Gypsum's Chemical Structure
The chemical formula for gypsum is CaSO4·2H2O. It contains calcium, sulfate, and water molecules bonded together in a crystalline structure. The abundance of water molecules makes gypsum soluble in water to a certain degree.
When gypsum is mixed with water, the water molecules work to break the bonds between the calcium, sulfate, and water molecules in the gypsum crystal structure. The freed calcium and sulfate ions then dissolve in the water.
Factors That Affect Solubility
Several key factors determine how much and how fast gypsum will dissolve in water:
- Temperature: Gypsum becomes more soluble as water temperature increases. At around 86°F, gypsum is about twice as soluble as at 32°F.
- Water concentration: The more concentrated or saturated the water is with gypsum, the less additional gypsum will dissolve. Concentrated solutions become saturated.
- Agitation: Stirring or otherwise agitating the water accelerates the dissolution process by exposing more gypsum surfaces to water.
- Particle size: The smaller the gypsum particle size, the faster it dissolves because of the greater surface area exposed to the water.
- pH: Gypsum's solubility decreases as pH rises over 7, becoming much less soluble above pH 9.
Gypsum's Solubility Level
At room temperature, gypsum has a relatively low solubility level of about 2.1-2.4 grams per liter of water. It takes about the same mass of water as gypsum to become saturated. By comparison, table salt has a solubility of about 359 grams per liter at room temperature, so gypsum's solubility is quite low.
This solubility level is considered "moderately soluble" on solvent solubility scales. Given enough time, gypsum will eventually dissolve, but the saturation limit keeps the amount in solution low.
Uses Utilizing Gypsum's Solubility
Gypsum's solubility allows it to be used to amend and fertilize soils. When gypsum dissolves in soil moisture, the calcium and sulfate improve nutrient availability and soil structure.
Gypsum's solubility in water also makes it valuable in the brewing industry. Added during brewing, the calcium ions improve enzyme activity and the sulfate ions accentuate hops flavor.
In dentistry, gypsum is used to make models and molds by mixing it with water, letting it solidify, then washing the solid cast away. Gypsum's solubility allows this without damaging the desired model shape.
So, gypsum does dissolve in water to a moderate extent, allowing its use in applications that depend on this useful property.
