Chemistry
Heating Curve for Water
The heating curve for water illustrates the relationship between temperature and heat energy as water undergoes phase changes. It shows the temperature changes as heat is added or removed, and the plateaus represent the phase transitions between solid, liquid, and gas. The curve helps to understand the energy required to change the state of water at different temperatures.
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4 Key excerpts on "Heating Curve for Water"
eBook - PDF- Richard L. Myers(Author)
- 2005(Publication Date)
- Greenwood(Publisher)
The heat needed to convert liquid water to steam is called the heat of vaporization of water. The heat of vaporization for water is 41 kJ per mole. This value is roughly seven times the heat of fusion and indicates it takes only 1/7 of the energy to melt water compared to vaporizing water. Vaporization takes significantly more energy than melt- ing because to convert liquid water to steam requires completely breaking the hydrogen bonds and separating the water molecules. In the gas phase, the water molecules can be thought of as independent molecules with minimal intermolecular attraction. The heat of vaporization of a substance is generally several times that of its heat of fusion. This is because the intermolecular forces present in the condensed phases must be overcome before a substance can be converted to gas. A couple of points should be made about the heating curve of water. The last section of the heating curve represents the situation when all the liquid water has been converted to steam. At this point, as the temperature of the steam begins to rise, superheated steam would be obtained. It should also be real- ized that phase changes can be considered in terms of a cooling curve. In this case, fol- lowing the curve from right to left shows that 41 kJ per mole of heat would have to be released to condense steam to liquid water, and 6.0 kJ per mole would have to be released by liquid water in order for it to freeze. Phase Diagrams The Heating Curve for Water shown in Figure 6.3 shows that as heat is added to a sub- stance it changes phases from solid to liquid to gas. The heating curve for other substances follows a similar pattern. While phase changes occur when heat is added or subtracted from a substance, pressure can also bring about phase Heat 91 changes. An increase in pressure compresses a substance and favors the solid state, while a decrease in pressure allows a substance to expand and favors the gaseous state.
eBook - PDF- P.F. Kelly(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
The hoar frost decorating lawns, trees, and rooftops on chilly mornings is deposited water vapour. Figure 38.1 shows the evolution of a thermally isolated sample of ice [ solid H 2 O ], with an initial temperature well below 0 C , receiving thermal energy at a constant rate. 3 The ice warms up at a constant rate, in keeping with the notion of specific heat capacity developed earlier in this chapter. This trend persists until the temperature reaches 0 C , whereupon it temporarily stops rising. In the plateau region at T = 0 C , the constituent fractions of relative phase are changing. The system passes from entirely solid, through decreasing amounts of solid together with increasing amounts of liquid, to all liquid, all the while remaining at a fixed temperature. Once the entire sample has melted, the temperature of the water rises at a constant pace governed by the specific heat capacity of liquid water. Another plateau corresponding to the phase transformation from liquid to gas occurs at 100 C . While the phase is changing, it is evident that heat is being added with no corresponding increase in the temperature of the [ two-coexisting-phase ] system. The energetics of these phase change plateaus are well-modelled by writing Δ Q = ML. 3 Constancy of the rate of heating means that the time parameter serves as a proxy for the total amount of energy added to the sample. Furthermore, it is implicitly assumed that the heat is added slowly enough that all parts of the sample are at [very nearly] the same temperature at each instant. 38–256 Properties of Materials 0 100 T t FIGURE 38.1 Temperature vs. Time for a Fixed Amount of Water Being Heated at a Constant Rate and Undergoing Phase Changes Here, the heat added [ the LHS ] is equal to the input power multiplied by the temporal duration of the plateau. The RHS is the product of the mass of the material undergoing the phase change, M , and the substance-and transition-specific latent heat, L .
eBook - PDF- Donald Olander(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
The simpler appearance of the p -T projection is due to the elimination of the volume as a represented variable in the diagram. The line labeled L/V in the p -T projection gives the temperature dependence of the vapor pressure of liquid water, also called the vaporization curve . The S/V line is the sublimation curve . It represents the equilibrium pressure of water vapor over ice. The S/L line is called the melting line and gives the combinations of pressure and temperature for which solid and liquid water coexist at equilibrium. Water is unusual among pure substances because its melting temperature is lowered as the pressure is increased. Most other substances behave in the opposite way; their melting * The line AB in the T -v plot of Figure 2.9 is hidden in the saturated-liquid curve. BZ BC v v v v xv x v v v v x f g f g f f g f = − − = + − − − = ( ) 1 Equations of State 67 curves tilt to the right from the triple point rather than to the left. The unusual behavior of water is due to the higher density of the liquid compared to that of the solid. Detailed analyses of the vaporization curve and the melting line are presented in Chapter 5. 2.7 THE STEAM TABLES The graphical representations of Figures 2.8 and 2.9 are valuable for understanding the general features of the p -v -T properties of water but are of little use for quantitative analysis. For this purpose, extensive tables of the thermodynamic properties of water have been compiled. Such tabular information is available for many condensable substances in addition to water. For the latter, the property listings are called the steam tables , although they contain data for liquid water and ice as well as for the vapor phase. The steam considered in the tables is pure, undiluted by other gases such as air. Mixtures of water vapor and noncondensible gases are considered in Chapter 5.
eBook - PDFThe Atmospheric Environment
Effects of Human Activity
- Michael B. Mcelroy(Author)
- 2021(Publication Date)
- Princeton University Press(Publisher)
4.2 Phase Diagrams The conditions for equilibrium between the three phases of a substance can be represented simultaneously on a graph known as a phase diagram. The phase diagram for water is illustrated by Figure 4.3a. Equilibrium between phases ex- Phase Diagrams 33 ists only for temperatures and pressures indicated by the lines separating the areas labeled “ice,” “liquid,” and “vapor.” The curves for the three phases intersect for pure water at a temperature of 273.0098 K (0.0098°C) at a pres- sure of 6.1 mb. For this unique configuration, known as the triple point, all three phases (ice, liquid, and vapor) are si- multaneously in equilibrium. Ice is unstable at temperatures higher than the triple point; it melts. Liquid is unstable at temperatures below the triple point; it freezes. The melting-point temperature (the solid line separating the liquid and solid phases) decreases slightly as the value of the total pressure applied to the ice increases; meaning, it is affected by the presence of other gases in addition to H 2 O. It is equal to 273 K (0°C) at a pressure of 1 atm (Figure 4.3b): it is this property of water that was initially used to set the zero point for the Celsius scale of temperature. We can un- derstand the shift in the melting point by recalling that the density of water ice is less than the density of its liquid: ap- plication of pressure to an ice crystal causes it to more easily revert to the more dense liquid form. The pressure effect on melting is relatively modest under atmospheric conditions; the melting point line in Figure 4.3a is nearly vertical.
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