17 Change in Phase

Matter around us exists in four common phases (or states). Ice, for example, is the solid phase of H 2 O. Add energy, and you add motion to the rigid molecular structure, which breaks down to form H 2 O in the liquid phase, water. Add more energy, and the liquid changes to the gaseous phase. Add still more energy, and the molecules break into ions and electrons, giving the plasma phase. The phase of matter depends on its temperature and the pressure that is exerted on it. Changes of phase almost always require a transfer of energy

Evaporation change of phase from liquid to gas that occurs at the surface of a liquid. Cooling

Condensation The opposite of evaporation the changing of a gas to a liquid. When gas molecules near the surface of a liquid are attracted to the liquid, they strike the surface with increased kinetic energy and become part of the liquid. In collisions with low-energy molecules in the liquid, excess kinetic energy is shared with the liquid, increasing the liquid temperature. Condensation is a warming process.

Condensation in the Atmosphere There is always some water vapor in the air. A measure of the amount of this water vapor is called humidity (mass of water per volume of air) Weather reports often use the term relative humidity—the ratio of the amount of water vapor currently in the air at a given temperature to the largest amount of water vapor the air can contain at that temperature. 1 Air that contains as much vapor as it can is saturated.

Why is it common for clouds to form where there are updrafts of warm moist air? Warm air rises. As it rises, it expands. As it expands, it chills and water-vapor molecules are slowed. Lower-speed molecular collisions result in water molecules sticking together. If there are larger and slower-moving particles or ions present, water vapor condenses upon these particles, and, with sufficient buildup, we have a cloud

Boiling 100°C for water at atmospheric pressure— molecules are energetic enough to exert a vapor pressure as great as the pressure of the surrounding water (which is due mainly to atmospheric pressure). The motion of water-vapor molecules in the bubble of steam (much enlarged) creates a gas pressure (called the vapor pressure) that counteracts the atmospheric and water pressure against the bubble.

The tight lid of a pressure cooker holds pressurized water vapor above the water’s surface, and this inhibits boiling. In this way, the boiling temperature of the water is increased to above 100°C. lowered pressure (as at high altitudes) decreases the boiling point of the liquid. So we see that boiling depends not only on temperature but on pressure as well.

Geysers A geyser is a periodically erupting pressure cooker. It consists of a long, narrow, vertical hole into which underground streams seep The column of water is heated by volcanic heat exceeding 100°C. As the water gushes out, the pressure on the remaining water is reduced. It then boils rapidly and erupts with great force

Boiling Is a Cooling Process Both Evaporation and boiling are cooling processes When 100°C water at atmospheric pressure is boiling, its temperature remains constant. That means it cools as fast as it warms. If cooling didn’t take place, continued input of energy to a pot of boiling water would result in a continued increase in temperature. The reason a pressure cooker reaches higher temperatures is because it prevents normal boiling, which, in effect, prevents cooling.

Boiling and Freezing at the Same Time We usually boil water by the application of heat. But we can boil water by the reduction of pressure. We can dramatically show the cooling effect of evaporation and boiling when room-temperature water is placed in a vacuum jar If the pressure in the jar is slowly reduced by a vacuum pump, the water will start to boil. The boiling process removes heat from the water left in the dish, which cools to a lower temperature. As the pressure is further reduced, more and more of the slower-moving molecules boil away. Continued boiling results in a lowering of temperature until the freezing point of approximately 0°C is reached. Continued cooling by boiling causes ice to form over the surface of the bubbling water.

Melting and Freezing Melting- If enough heat is absorbed, the attractive forces between the molecules will no longer be able to hold them together. Freezing is the converse of this process. As energy is withdrawn from a liquid, molecular motion diminishes until finally the molecules, on the average, are moving slowly enough so that the attractive forces between them are able to cause cohesion. The molecules then vibrate about fixed positions and form a solid. At atmospheric pressure, water freezes at 0°C—unless such substances as sugar or salt are dissolved in it. Dissolved substances cause a lower freezing point Solute ions grab electrons from the hydrogen atoms in H 2 O and impede crystal formation. The result of this interference by “foreign” ions is that slower motion is required for the formation of the six-sided ice-crystal structures. The ice first formed is almost always pure H 2 O.

Regelation This phenomenon of melting under pressure and freezing again when the pressure is reduced It is one of the properties of water that distinguishes it from other materials. The making of snowballs is a good example of regelation. When we compress the snow with our hands, we cause a slight melting of the ice crystals; when pressure is removed, refreezing occurs and binds the snow together. Making snowballs is difficult in very cold weather because the pressure we can apply is not enough to melt the snow The wire gradually passes through the ice without cutting it in half.

Energy and Changes of Phase

Heat pumps A refrigerator is a “heat pump.” It transfers heat out of a cold environment and into a warm environment. When the process is reversed, the heat pump is an air conditioner. In both cases, external energy operates the device. This is accomplished by a liquid of low boiling point, the refrigerant, which is pumped into the cooling unit, where it turns into a gas. can extract heat from water that is pumped in from nearby underground pipes. Water underground is relatively warm. In the Midwest and the Central Plains, subsoil temperature below a meter deep is about 13°C (55°F) year-round—warmer than the air in wintertime. Heat is extracted from the water (just as from food in a refrigerator) by the vaporization of a common refrigerant. The vaporized refrigerant is then pumped to condensation coils, where it condenses and gives off heat to warm the home. The cooled water is returned to the ground outside, where it warms back up to ground temperature and repeats the cycle.

A graph showing the energy involved in the heating and the changes of phase of 1 g of H 2 O.

energy involved in the heating and the changes of phase of 1 g of H 2 O. a 1-gram piece of ice at a temperature of –50°C in a closed container that is placed on a stove to heat. A thermometer in the container reveals a slow increase in temperature up to 0°C. Then an amazing thing happens. The temperature remains at 0°C even though heat input continues. Rather than getting warmer, the ice begins to melt. In order for the whole gram of ice to melt, 80 calories (335 joules) of energy is absorbed by the ice, not even raising its temperature a fraction of a degree. Only when all the ice melts will each additional calorie (4.18 joules) absorbed by the water increase its temperature by 1°C until the boiling temperature, 100°C, is reached. Again, as energy is added, the temperature remains constant while more and more of the gram of water is boiled away and becomes steam. The water must absorb 540 calories (2255 joules) of heat energy to vaporize the whole gram. Finally, when all the water has become steam at 100°C, the temperature begins to rise once more. It will continue to rise as long as energy is added.

Heat of vaporization is either the energy required to separate molecules from the liquid phase or the energy released when gas condenses to the liquid phase. Heat of fusion is either the energy needed to separate molecules from the solid phase or the energy released when bonds form in a liquid that change it to the solid phase.

Water’s heat of vaporization is huge. The energy needed to vaporize a quantity of boiling water is nearly seven times the energy needed to melt the same amount of ice. The amount of energy required to change a unit mass of any substance from solid to liquid (and vice versa) is called the latent heat of fusion for the substance. (The word latent reminds us that this is thermal energy hidden from the thermometer The amount of energy required to change any substance from liquid to gas (and vice versa) is called the latent heat of vaporization for the substance. For water, we have seen this is a whopping 540 calories per gram (2255 joules per gram). 6 It so happens that these relatively high values are due to the strong forces between water molecules—hydrogen bonds.

On a cold day, hot water freezes faster than warm water because of the energy that leaves the hot water during rapid evaporation The rate of cooling by rapid evaporation is very high because each evaporating gram of water draws at least 540 calories from the water left behind. This is an enormous amount of energy compared with the 1 calorie per Celsius degree that is drawn from each gram of water that cools by thermal conduction.

Paul Ryan tests the hotness of molten lead by dragging his wetted finger through it. How can they do this? The low conductivity of wooden coals, however is the principal reason the feet of barefoot firewalkers are not burned.