sand is insoluble in water & if something is insoluble in something , then they will remain separated ->  sand will remain as solid particles in water  salt is soluble in water & if something is soluble in something , then that something can be dissolved in something forming a solution ->  salt will dissolve in water  sand will remain as solid particles in water & salt will dissolve in water ->  adding water to salt and sand will produce a mixture of liquid and solid particles  adding water to salt and sand will produce a mixture of liquid and solid particles & a filter is used for separating liquid from solid particles -> a filter can separate sand from salt water