water and salt, since the salt has dissolved into the water, assumingly, and sand isn't dissolvable so it can't pass through. If the water were to dry up, then you'd get the salt back, then it won't be able to go through the filter
A filter can be used to only separate sand and water but not salt. In order to separate salt and water, you need to use the process of distillation. This process involves boiling and evaporation of water occurs, leaving the salt behind as a solid. This works because salt has a higher boiling point than water.
