# Why is ice less dense than water?

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?

• 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 '14 at 15:33
• That page is better than the below answers. – phs Nov 6 '14 at 22:05
• Possible duplicate: physics.stackexchange.com/q/78414/2451 – Qmechanic Jan 16 '16 at 8:32

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