Membranes can be classified as impermeable, permeable, semipermeable or selectively permeable. An impermeable membrane is that through which no substance can pass. Semipermeable membranes are those that let only solvent, like water, to pass through it. Permeable membranes are those that let solvent and solutes, like ions and molecules, to pass across it. There are still selectively permeable membranes, i.e., membranes that besides allowing the passage of solvent let only some specific solutes to pass blocking others.