Why does freezing water expand

In liquid water, these "hydrogen bonds" form, break, and re-form. But by 39 degrees, molecules have formed more stable bonds. And at 32 degrees, water molecules have linked up into hexagonal crystal lattices. Unlike the slip-sliding molecules in your just-filled ice cube tray, the molecules in solid ice are held rigidly apart, with an empty space in the center of each lattice. This extra space is the reason a parcel of water is about 9 percent bigger when frozen solid.

And why cups of water make nearly cups of ice. But ice's expansion has a dark side. In a late-spring frost, plant cells may burst as their water turns to ice, killing fruits and flowers.

Indoors, if the heat goes out, water can freeze in a building's plumbing system, bursting cast-iron pipes as if they were cheap plastic.

Scientists say that the force of expanding water depends on the exact form ice takes as it freezes. But some estimate that water freezing in a pipe can exert a force of about 3, pounds per square inch.

On heating, liquids expand since the molecules move with greater energy overcoming the intermolecular attraction. On the contrary, liquids usually contract on cooling. It is because the molecules move slower and are not able to overcome the force of attraction between them. When they freeze, they contract some more to form a rigid solid structure with minimal intermolecular spaces between them.

But that is not the case with water. Instead of contracting, it expands. The water molecule , consisting of 2 atoms of hydrogen and one of oxygen forms a Mickey Mouse head-like structure such that the ears are the hydrogen atoms and the head is represented by the oxygen atom.

The oxygen atom side of the molecule is slightly negative while the hydrogen atoms side has a slight positive charge. This makes the water molecules drawn towards each other forming hydrogen bonds. Upon freezing, the molecules set themselves in an arrangement that is very open in nature and contains more space than the water in the liquid state.

Hence, water is said to expand on freezing and becomes less dense. Demonstrate that water expands when it freezes by showing how it can break a bottle. Equipment Apparatus Glass bottle with a lid see note 2 below Plastic zip-lock type bag see note 3 below Chemicals Water Health, safety and technical notes Read our standard health and safety guidance.

A thin-walled glass bottle with a screw-top lid is ideal. A thick plastic bag is best as it will need to contain broken glass. The bag needs to be see-through. Procedure Lesson 1 Fill the bottle as full as you can and attach the lid. Dry the outside of the bottle, place into the plastic bag and seal it. Put into the freezer at least overnight. Lesson 2 Remove the bag from the freezer and observe the broken bottle. Teaching notes This demonstration shows the very unusual property which water has of expanding when it freezes.

Additional information This is a resource from the Practical Chemistry project , developed by the Nuffield Foundation and the Royal Society of Chemistry. Level years years. Use Demonstrations. Category Properties of matter Structure and bonding. Chemical changes and structure b Structure and bonding ii Intermolecular forces Hydrogen bonding between molecules in ice results in an expanded structure that causes the density of ice to be less than that of water at low temperatures.

Develop and use models to describe the nature of matter; demonstrate how they provide a simple way to to account for the conservation of mass, changes of state, physical change, chemical change, mixtures, and their separation. Leaving Certificate Chemistry 2. Chemical bonding 2. Intermolecular forces: van der Waals', dipole-dipole, hydrogen bonding.