When liquid water freezes to form ice, how does its density change?

Introduction / Context:
This question addresses a very important and unusual physical property of water. Unlike most substances, water expands on freezing, and this has direct consequences for density, floating of ice and many environmental phenomena.

Given Data / Assumptions:

• We consider pure water at normal atmospheric pressure.
• The change is from liquid water to solid ice.
• We are asked what happens to density during this phase change.
• Density is defined as mass per unit volume.

Concept / Approach:
Most substances contract on cooling and thus increase in density when they solidify. Water behaves unusually because of its hydrogen bonding. As water freezes, molecules arrange themselves in an open hexagonal lattice that occupies more volume than the liquid. Since mass remains the same while volume increases, density decreases. This lower density of ice compared with liquid water causes ice to float on water, an effect with major environmental significance.

Step-by-Step Solution:
1) Density is given by density = mass / volume.2) When water at around 4 degrees Celsius cools and freezes, its molecules arrange into a rigid structure with more open space.3) The mass of the sample is unchanged during freezing.4) However, the volume increases due to the open lattice structure of ice.5) With a larger volume and the same mass, the density decreases, which means ice is less dense than liquid water.

Verification / Alternative check:
Numerical values from data tables show that the density of water at around 4 degrees Celsius is close to 1.0 gram per cubic centimetre, while the density of ice at 0 degrees Celsius is about 0.917 gram per cubic centimetre. This confirms that the density is lower in the solid state. The everyday observation that ice cubes float on water is direct evidence of this density difference, since floating occurs when the solid is less dense than the surrounding liquid.

Why Other Options Are Wrong:
It remains constant: If density were constant, ice would neither sink nor float and there would be no expansion on freezing, which is contrary to observation.
It increases: This is what happens for most substances but not for water; if density increased, ice would sink instead of floating.
It becomes zero: Density can never become zero for a real substance with non zero mass and finite volume.
It fluctuates randomly: The density change is systematic and predictable, not random.

Common Pitfalls:
Some students assume that all substances become denser when they solidify because this is generally true. Water is a famous exception due to hydrogen bonding and its special molecular structure. Forgetting this anomaly leads to wrong answers. Remember the key visual clue: ice floats on water, so its density must be lower than that of liquid water. This simple observation should guide you to the correct choice in exam questions.

Final Answer:
When water freezes to form ice, its density decreases.