Explain why the arrangement of water molecules is different in ice and water

Ice is less dense than water.

First of all, we must bear in mind that ice is a solid and water is a liquid. In ice, the molecules of water are arranged into regular patterns held together by hydrogen bonds forming a rigid crystal lattice with spaces between these lattices. This gives a solid structure in which each water molecule form four hydrogen bonds to other water molecules.  This arrangements takes up a greater volume hence ice is less dense than an equal mass of water.

Water on the other hand is a liquid and the molecules of water are closer to each other and are also held together by hydrogen bonds. Water molecules stick together and take up less space hence water is denser  than an equal mass of ice.

To learn more about hydrogen bonding in ice see:

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