydration is a chemical change that changes both the substance’s composition and its crystal structure, and it is endothermic, absorbing heat in the process. The physical effects include loss of mass, formation of a more porous or brittle layer, and potential cracking or spalling of the board as the core dries out. Other concepts like oxidation or combustion describe burning of material or organic matter, which gypsum itself does not undergo in the same way, and saponification is a reaction of fats with bases, so those do not fit the scenario.