Epsom salt is a common household item, often used in warm baths to soothe sore muscles. This crystalline substance dissolves readily in water. Since it is frequently mixed with warm or hot water, a natural question arises: what happens when this substance is subjected to boiling temperatures?
What Exactly Is Epsom Salt?
Epsom salt is chemically known as magnesium sulfate heptahydrate (\(\text{MgSO}_4 \cdot 7\text{H}_2\text{O}\)). This means the core compound, magnesium sulfate, has seven water molecules bound within its crystal structure. Unlike table salt (sodium chloride), Epsom salt is a water-soluble mineral composed of magnesium cations and sulfate anions. The term “heptahydrate” signifies that this common form incorporates water molecules into its crystalline lattice.
The Direct Answer: Boiling and Solubility
Epsom salt can be boiled when fully dissolved in water. Boiling the solution does not chemically change the salt itself but dramatically increases the amount that can be dissolved. Magnesium sulfate is highly soluble, with about 71 grams dissolving in 100 milliliters of water at room temperature (20°C).
Boiling elevates the water’s temperature to 100°C, which significantly increases the salt’s saturation point. At this boiling temperature, solubility jumps to approximately 73.8 grams per 100 milliliters of water. This higher solubility allows a concentrated solution to hold far more dissolved salt than is possible at lower temperatures.
As the water boils, it evaporates, concentrating the solution further until it may form a supersaturated state. This state is unstable, and crystals will rapidly form again as the solution cools. The liquid water boils away completely before the dissolved salt undergoes any chemical decomposition.
Safety and Chemical Stability Under High Heat
The main chemical transformation that occurs with Epsom salt at high heat is dehydration, not a dangerous breakdown. If a solution is boiled until all the liquid water has evaporated, the remaining solid magnesium sulfate heptahydrate begins to lose its seven bound water molecules. This process begins around 150°C and converts the salt into lower hydrate forms.
At temperatures above 320°C, the salt eventually becomes anhydrous magnesium sulfate (\(\text{MgSO}_4\)). This loss of water is a benign chemical transformation that results in a very dry powder.
While toxic fumes are not a concern, practical safety risks exist when boiling highly saturated solutions. As the water evaporates, the concentrated solution can splatter and leave a rapidly crystallizing residue on surfaces.
A significant risk arises if the container is allowed to boil completely dry and is left on a heat source. Heating the dry salt to extremely high temperatures—around 1124°C—would be necessary for the anhydrous salt to decompose into magnesium oxide and sulfur trioxide gas. Therefore, the only real hazard in a kitchen setting is container damage or splattering, not a toxic chemical event.