The density of something (how much it weighs for the same volume) is what decides if it floats or sinks. Density is determined by how closely the molecules pack together. Water is densest at 4 degrees celcius – just before it freezes. Ice is a crystal structure that isn’t as dense as cold water.
Fun fact – water over about 25 degrees is less dense than ice from the freezer. If you put ice into very warm water, it will sink before it melts. There’s a way to win a bet with your friends.
As the other scientists have said it is because it is less dense. In ice, the water molecules are arranged in a crystal structure and are further apart than in water, where they are hydrogen bonded and more tightly packed together.
Hello again willemh! Lots of good questions from you today =)
Yup, Drew’s got it right again! Ice is less dense than water so it floats. Its a very special liquid/solid system for that reason – for most the solid is more dense! I think everyone’s done a great job of describing how that’s based on the special crystalline structure of water-ice. And I really want to try the neat trick Drew has described =D