Water Crystallization & Hydrogen Bonding
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Hits the standard: ESS2-5: Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.
Lesson Brief: Among the many ways that water influences the surface of the earth are freeze-thaw cycles that contribute to weathering and soil formation, and the accumulation of snow and ice, and the way that ice forms to create a floating solid. Understanding the way that water molecules join to form a crystalline structure is a key to understanding how water can break bedrock and form complex geometric shapes. This lesson examines how the change from unordered water molecules into the regular crystal lattice structure of frozen water gives ice properties that lead to geological processes such as soil formation and have helped shape the biology of Earth.
Background: Hydrogen bonding that occurs when water freezes causes strong forces to orient the water molecules into hexagonal rings. Multiple stacked rings can be visualized as hexagonal prisms with oxygen molecules at the vertices. When additional hexagonal prisms bond together, they form shapes that radiate outwards at 60° angles to adjacent projections. Some units bond to the upper and lower surfaces and thus form additional layers or very straight needle-like structures of ice. The forces bonding the molecules together are very strong and cause the unordered and densely packed molecules to spread apart slightly as they take on the hexagonal structure during freezing. This slight (approximately 8% less dense) decrease in how tightly packed together the water molecules are causes ice to float on water and causes the water to expand as it freezes. This expansion causes water filled cracks in rocks to increase in volume, splitting bedrock. This expansion causes containers full of water to split open when frozen. Mineral particles in soil are partially derived from the freeze-thaw action of water on the underlaying bedrock. The decreased density that allows ice to float allows ice to insulate standing fresh water such as lakes and has likely played an important role in the evolution of life on the surface of the earth.
Activity: Print the water crystallization model. This can be used to highlight how water crystallizes into ice. The change from “jumbled” water molecules into a hexagonal prism-based solid form can be used to show why ice is less dense than water. The abundance of 60° angles in ice crystals and snowflakes can be highlighted with the steps modeled as well.
Figure 1: Render of the 3-D printed water crystallization model.