Question
Why does ice float on water? Most solids sink in their own liquid — so why is ice an exception?
This is a classic NCERT Class 7 question and a favourite in board exams. The answer ties together density, molecular structure, and hydrogen bonding.
Solution — Step by Step
Any object floats if its density is less than the liquid it’s placed in. Ice floats on water, so we need to show that ice is less dense than liquid water — which is unusual because solids are normally denser than their liquids.
In liquid water, molecules are loosely packed and constantly moving. They don’t follow a fixed pattern, so they can fit close together. This gives liquid water a relatively high density: 1.0 g/cm³.
When water cools below 0°C and becomes ice, hydrogen bonds lock every water molecule into a rigid, hexagonal (honeycomb-like) lattice. This open structure forces molecules farther apart than they were in the liquid state.
Because the molecules spread out in ice, the same mass now occupies more volume. Density = mass/volume, so density goes down. Ice has a density of about 0.917 g/cm³ — less than liquid water’s 1.0 g/cm³.
Since density of ice (0.917) < density of water (1.0), ice floats. About 9/10ths of an iceberg sits below the surface and only 1/10th shows above — because the densities are very close.
Answer: Ice floats because hydrogen bonding creates an open hexagonal structure in ice, making it less dense than liquid water.
Why This Works
Water is one of the very few substances where the solid is less dense than the liquid. This happens because of hydrogen bonds — the attraction between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another.
In liquid water, these bonds keep breaking and reforming, so molecules stay fairly close together. In ice, the bonds freeze into place and force every molecule to sit at a fixed distance from its neighbours in a spread-out hexagonal lattice. More space between molecules means lower density.
This property has enormous consequences for life on Earth. Because ice floats, it forms an insulating layer on top of lakes and ponds in winter. The water beneath stays liquid, and aquatic life survives. If ice sank (like most solids), lakes would freeze solid from the bottom up and most aquatic life would die.
Alternative Method
We can also think about this using volume change:
Take 1 gram of water. Its volume is exactly 1 cm³. Now freeze that same 1 gram. The mass hasn’t changed (it’s still 1 gram), but the volume increases slightly because the hexagonal lattice takes up more space. So the same mass now occupies, say, 1.09 cm³.
Same answer, different angle. For board exams, the hydrogen bonding explanation scores full marks.
Remember the number: ice is roughly 10% less dense than water. That’s why exactly about 1/10th of an iceberg sits above the waterline. This ratio appears in application-based questions.
Common Mistake
Many students write: “Ice floats because it is lighter than water.” This is incomplete and loses marks. Lighter alone means nothing — a truck is heavier than a coin, but steel sinks and both weigh different amounts. The correct term is less dense. Always compare densities, not masses. The examiner specifically looks for “density of ice is less than density of water” in the answer.
A second slip: students say water expands on cooling. Be careful — water actually contracts from 100°C down to 4°C, reaches maximum density at 4°C, and then expands as it freezes from 4°C to 0°C. This expansion-on-freezing is the key step, and it’s caused by the hydrogen bond lattice forming.