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How does water freeze?

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Quick answer

Water freezes when its molecules slow down enough for hydrogen bonds to lock them into a hexagonal crystal lattice. This typically happens at 0°C under normal pressure, but dissolved substances like salt can lower the freezing point. The open lattice makes ice less dense than liquid water, so ice floats.

Water freezes when its molecules slow enough to lock into a rigid, repeating lattice held together by hydrogen bonds. This happens at 0°C under normal pressure, but impurities and dissolved substances can lower that temperature.

Step by step

  1. Water molecules are attracted to each other through hydrogen bonds between partial positive and negative ends.
  2. As the temperature drops, molecules lose kinetic energy and move more slowly, allowing bonds to persist longer.
  3. At the freezing point, molecules arrange into a hexagonal pattern, forming a tiny ice crystal nucleus.
  4. The crystal grows outward as more molecules join the lattice, releasing heat in the process.
  5. Impurities or dissolved substances disrupt the lattice, requiring lower temperatures to freeze.

Key things to understand

  1. Water molecules are polar, with partial negative oxygen and partial positive hydrogen ends that attract each other.
  2. As water cools, its molecules lose kinetic energy and move more slowly, allowing hydrogen bonds to hold them in place.
  3. At 0°C, the molecules arrange into a hexagonal crystal lattice, which is why ice is less dense than liquid water.
  4. Freezing requires nucleation, a starting point like a dust particle or a tiny ice crystal, to begin the crystal structure.
  5. Dissolved salts or sugars interfere with the lattice, lowering the freezing point, which is why seawater freezes below 0°C.

Labelled diagram

  1. Liquid water

    Molecules move freely, constantly breaking and reforming hydrogen bonds.

  2. Cooling

    Molecules slow down as kinetic energy decreases, so bonds last longer.

  3. Nucleation

    A tiny cluster of molecules forms a stable hexagonal arrangement.

  4. Crystal growth

    More molecules join the lattice, expanding the ice crystal.

  5. Ice

    Rigid hexagonal lattice with open spaces makes ice less dense than water.

How water molecules freeze into ice

Real-world examples

  • Ice cubes in a freezer freeze from the outside in because the surface cools first and nucleation starts there.
  • Seawater freezes at about -2°C because dissolved salt interferes with the ice crystal lattice.
  • Frost forms on grass on a cold night when water vapor deposits directly as ice crystals without becoming liquid.
  • Snowflakes are hexagonal ice crystals that grow in clouds when water vapor freezes around dust particles.

Common mistake

✗ Myth: Water always freezes at exactly 0°C.

✓ Fact: Pure water freezes at 0°C under normal pressure, but dissolved substances like salt lower the freezing point. Also, very pure water can be supercooled below 0°C without freezing until nucleation occurs.

People also ask

What causes water to freeze?
Water freezes when its molecules lose enough kinetic energy to be held in place by hydrogen bonds, forming a rigid hexagonal lattice. This typically happens at 0°C under normal pressure. The process requires nucleation, a starting point like a dust particle or tiny ice crystal, to begin the crystal structure.
Why does ice float on water?
Ice floats because its hexagonal crystal lattice is less dense than liquid water. When water freezes, hydrogen bonds hold molecules in an open arrangement with more space between them, so solid ice occupies about 9% more volume than the same mass of liquid water. This is why ice forms on the surface of lakes and oceans.
Can water freeze at room temperature?
No, pure water cannot freeze at room temperature under normal pressure. Freezing requires the molecules to slow down enough to form a stable crystal lattice, which only happens at or below 0°C. However, water can be supercooled below 0°C if it is very pure and undisturbed, but it will freeze rapidly when nucleation occurs.
Can water freeze at temperatures above 0°C?
No, pure water cannot freeze above 0°C at normal atmospheric pressure. However, water can remain liquid below 0°C if it is very pure and undisturbed, a state called supercooling, until a nucleation site triggers sudden freezing.
How does salt melt ice on roads?
Salt lowers the freezing point of water by dissolving into sodium and chloride ions that interfere with the formation of the ice crystal lattice. This makes the ice melt at temperatures where pure water would stay frozen, but the effect weakens as the salt becomes diluted.
Does water freeze faster if it starts hot?
In some conditions, hot water can freeze faster than cold water, an effect named after Aristotle and later studied by Francis Bacon and Erasto Mpemba. The exact cause is still debated, but factors like evaporation, dissolved gases, and convection currents may contribute.

Part of the guide: Everyday Science: Light, Heat & Matter →