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Lesson 10

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  Lesson 10: Properties of Water Objective  In this lesson we will answer the following question:    What are the unique properties of water? Reading Assignment  Along with the online lecture, read chapter 9 in  Basic Chemistry for Water and Wastewater Operators . Lecture The Basics Introduction  Water covers about three quarters of the earth's surface and makes up about three quarters of our body weight. In fact, without water, life would not be possible. This simple fact is why scientists are constantly looking for water on other planets - the  presence of water could indicate the presence of life. We have discussed some of the properties of water in previous lessons. This lesson will consider water's properties in more depth. As you read about the characteristics of water, take some time to think about how these characteristics relate to the fields of water and wastewater treatment. Water Molecule     As you should remember, water is represented by the formula H 2 O. The picture  below can also be used to represent water. Lesson 5 explained how hydrogen bonds form between water molecules. The ability of water molecules to form hydrogen bonds, as shown below, causes many of water's unique characteristics. For example, you should recall that hydrogen bonding makes water an excellent solvent. Part 2: Temperature  Lesson 10: Properties of Water   Temperature Introduction   Water is unique in that it is found as a gas, a liquid, and a solid at natural earth temperatures. In contrast, most other substances are naturally found in only one or two states. This property of water is integral to our daily lives, and is especially important in the hydrologic cycle. On this page, we will consider how water is influenced by temperature. The surrounding air temperature can change the temperature of water, change water's state, and change water's density. Specific Heat Capacity   Specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. Every substance has its own specific heat capacity, with the specific heat capacity of water being 1 cal/(g°C). The specific heat capacity of water is much higher than that of other common substances. For the sake of comparison, the specific heat capacity of oil is about 0.5 cal/(g°C) and the specific heat capacity of aluminum is about 0.2 cal/(g°C). This means that it takes a lot more heat to raise the temperature of water compared to the amount of heat it would take to raise the temperature of oil or aluminum. The high specific heat of water helps the earth's temperature remain moderate since water traps heat during the day and releases it slowly at night. As a result, the temperature on earth's surface does not vary very widely, ranging from extremes of 134°F to -129°F. For comparison, the moon has no liquid water and its temperatures can range from 240°F to -290°F. (The lack of atmosphere on the moon, along with other factors, also contributes to the wide range of temperature.) Boiling Point  Water also has a very high boiling point, meaning that liquid water turns into water vapor at a higher temperature (212°F) than would be expected due to the size and weight of the molecule. The high boiling point of water is due to the hydrogen bonds which tend to hold water molecules together, preventing them from breaking apart and entering the gaseous state. Since it takes such a large amount of energy to change the state of water, sweating is a  very effective method of cooling the body. In order to evaporate, the sweat requires the input of a great deal of heat energy, some of which comes from our bodies. So, as our sweat evaporates, we begin to feel cooler. Density and Expansion  As you should remember from ENV 110, density  is the ratio of mass to volume. Dense objects feel heavier and tend to sink while less dense objects feel lighter and tend to float. The density of most objects changes slightly as the temperature changes. In general, warmer temperatures tend to make substances less dense because the greater random kinetic energy makes the molecules spread out. The amount that objects expand when heated is known as the coefficient of expansion . The density of water, once again, is a special case. Water is most dense at 39°F, and as it cools or warms from this temperature, the water expands slightly. This means that ice is slightly less dense than cold water, which is why ice floats on the surface of  bodies of water. The floating ice slows the freezing process by insulating the water underneath, which contributes to the moderate temperatures on earth. In addition, the layer of ice prevents many lakes from freezing solid, allowing fish and other organisms to survive under the ice.
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