Re: How do water particles get tightly packed?

Because I was not in your class I can only guess that you were discussing the differences between solids, liquids, and gases, and probably using water as an example. And "getting tightly packed" is sometimes the description used to talk about what happens when liquid water freezes into solid water (ice) or when gaseous water condenses into liquid water.

Please spend some time starting at this excellent chem4kids page and go through all the pages they have there about States of Matter.

So, liquids and solids are much more dense than gases. The motion of the individual molecules of a gas, which we indirectly measure as temperature, keeps them bouncing around and not staying close to each other for long when they collide. If conditions allow the molecular motion to be reduced (the gas cools), the molecules can "hang on" to each other and become liquid (There are attractive forces between the molecules.). The molecules are "more tightly packed" than the gas state because the molecules don't have enough energy (motion) to stay far from each other. However, the molecules in liquids do not have fixed, orderly positions, they "slide" past each other, and so the matter is not fixed in shape. That's one of the main differences between liquids and solids: solids maintain a shape; liquids do not.

If conditions allow the substance to cool even more, then the molecules will get to the state where there is not enough motion to avoid being "stuck" (attracted) to enough close neighbors, that they do not move easily past each other. This is the solid state, and in most solids the molecules are just a little bit closer to each other than they were in the liquid state, and that is often described as "more tightly packed". They get that way because the molecules attract each other somewhat and so they "settle" together.

Water, though, is a very unusual substance, and one of the differences between water and other substances is that solid water is less dense than liquid water at its freezing temperature. This is why ice floats. The "sliding past each other" of water molecules in the liquid state allow them to get just a little bit closer to each other than when they are in the orderly arrangement of the solid. The reason for that is due to the "shape" of the water molecule itself, and you may learn more about that if you get into a chemistry class in high school. Hosted at Estrella Mountain Community College, the Online Biology book is an extremely good source of information about the type of things I have been discussing in only the briefest form. There is also a great page at iapws, and, as usual, there is also a huge amount of information at Wikipedia.

John Link, MadSci Physicist