Changes in State
The cooling curve A plot of the temperature of a substance versus the heat removed or versus the cooling time at a constant rate of cooling. Although we might expect the cooling curve to be the mirror image of the heating curve in Figure At this temperature, the steam begins to condense to liquid water.
No further temperature change occurs until all the steam is converted to the liquid; then the temperature again decreases as the water is cooled. This region corresponds to an unstable form of the liquid, a supercooled liquid A metastable liquid phase that exists below the normal melting point of a substance.
If the liquid is allowed to stand, if cooling is continued, or if a small crystal of the solid phase is added a seed crystal A solid sample of a substance that can be added to a supercooled liquid or a supersaturated solution to help induce crystallization. As the water freezes, the temperature increases slightly due to the heat evolved during the freezing process and then holds constant at the melting point as the rest of the water freezes.
Subsequently, the temperature of the ice decreases again as more heat is removed from the system. For example, supercooling of water droplets in clouds can prevent the clouds from releasing precipitation over regions that are persistently arid as a result. Clouds consist of tiny droplets of water, which in principle should be dense enough to fall as rain.
In fact, however, the droplets must aggregate to reach a certain size before they can fall to the ground. Usually a small particle a nucleus is required for the droplets to aggregate; the nucleus can be a dust particle, an ice crystal, or a particle of silver iodide dispersed in a cloud during seeding a method of inducing rain. One approach to producing rainfall from an existing cloud is to cool the water droplets so that they crystallize to provide nuclei around which raindrops can grow. This is best done by dispersing small granules of solid CO 2 dry ice into the cloud from an airplane.
Solid CO 2 sublimes directly to the gas at pressures of 1 atm or lower, and the enthalpy of sublimation is substantial As the CO 2 sublimes, it absorbs heat from the cloud, often with the desired results. Assume that no heat is transferred to or from the surroundings. The density of water and iced tea is 1. Substitute the values given into the general equation relating heat gained to heat lost Equation 5.
Recall from Chapter 5 "Energy Changes in Chemical Reactions" that when two substances or objects at different temperatures are brought into contact, heat will flow from the warmer one to the cooler. The amount of heat that flows is given by.
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Eventually, the temperatures of the two substances will become equal at a value somewhere between their initial temperatures. Calculating the temperature of iced tea after adding an ice cube is slightly more complicated. The general equation relating heat gained and heat lost is still valid, but in this case we also have to take into account the amount of heat required to melt the ice cube from ice at 0.
Suppose you are overtaken by a blizzard while ski touring and you take refuge in a tent. You are thirsty, but you forgot to bring liquid water. Use the data in Example 8. Changes of state are examples of phase changes , or phase transitions. All phase changes are accompanied by changes in the energy of a system. Changes from a more-ordered state to a less-ordered state such as a liquid to a gas are endothermic. Changes from a less-ordered state to a more-ordered state such as a liquid to a solid are always exothermic. The conversion of a solid to a liquid is called fusion or melting.
The direct conversion of a solid to a gas is sublimation. Plots of the temperature of a substance versus heat added or versus heating time at a constant rate of heating are called heating curves.
6 CHANGES OF STATE
Heating curves relate temperature changes to phase transitions. A superheated liquid , a liquid at a temperature and pressure at which it should be a gas, is not stable. A cooling curve is not exactly the reverse of the heating curve because many liquids do not freeze at the expected temperature. Instead, they form a supercooled liquid , a metastable liquid phase that exists below the normal melting point.
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Supercooled liquids usually crystallize on standing, or adding a seed crystal of the same or another substance can induce crystallization. In extremely cold climates, snow can disappear with no evidence of its melting. How can this happen? What change s in state are taking place? Would you expect this phenomenon to be more common at high or low altitudes? Why do car manufacturers recommend that an automobile should not be left standing in subzero temperatures if its radiator contains only water?
Car manufacturers also warn car owners that they should check the fluid level in a radiator only when the engine is cool. What is the basis for this warning? What is likely to happen if it is ignored?
Which causes the greatest temperature change in the water? When frost forms on a piece of glass, crystals of ice are deposited from water vapor in the air. How is this process related to sublimation? Describe the energy changes that take place as the water vapor is converted to frost. What phase changes are involved in each process? Which processes are exothermic, and which are endothermic?
Why is the enthalpy of vaporization of a compound invariably much larger than its enthalpy of fusion? What is the opposite of fusion, sublimation, and condensation? Describe the phase change in each pair of opposing processes and state whether each phase change is exothermic or endothermic. There isn't an increase in temperature because the energy is all used in changing the particles from a solid to a liquid. The same thing happens when the particles change from a liquid to a gas - at first the heat given to the liquid just causes it to warm up.
Then any more heat given does not cause the temperature to rise because it is all used to change state from liquid to gas. Interactive Learning at ewart. Changes in State There are three states of matter that we know about: State Changes All solids, liquids and gases are made of particles. Now answer these questions What are solids, liquids and gases made out of? Are the particles moving or still? Do the gas particles move more or less than in a liquid? Do the gas particles move more or less than in a solid? A change from a solid to a liquid is called what?
Pressure is one of those effects. When the pressure surrounding a substance increases, the freezing point and other special points also go up. It is easier to keep things solid when they are under greater pressure. Generally, solids are more dense than liquids because their molecules are closer together. The freezing process compacts the molecules into a smaller space. There are always exceptions in science.
Water is special on many levels. It has more space between its molecules when it is frozen. The molecules organize in a specific arrangement that takes up more space than when they are all loosey-goosey in the liquid state. Because the same number of molecules take up more space, solid water is less dense than liquid water.