How does salt make water denser?
If one liquid is “denser” than another, it means that for a given volume (say, a bucket full) it has more mass. Imagine a bucket of fresh water. If you put a rock in that bucket, some water will spill, but the result is still that your bucket will become heavier, because the rock is much heavier/denser than water and water + rock is heavier than water alone. Salt is a kind of rock too, and heavier/denser than water. Imagine the rock you dropped into your bucket of water was rock salt. That it dissolves after a while doesn’t change the fact that salty water is basically water carrying a heavy rock. Here are some relevant sites. The text is very scientific but there are some very illustrative pictures.
