Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Water Expansion Upon Freezing
Name: Emily
Status: student
Grade: 6-8
Location: ME
Country: USA

When water freezes, why do its molecules move apart?

Hi Emily,

The main reason for this is that the force that holds molecules in the liquid state is different from the forces that hold molecules together in the solid state, and it so happens that in the case of water, the force holding the water molecules together in the liquid state is actually stronger than that in the solid state.

In the liquid state water is held together by a force called hydrogen bonding - this is an unusually strong attractive force between molecules and particularly strong in water. Hydrogen bonding happens when one of the hydrogens of the water is transferred to another water molecule -which in turn has one of its hydrogen atoms transfer to a different water molecules . . . and so on. This very fast transfers act to bridge the water molecules together so that they get very close to each other.

In the solid state, water is held together by the fact that as heat is given off (as water freezes), the water molecules have to "fit" together - like a jigsaw puzzle, and only certain orientations are low enough in energy or stable. This fitting together limits the way the water molecules can get close to each other and as it turned out, is not as close as when water does hydrogen bonding.

Hope this helps,
Greg (Roberto Gregorius)

Click here to return to the Material Science Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory