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The answers to this question explain that ice is less dense than water because it has a "crystal structure", but they dont explain what exactly that is and why this happens, also I saw this answer from another site stating that not all ice is less dense than water.

What is the "crystal structure" that ice has? Why is ice structured that way? Can ice be more dense than water, and if yes, how and when?

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See en.wikipedia.org/wiki/Ice#Phases, and especially the table detailing Ice I through Ice XV. There are actually 15 different types of ice, some of which are denser than water and some of which are not. –  DumpsterDoofus Apr 20 at 15:33
    
That page is better than the below answers. –  phs Nov 6 at 22:05

3 Answers 3

up vote 3 down vote accepted

I'm sure you have seen photographs of snowflakes up close. You will notice that there are hundreds of small crystals of ice. This is the crystal structure of ice. You don't see ice cubes with a crystal structure because they freeze too fast. The water doesn't have enough time to move into the crystal lattice when you freeze the water. This web site shows how the molecules line up in the crystals.

Yes, some ice is denser than water. If you put pressure on regular ice, and give it time to rearrange, the molecules will move into a new crystal lattice which results in the ice being more dense than water. In the first ice crystal, there are spaces between some of the molecules which is not there in the second crystal structure.

With extreme pressure, you can have frozen water at 100 °C.

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To complete LDC3 answer, water molecules can form Hydrogen bonds that are extremely strong compared with any other intermolecular force. While in liquid state, this bonds are formed and destroyed because of the kinetic energy of the molecules; however, as the temperature drops, molecules will start to arrange themselves in such a way that the Hydrogens bonds are locked into a lattice, leaving empty space between the molecules and thus expanding the volume.

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Water is known as $H_2O$. In it's liquid state this molecules wiggles around and needs more space.

Hexagonal lattice structure

Cooling water down slows this wiggling motion. After the phase transition to it's solid phase it is called ice. The solid phase features a hexagonal lattice structure caused by strong hydrogen bonds. The picture depicts red oxygen and small gray hydrogen atoms. Due to the compact lattice density of water ice is less than it's fluid phase. Concluding icebergs and ice in your drink can float in water.

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