Answer:
A pure substance
Step-by-step explanation:
Epsom salt is named after Epsom, the town in England where it was discovered. Epsom salt is an inorganic salt, Magnesium Sulfate, its formula is MgSO₄.
It is more commonly found in its heptahydrate state: MgSO₄·7H₂O. However the water (or hydration) refers only to the structure of its crystals, as such the hydrated salt is still considered a pure substance.