4.1 The Changing States of Water
Changing States of Water
Water is an essential atmospheric element in transferring heat, determining the stability, and creating active weather. We briefly examined the states of water associated with latent and sensible heat in Module 2. Due to the importance of water, the changes associated with its state are reviewed and expanded in this section.
4.1.1 Evaporation, Condensation, and Saturation
To examine the properties of water (liquid) and water vapor (gas), as an experiment we can fill half a container with water. A barrier is placed between the water the remaining space in the container. All of the water vapor molecules are removed from the “empty” portion of the container. The experiment begins by removing the barrier between the two sections of the container.
Water molecules leave the liquid and become airborne (water vapor) which is called evaporation. The rate of molecules leaving the liquid is the evaporation rate. Likewise, water molecules can again become liquid which is called condensation. Initially, the evaporation exceeds the condensation rate. Eventually, an equilibrium is reached where the evaporation and condensation rates are the same. This equilibrium between the two rates is called saturation.
Saturation, evaporation rate, and condensation rate are related to the kinetic energy of the water molecules. Temperature is a measure of this kinetic energy. Water vapor (gas) has the highest kinetic energy and has the highest temperatures. Like any gas, water vapor is compressible. At the molecular level, water consists of two hydrogen atoms and one oxygen atom. The hydrogen atoms have a positive charge while the oxygen has a negative charge.
These charges determine the structure of water as it undergoes changes in state. Water vapor as a gas consists of individual molecules mixed in with the other atmospheric gases. Water as a liquid has the molecules aligned with positive and negative charges adjacent to each other. The liquid form is very compact and water is densest at 4oC or 39oF.
Water as a solid (ice) has a six-sided crystalline structure. This structure requires more space as the hydrogen and oxygen molecules align themselves into “fixed” locations. Hence, the reason that water pipes burst when frozen. The density of ice is less than the density of water. Hence, the reason ice cubes float in a cold drink. Water is the only substance on our planet that naturally exists in all three states. This illustration depicts water when frozen as ice.
4.1.2 Sublimation and Deposition
The orderly change of state for water is to go from ice to liquid to vapor and vice versa. However, once a water molecule has enough energy it can skip this sequence of steps. The phase change of water going directly from ice to water vapor is called sublimation. Deposition is going straight from water vapor to ice. Deposition is a crucial step in creating precipitation which will be examined later in this course.
4.1.4 Latent Heat
Latent heat is the energy required to change the phase (gas, liquid or solid) of a substance. This definition was given in Module 2. This heat energy is labeled as latent because it is hidden. Water can exist as both ice and liquid at 0oC, although latent heat is required to go from one state to another. The latent heat associated with these phase changes is given in this illustration. The energies associated with the phase or temperature change are for 1 gram of water.
Melting or freezing takes 80 calories. The temperature change for liquid water to go from 0oC to 100oC is 100 calories. The latent heat of evaporation (or vaporization) is a whopping 540 calories. Likewise, the latent heat of condensation is 540 calories. More important than these values is how latent heat changes the environment. The environment in our case is the atmosphere or the Earth’s surface.
The latent heat of condensation releases 540 calories per gram of water, which is an enormous amount of heat. One gram of water equals a milliliter. A normal size bottle of water contains 500 ml. The energy required to evaporate this bottle or condense water into it is 270,000 calories. A person’s normal food calorie intake in a day is 3,000 calories or less. When one bottle of water precipitates out of the atmosphere, the atmosphere has gained the heat energy comparable to over 3 months of food (calories) for one person. The latent heat released through condensation is a fundamental driver in hurricanes and strong thunderstorms.
Video: Mod 4.1.4 Water changes of state and latent heat (8:19 min.)
This video examines the importance of water’s phase changes and the latent heat associated with these changes.
References:
water_molecule.png – Public domain from WikiCommons at https://commons.wikimedia.org/wiki/File:Water_molecule_3D.svg
water_molecule_with_signs.png – Public domain from WikiCommons at https://commons.wikimedia.org/wiki/File:Water_molecule_3D_with-signs.svg
ice molecules.png – Public domain from WikiCommons at https://commons.wikimedia.org/wiki/File:H-Br%C3%BCcken_f%C3%BChren_im_Eis_zu_regelm%C3%A4%C3%9Figen_Anordnung_im_Eis-Meolek%C3%BCl_(Kalottenmodell).png
