Question

Why is ice less dense than water?

Short answer

Ice is less dense than water because when water freezes, the water molecules arrange themselves in a structure that leaves more open space compared to the random arrangement in liquid water. Consequently, fewer molecules can fit into the same space, resulting in lower density.

Step-by-step solution

Explanation of density

Density is defined as the mass per unit volume. In short, it's how much stuff is packed into a certain amount of space. If an object is dense, it means there's a lot of mass in a small volume. If it's less dense, there's less mass in the same amount of volume. The concept of density applies to all states of matter, including solids, liquids, and gases.

The structure and behavior of water molecules in liquid state

Water molecules are made up of one oxygen atom and two hydrogen atoms, bonded together by covalent bonds. In a liquid state, water molecules are constantly moving about and can be close together or farther apart at any given time. The varying distances between the molecules mean that they can pack together quite compactly, making water more dense.

The structure and behavior of water molecules in solid state

When water freezes and becomes ice, the movement of the water molecules slows down. They begin to arrange themselves in a distinct, repeating pattern, with each oxygen atom loosely bonded to four other hydrogen atoms, forming a tetrahedral shape. This arrangement leads to open spaces in the structure of the ice, meaning fewer molecules can be fitted into the same space. Hence, the mass per unit volume (density) decreases, leading to ice being less dense than water.