Chapter 8: Thermodynamics of Water

8.1 Three States of Water Under normal conditions, water exists in one of three states: solid, liquid, and gas. Water in its solid state is what we call ice. Pure water at sea level freezes at 0°C. Liquid water is most often used in cooking, and the temperature varies between 0°C and 100°C. Water boils at 100 °C (at sea level) and becomes vapor. The vapor state is more commonly called steam. The transition between two states is called a phase transformation. If you take a cup of water and heat it under atmospheric conditions, the heating curve will look like one shown in Figure 8.1. In the figure, the horizontal axis shows time (or more energy added), and the vertical axis is temperature. Notice that the temperature of water stays constant with added energy during phase transformation. This is because all of the energy is used to transform one state to another that no energy is available for heating the water. Figure 8.1 Heating curve of water 8.2 Boiling Boiling water is one of the most commonly used heat source in cooking. Boiling water is undergoing liquid-to-gas transition, and because of this it stays at a constant temperature of approximately 100°C. This provides a convenient standard for us to control the cooking process. As liquid water is heated, the molecules become increasingly mobile. Some of the molecules acquire enough energy to escape as vapor. These molecules exert a force on the atmosphere, called the vapor pressure. The vapor pressure is opposed by another force, created by a column of air pushing down on the pan. This pressure is the atmospheric pressure. The two pressures are illustrated in Figure 8.2. Water begins to boil when the vapor pressure overcomes the atmospheric pressure. This means that the majority of water molecules have become energetic enough to escape the surface. Figure 8.2 Pressure and boiling The temperature at which water boils is related to the vapor pressure required for boiling, which is equal to the atmospheric pressure. The implication of this is that as the atmospheric pressure changes, the boiling point of water changes as well. When you go up a mountain, the air pressure is lower (the column of air pushing down is smaller). Therefore, water boils at a lower temperature, and food takes longer to cook. For every 1000 ft. in altitude, the boiling point of water decreases by about 1 °C. A clever appliance designed to take advantage of the pressure-boiling point relation is the pressure cooker. A pressure cooker is a tightly sealed pot which uses the steam from water to increase the internal pressure of the pot. As the pressure inside the pot increases, the boiling point rises, and a higher temperature can be achieved for cooking. The internal temperature can be determined by controlling the vapor pressure inside the pot. Since the internal temperature can be raised substantially above 100°C, food cooks faster. The boiling point of water can also be changed by adding impurities in the water. Impurities include salt, sugar, and other dissolving molecules. Generally, impurities increase the boiling point of water. A simple explanation of this is that the impurities dilute the concentration of water (the number of water molecules per unit volume decreases), and the number of molecules that can vaporize at any give temperature decreases. The result is that a higher temperature is required to achieve the same vapor pressure. Concentrated sugar-water solutions that are used for making candies and caramel boil at temperatures exceeding 150 °C. 8.3 Freezing Water freezes when the molecules have slowed down enough to develop bonds upon collision. The rate at which freezing occurs is governed by nucleation and growth. Nucleation is the formation of small solids in a liquid. The clusters of solids are called the nuclei. The rate at which new nuclei form (number of nuclei per second) is the nucleation rate. Once the nuclei have formed, they become the landing sites for other molecules to attach onto. The growth rate is the rate at which the radius of a nucleus grows after formation. The solidification rate is determined by the combination of nucleation and growth rates. The size of crystals formed during solidification is determined by the nucleation/growth processes. A solidification process with fast nucleation rate and/or slow growth rate will result in many small crystals forming. Larger crystals form from slow nucleation rate. Most liquids decrease in volume upon solidification. Water, however, has a rather unique property of expanding during liquid-to-solid transformation. This property comes from the hexagonal structure of ice crystals; water molecules form a hexagonal crystal structure, which actually takes up more volume than if the molecules were freely slipping past one another. Consequently, ice cubes float in water. The freezing point of water at sea level is 0°C. This temperature can be changed, however, by adding impurities in water. Sprinkling salt on road surfaces on an icy day melts the ice by lowering the melting temperature. Salt is also used in simple ice cream machines during cooling of the cream. In an ice cream machine, the vessel containing the ice cream mixture is cooled by concentrated brine (salt-water solution) which has a temperature that is lower than the freezing point of ice cream mixture. Another consequence of the decrease in freezing point due to impurities is the soft texture of ice cream. As ice cream freezes, the remaining liquid becomes more and more concentrated with sugar and other impurities. The concentrated liquid has a much lower freezing temperature than water. As a result, ice cream never completely freezes, and retains the characteristic soft texture. 8.4 The Refrigeration Cycle Refrigerators and freezers use refrigeration cycle to draw heat away from the compartments to keep foods cool. The cycle has four major components: evaporator, compressor, condenser, and an expansion valve, as shown in Figure 8.3. A working fluid, typically a refrigerant such as chlorofluorocarbons (CFC’s), flows through the cycle in a closed loop. At point A, the fluid is at a low-pressure liquid state. As it enters the evaporator, which is located inside the cooling compartment, it expands into its vapor state. The phase change is accompanied by absorption of heat from the refrigeration compartment, cooling the refrigerator. An ideal temperature for a refrigerator is ~2 °C, or just above freezing. For freezers, it is typically ~ -15 °C. Figure 8.3 Refrigeration Cycle After all the working fluid has turned into vapor in the evaporator, the gaseous fluid then enters the compressor. The compressor takes the gas and pumps it to the condenser. The pressure of the fluid increases during this process. The condenser is a series of tubes that you typically see behind the refrigerator. As the fluid passes through these tubes, it cools by releasing heat into the surrounding air and condenses to a liquid state. The heat released by the fluid is what you feel behind the refrigerator. The heat, which was taken from the refrigeration compartment, is now released into the room. Finally, an expansion valve is used to carry the condensed fluid back to the evaporator. The tube has a very small diameter compared to the condenser, because the tube only has to carry a small amount of liquid refrigerant as opposed to a less dense gaseous fluid. Refrigeration keeps foods from spoiling by reducing the growth of bacteria. The cool temperature slows down the metabolism of bacteria in foods. This, however, is not the only reason why refrigerators have become such integral part of our lives. Some foods, such as fruits, taste better when they are cold. Furthermore, some foods need to be cooled to develop certain texture. An example of such case is Jell-O, which needs to be refrigerated in order to solidify. An inevitable result of keeping the food at a low temperature is the tendency to dry out if left uncovered. Cold air cannot support as much moisture as warm air, as evident from the dry atmosphere in the winter. The air inside the refrigerator is much drier than in the room. Foods with high water contents must be protected from the dry air using covered containers or plastic wraps. Vegetable and fruit compartments in the refrigerator help keep the foods’ moisture within the drawer to prevent drying. Freezing allows foods to be kept even longer than refrigerating by slowing the bacteria even further. In addition, freezing tend to lock in the water molecules in the form of ice, preventing foods from drying. After time, however, fluid loss will occur through sublimation (phase transition from ice to vapor), causing freezer burns. Another effect of freezing is the loss of texture in many vegetables and fruits. Plants have a high water content, much of which are trapped within cells. Water expands upon freezing and ruptures the cell walls, leading to the loss of structure and texture in the foods. Some say that warm water placed in the freezer will freeze quicker than cold water that is placed in the same freezer. This strange phenomenon has been observed in many cases. The common explanation of this is that if the freezer is covered in a thick layer of frost, then the warm water in an ice cube tray will slightly melt the frost when placed in the freezer, establishing better contact between the freezer floor and the ice cube tray than if a tray of cold water were placed in the freezer. Another explanation argues that the convection currents, which are stronger in the warm water than cold water, stirs the water as it cools in the freezer, allowing more even cooling of water. Useful Links How Refrigerators Work by HowStuffWorks.com Click here for References TOC     << | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | >> HOME | GENERAL INFO | LECTURE NOTES | SYLLABUS | HOMEWORK | LAB INFO Send questions and comments to Haruna Tada ©2002 Mechanical Engineering Department*Tufts University*Medford, MA 02155