Precipitation. Magnesium hydroxide is insoluble in water and is a precipitate. Magnesium hydroxide is a white crystal or powder. Magnesium hydroxide is insoluble in water and alcohol, soluble in dilute acid and ammonium salt solution, and the aqueous solution is alkaline. The solubility in water is very small. Generally speaking, in addition to sodium hydroxide, potassium hydroxide, barium hydroxide, ammonia water is soluble in water, calcium hydroxide is slightly soluble in water, and other hydroxides are hardly soluble in water.