OC103 Lesson #12: Properties of Seawater This lesson covers the water in the ocean: where it came from, what’s in it, and how its composition affects its behavior. Where did the water on Earth come from? • Mostly from outgassing of Earth during volcanic activity early in its history. Remember how Earth differentiated early in its history, and some of the heavy chemical elements such as iron and nickel sank to form the core? Well for the same reason, the lightest elements rose to the surface, and then accumulated there to form our atmosphere and oceans. This process was not complete, and 90% of the water on Earth is actually still dispersed in minute concentrations deep in the crust and mantle within Earth. Even though the oceans are almost 100% water, they are only about 4–5 km thick (deep), whereas the crust and mantle mostly contain less than 1% water but are thousands of km thick. In this class we will ignore the water deep inside Earth since it has little to do with oceanography. So when we talk about the amount of water on Earth, we are only referring to the water (whether solid, liquid, or gas) at or very near to Earth’s surface, not locked up deep in Earth. • A much smaller amount comes from comets that fall to Earth. Many comets are made of rocky ice, and when those comets strike Earth (a common occurrence, but they are usually so small that we do not notice them), their ice melts and makes a small contribution to the water on Earth. Over millions of years this water adds up, but is still a small amount compared to what is already here on Earth. Where’s the Water Now? So almost all of the water on Earth is in the oceans, and when you add that to how much is in the polar ice caps, less than 0.5% is left for everything else. All six of these reservoirs are connected by something called the Hydrologic Cycle. Reservoir Percent Volume (million km3) Oceans 97.5 1320 Ice (polar) 2.1 29 Groundwater 0.4 5 Lakes 0.01 0.1 Atmosphere 0.001 0.01 Rivers 0.0001 0.001The Hydrologic Cycle The Hydrologic Cycle is a description of how water molecules move around between the six reservoirs in the table on the previous slide (see figure below). As the sun warms the surface of the land and ocean, it causes water to evaporate into the atmosphere. That water vapor circulates around until it condenses into clouds and rain or snow that falls back down to Earth. If it falls onto land, the water runs back into the ocean via rivers and groundwater, completing the cycle. If the rain falls as snow, most of that snow usually melts at the end of the winter and the water flows back to the ocean. Water’s unique ability to pass through this cycle (it is the only substance on Earth that exists naturally as solid, liquid, and gas) is explained by some exceptional properties of the water molecule that also explain many properties of the ocean.H2O: The Water Molecule The water molecule is made up of 2 positively-charged hydrogen atoms and 1 negatively-charged oxygen atom joined in a shape that looks like a Teddy Bear’s head (or maybe Mickey Mouse with stunted ears), with the H atoms forming the ears (see left side of figure below). This interesting shape is significant because its asymmetry means one end of the molecule (the end with the exposed O) has a slightly negative charge, and the other end (the end with the H's) has a slightly positive charge. Consequently, water has some very special properties.Water has High Melting and Boiling Points The melting and boiling points of water are high compared to H and O alone (H & O occur only as gasses on Earth, except where humans have made them into liquids at extremely low temperatures not naturally found on Earth). The high melting and boiling points of water can be explained by properties of that Teddy Bear molecule. The positively and negatively charged ends of the water molecules have natural attractions for the oppositely-charged end of other water molecules (called hydrogen bonding) that makes them cluster together and resist boiling to a gas (see right side of figure on the previous slide). At low temperatures, these opposite charges help the water molecules bond together to resist melting (see left side of figure below). It requires extra thermal energy to break these bonds, which is why relatively high temperatures are needed to make ice melt and water boil.Water has Unusual Density Properties The density of solid water is less than that of liquid water. Most substances are denser in their solid form than in their liquid form because the molecules are closer together in the solid state, but water bucks this trend. Up to a point, as water gets colder its density increases because the molecules move closer together, as is typical with most substances, but when water gets within about 4 degrees (Celsius) of freezing, the molecules begin to organize in a structured formation to make ice. In that ice structure the water molecules are slightly farther apart than they are in cold water (see how the molecules in part ‘a’ of the figure on the previous slide are farther apart than in part ‘b’). So ice is slightly less dense than cold water, which is why it floats. The right side of the figure below shows on a graph how pure water is densest at about 4°C, and becomes less dense at warmer as well as colder temperatures. This property of water has some subtle but significant ramifications. For example, if water was “typical” (i.e., if ice was heavier than liquid water), then many of the lakes in the world would freeze solid in the winter, rather than just form a crust of ice on top.Heat capacity is defined as how much heat energy it takes to raise the temperature of a substance. Some materials, like most metals, warm up very easily and rapidly when a little bit of heat is added. Other materials, such as water, require substantially more heat to increase their temperature by the same number of degrees. This is important for the oceans because it means that water does not change temperature easily. For example, while the air temperature over the ocean often changes by 20°C or more between day and night, the temperature of the ocean water changes very little, if at all. Similarly, the ocean can significantly warm up (give heat to) or cool down (take heat from) the atmosphere above it without changing its own temperature much. This high heat capacity is also explained by the fact that water stores extra