Why is ice less dense than liquid water?

• A. Ice has more molecules
• B. Ice traps air increasing density
• C. Hydrogen bonds form a lattice that expands, making ice less dense
• D. Ice is made of heavier molecules

Answer: (C)

The main idea is that density depends on how tightly packed the molecules are. In ice, the water molecules are held in a rigid, open lattice by hydrogen bonds, which forces them to stay further apart. This expanded structure means a given mass takes up more space, so ice has a lower density than liquid water. When water is liquid, those hydrogen bonds constantly break and reform, letting molecules slide past each other and pack more densely, increasing density. Because density is about how much mass fits in a volume, not about having more or heavier molecules, the other ideas don’t explain why ice is less dense.