Chloride ions do not cause water to pass out of cells into mucus. In fact, the movement of chloride ions is what causes water to move into epithelial cells. This process is known as electroneutral chloride secretion.

When chloride ions move out of a cell, they are usually accompanied by sodium ions. This creates an electrical gradient that causes water molecules to move into the cell to balance the charge. The movement of water into the cell is what causes mucus to become thin and watery.

In some cases, chloride ions can also cause water to move out of cells. This can happen when the chloride ion concentration in the surrounding fluid is very low. In this case, the chloride ions inside the cell will move down their concentration gradient and out of the cell. This will cause water to follow them, resulting in dehydration of the cell.