First, it's a liquid. Now you might be wondering why concrete and salt water are not new compounds when they are all mixed together. The special thing is that the basic parts can still be removed by physical forces. You can take the solid concrete and grind it up again. The individual components can then be separated and you can start all over.
Salt water is even easier.
All you have to do is boil the water off and the salt is left over, just like when you started. The thing to remember about mixtures is that you start with some pieces, combine them, and then you can do something to pull those pieces apart again. Andrew Rader Studios, All rights reserved.