Final answer:
The water level in a glass does not change when the ice melts because ice has a lower density than liquid water due to the arrangement of water molecules, and it displaces an equal amount of water as both a solid and a liquid.
Step-by-step explanation:
When a piece of ice floating in a glass of water melts, the level of water in the glass does not change. This phenomenon is due to the unusual property of water, whereby ice has a lower density than liquid water because of its molecular structure. The solid form of water, ice, is less dense than its liquid form because of the way water molecules are arranged. This molecular arrangement means that as the ice melts, it contracts, and the volume it displaces as a solid is equal to the volume of water it adds when it transitions to a liquid. Therefore, the water level does not change when ice floating in it melts.