Plaster of Paris and Gypsum

Introduction

Gypsum and plaster of Paris contain the same calcium sulfate framework but different amounts of crystal water. Gypsum is CaSO₄·2H₂O; plaster of Paris is the hemihydrate CaSO₄·0.5H₂O. Gentle dehydration of gypsum can produce the powder, and adding water lets the hemihydrate set as a harder dihydrate structure.

Core explanation

The formula CaSO₄·2H₂O means one calcium sulfate unit is associated with two water molecules in the ideal gypsum crystal composition. The plaster-of-Paris formula CaSO₄·0.5H₂O is a conventional way of saying one water molecule per two CaSO₄ units; it does not imply that a physical half-molecule of water exists. Multiplying by two gives 2CaSO₄·H₂O for whole-number formula bookkeeping.

The dehydration can be written CaSO₄·2H₂O → CaSO₄·0.5H₂O + 1.5H₂O, or without fractional coefficients, 2CaSO₄·2H₂O → 2CaSO₄·0.5H₂O + 3H₂O. Heating removes some crystal water. When suitable water is added back, the hemihydrate reacts toward the dihydrate: CaSO₄·0.5H₂O + 1.5H₂O → CaSO₄·2H₂O. Crystallisation of gypsum as the mixture sets gives the hardening effect. The word “reversible” describes the hydration change in principle, not a promise that an old cast can be effortlessly returned to identical powder without controlled processing.

This is salt chemistry even though it is not a simple acid–base neutralisation in the setting step. CaSO₄ is an ionic compound with Ca²⁺ and SO₄²⁻; the dot formula describes water incorporated into the solid crystal. Crystal water affects molar mass, appearance and behavior on heating. It should not be counted as free OH⁻ or as an acid proton in a solution pH calculation.

For mass calculations, the two forms differ substantially. Using approximate molar masses CaSO₄ = 136.1 g mol⁻¹ and H₂O = 18.0 g mol⁻¹, one mole of gypsum is about 172.1 g, while one mole of hemihydrate formula units is about 145.1 g. The difference, about 27.0 g, equals 1.5 moles of water per CaSO₄ unit. A measured mass loss on controlled heating can therefore test whether the expected water amount was removed, subject to purity and complete conversion.

Plaster of Paris can form casts because a paste is workable before gypsum crystals interlock during setting. The hydration is exothermic, so heat can be released. Conditions such as powder fineness, water amount and additives influence setting time and final material properties. The formula relationship supplies the chemical core, while material performance depends on crystal growth and processing.

Step-by-step reasoning

1. Write the correct formulas for gypsum and hemihydrate, including dot water. 2. Subtract crystal-water counts: two minus one half equals one and one half waters per CaSO₄. 3. Write dehydration or hydration in the stated direction and balance water. 4. Use the full hydrate molar mass for any sample-mass calculation. 5. Connect setting to formation and interlocking of the dihydrate crystals, not merely to drying.

Visual explanation

Draw a gypsum crystal box labelled CaSO₄·2H₂O. An arrow labelled controlled heating removes 1.5H₂O and leads to hemihydrate powder. A return arrow labelled add suitable water points to a set gypsum network. Show the water count beside each arrow so the fractional notation has a clear whole-number interpretation.

Real-world analogy

A building structure can include a specified number of removable support pieces; taking some out changes how the pieces fit, and replacing them lets a new rigid structure form. Crystal water similarly changes the solid form. The analogy captures composition and structure change, but real setting involves dissolution and recrystallisation processes.

Real-world example

Plaster of Paris is used for moulds and casts because it can be mixed into a shapeable paste and then set. The final hard material is associated with gypsum formation. The same chemical family appears as natural gypsum rock, illustrating how hydration state changes a useful material's handling properties.

Why?

Why does adding water make plaster hard rather than simply wet? The hemihydrate converts toward gypsum and new dihydrate crystals grow and interlock. The solid network develops strength; the change is more than liquid water evaporating from a powder.

Common misconception

“CaSO₄·0.5H₂O contains half of an individual water molecule.” The fraction is a composition ratio per CaSO₄ unit. Two formula units of hemihydrate contain one whole water molecule in the ideal formula bookkeeping.

Worked example

What mass of water is released ideally when 0.20 mol gypsum becomes hemihydrate? Each mole CaSO₄·2H₂O loses 1.5 mol H₂O, so 0.20 mol gypsum loses 0.30 mol water. At 18.0 g mol⁻¹, water mass is 5.4 g. The same amount of CaSO₄ units remains. This is the theoretical chemical water loss, not a prediction of additional surface moisture in an impure sample.

Quick check

1. How many moles of water are added per mole of CaSO₄·0.5H₂O to form gypsum? Answer: One and a half moles of water are added per mole of hemihydrate formula units to reach CaSO₄·2H₂O.

Exam focus

Keep dot water in hydrate formulas and molar masses. Balance a 1.5-water change per CaSO₄ unit or multiply the equation by two for whole numbers. Explain setting through dihydrate crystal formation, not only evaporation.

Advanced insight

Calcium sulfate has multiple solid forms, and heating conditions can drive beyond the hemihydrate toward more dehydrated material. The familiar reversible classroom pair is a controlled simplification. Real setting rates depend on nucleation, dissolution and growth of the gypsum phase.

Summary

Gypsum is CaSO₄·2H₂O and plaster of Paris is CaSO₄·0.5H₂O. Removing 1.5 waters per calcium sulfate unit produces hemihydrate; rehydration forms gypsum and sets a cast. Crystal water must be included in formulas and masses, and the hardening reflects a crystal-structure change.

Practice questions

1. Give a whole-number dehydration equation for two gypsum formula units. Answer: 2CaSO₄·2H₂O → 2CaSO₄·0.5H₂O + 3H₂O, conserving all water units. 2. Does plaster setting occur solely because excess liquid water evaporates? Answer: No. Hydration toward gypsum and growth of interlocking dihydrate crystals are central to setting. 3. Why is hemihydrate molar mass larger than anhydrous CaSO₄ molar mass? Answer: Each formula-unit ratio includes half a mole of crystal water per mole of CaSO₄, adding about 9 g mol⁻¹.