This is because as soon as salt is added into water it starts breaking down into smaller particles that are difficult to be seen by naked eyes. These salt particles get inside the water. At this stage, the salt starts dissolving in the water and a stage comes when the entire salt gets dissolved. Thus, no more salt could be seen.
The salt got broken into smaller particles that occupy the position in between the particles of water.
Out of the two substances, we would be able to separate chalk powder from water by straining with a cloth. This is because salt particles cannot be seen in water whereas chalk powder can be.
Disclaimer: The second part of the question aims to increase the process of scientific thinking in the students. Keeping this process of participation in mind, the answer to this question has not been provided.