The Structure of Water Chapter Notes

Open Crystalline Structure When water freezes, water molecules arrange themselves into a six- sided crystalline structure with many open spaces Water molecules in liquid form can get relatively close to one another When water freezes, the crystalline structure causes water in the solid state to occupy more space than water in the liquid state This is why ice cubes float in water—the increase in volume from liquid to solid causes solid water to be less dense than liquid water This is a rare property of water—most atoms of frozen solids pack together in a way that causes the solid state to occupy a smaller volume than the liquid phase

Structure of H 2 O Hexagonal shape causes snowflakes to all have a similar shape Applying pressure to ice causes the open space to collapse and turn into liquid water Ice is covered by a thin film of liquid water even at temperatures that are below freezing Ice’s hexagonal structure requires support; at the surface, the structure is not supported so it collapses into a thin film of liquid—this is why ice is so slippery

Melting and Freezing Melting and freezing occur at the same time Melting: water changing from a solid to a liquid Freezing: water changing from a liquid to a solid At 0°C, water is both melting and freezing—the rate of ice formation is equal to the rate of liquid water formation In the liquid state, water molecules are moving slowly enough that they will clump together and form ice crystals In the solid state, water molecules are vibrating with enough commotion to break free from the crystalline structure and form liquid water

Melting and Freezing At exactly 0°C, the two phases of solid and liquid water will coexist indefinitely If we want solid ice to form, heat needs to be removed—this process causes the formation of hydrogen bonds If we want a mixture of ice and liquid to melt completely so that we only have liquid water, heat needs to be added—this causes hydrogen bonds to be broken apart

Solutes and Water Adding solutes to water will prevent water molecules from forming crystalline structures For example, when you add salt or sugar to an ice and liquid water mixture, the solute molecules take up space and prevent water molecules from joining ice crystals The rate of ice formation decreases Hence, why we use salt on icy roads in the winter!

Water’s Density For most substances, when the temperature is increased, molecules will vibrate faster and move farther apart Solids, liquids, and gases expand when heated and contract when cooled Water also expands with increasing temperature and contracts with decreasing temperature Exception—near 0°C, crystals of ice can form within liquid water, causing the liquid’s volume to increase. When the temperature rises, the crystals begin to melt, and the volume of the liquid water decreases Between 0°C and 4°C, liquid water contracts as temperature increases When temperatures are greater than 4°C, expansion will override contraction Liquid water has its smallest volume and greatest density at 4°C

Benefits of Water’s Unique Properties Since water at 4°C is slightly more dense than water at 0°C, solid ice floats on top of liquid water Imagine if this didn’t occur and ponds froze from the bottom up? All organisms living in the water would die Because solid ice is less dense than liquid water, organisms in ponds and lakes can live in the warmer water beneath the frozen surface Water also has vertical movement, which creates vertical currents— organisms benefit from the cycling of oxygen-rich surface water being moved to the bottom and nutrient-rich bottom water being moved to the surface