When one substance dissolves in another, it seems to disappear. But if you taste water that has salt dissolved in it, it still tastes salty, so you know that the salt is simply mixed up with the water. This means that salty water contains both water and salt. It must take up more space than the water alone.
Dissolving can be hard to imagine, so it can be easier to see what is going on by using glass beads of different colours, as this diagram shows.
