Why is salt water denser than normal water?
Density is a unit of measurement defined by how much matter takes up a certain amount of volume or space, therefore, when salt is added to water the following occurs:
A) The volume or space that the water takes up has very slightly increased but
B) The mass of the water has very greatly increased due to the salt ions (Ions being atoms with either extra electrons or missing electrons) having been attracted to the water and bonded themselves very closely to the water molecules.
The mass of the water has increased faster than the volume or space that it requires to take up, therefore the density of the water with salt in it is greater than normal water because for a certain volume of water there is more material or matter in salt water than there is in the same volume of normal water. So basically, in the very simplest answer would be "Because there's more stuff than just water in it." And most of that other "stuff" is heavier than pure water.
If you need something visual, picture (or try) this: You have 2 glasses filled with the same amount (volume) of freshwater. They both have the same density. Now, to one of the units, add 25 grams of table salt and stir it until it is all dissolved. Once dissolved, you will find that the volume of this unit is still the same (one unit) but if you weigh it, it is now 25 grams heavier than the remaining freshwater sample. Therefore the unit with the salt is now denser than the freshwater unit.
(Basically, think of fresh water as marbles in a cup. There are gaps between the marbles. When you add salt, the gaps are filled in, making it denser.)
Need something more scientific than just "because there's more stuff, it's heavier"? Check out what this science teacher has to say:
Firstly, you need to understand that when salt (sodium chloride NaCl) is added to water it disassociates into two charged ions; Na+ and Cl-
The water molecule is shaped much like this:
The oxygen has two lone pairs of electrons exposed on the top, so it attracts anything positive (e.g. Na+). The Hydrogen atoms are sharing their one electron with each of the covalent bonds with the oxygen atom. This exposes the positively charged nucleus and attracts negatively charged particles (e.g. Cl-).
This has the overall effect of pulling the water molecules closer together (but not chemically bonding) in salt water than they would normally be in fresh water. We define density as being related to the number and size of particles in a given volume. The overall pulling together of the water molecules means that more water molecules will fit into a smaller space.