What differences would be observed between the mixture of salt and sand in the 250-mL beaker and the same materials after the water was added?
You are asking the wrong man, (and the wrong woman!)....
You did the experiment, we did not. What did you observe? Now clearly, sand is insoluble in water; sodium chloride is SOLUBLE. You had a volume of what with respect to sand and salt? How much water did you pour in? Of course, sand gives a fairly dense slurry when water is added.........i.e. you go to the beach, and the sand by the water's edge is dense, and packed, and easy to walk on. Dry sand is harder to walk on.
Anyway, it's your experiment. You tell us what you observed and recorded!