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Latent Heat, Phase Change Fundamental Physics I by Dr. Abhijit kar Gupta (email: [email protected]) 1 Latent Heat, Phase Change Phase Change: Basic concept, definitions and facts Matter normally exists in one of the three phases: solid, liquid and gas1 . Solid is melted into liquid; liquid is boiled or evaporated into gas as the heat is added on to the system. Conversely, gas condenses into liquid and liquid freezes into solid if the heat is taken out of the system by some means. The heat that is required to be added to a system or that is given up by the system for any kind of phase change to occur is the usual sensible heat as discussed in the previous chapter. However, during the time of phase change, the temperature of a system remains unchanged, although the heat is being transferred to or from the substance. The thermometer shows no increase (or decrease) of temperature. This is the concept of latent heat which is a ‘hidden’ heat. Sensible heat turns to latent heat during a phase change. Where does the heat go? Addition of heat causes increased vibration of molecules about their equilibrium positions in a solid and the motion of molecules in a liquid. As a result of this, the temperature of a substance increases. During a phase change (or phase transition), the heat energy goes into the work of breaking bonds or separating molecules etc. rather than increasing the temperature of a substance. Where do we commonly observe phase change? Melting of ice and boiling of water are common things. Ice starts melting at 0 0 C . The temperature remains fixed at 0 0 C until the entire ice melts and is converted to water. When the phase change from solid ice to liquid water is over, the temperature of water is seen to rise further if heat is being added. Again as the temperature of water reaches 100 0 C , the liquid water starts boiling and another phase change process starts. The temperature of boiling water remains fixed at 100 0 C during this time until the entire water is converted to vapour. If now more heat is added, the vapour/ steam can only be superheated (temperature becomes higher than 100 0 C ). 1 Plasma is often considered a fourth state of matter. Fundamental Physics I by Dr. Abhijit kar Gupta (email: [email protected]) 2 d e 100 Temp. c ( 0 C ) b 0 a -50 Time The above figure demonstrates how the temperature changes with time during phase transitions (ice → water → vapour): a. Temperature rises up to 00 C as the ice absorbs heat. b. Latent heat is absorbed by ice and it starts melting (Temp. remains fixed) c. Temperature rises again as the liquid water absorbs heat. d. Water absorbs latent heat of vaporization and starts boiling at1000 C (Temp. remains fixed) e. Heat is absorbed by steam and thus the temperature increases again. Table: 1 Phase Changes of various types: • Melting/ Fusion: Solid Liquid • Vaporization: Liquid Vapour (gas) 1. Evaporation (slow) 2. Boiling (fast) • Sublimation: Solid Vapour • Condensation: Vapour Liquid • Freezing/ Solidification: Liquid Solid • Deposition: Vapour Solid Concept of Critical Temperature: For every pure substance, there is a critical temperature (corresponding to a fixed pressure) at which the process of phase change begins. This is termed as critical point. The critical temperature at which the ice starts melting is 0 0 C (at normal atmospheric pressure) and this is called melting point of ice. The critical temperature at which water starts boiling is 100 0 C (at normal atmospheric pressure) and this is called boiling point Fundamental Physics I by Dr. Abhijit kar Gupta (email: [email protected]) 3 of water. Similarly, there is freezing point at a temperature when freezing (or solidification) process starts and so on. For every pure substance and for every kind of phase change there is a definite critical point (the value of which depends on the atmospheric pressure). However, the impure or mixed substances have no definite melting or freezing point. Solids of different kinds are mixed together in different proportions to prepare mixtures whose melting points are estimated for practical and experimental purposes. Similarly, different liquids can be mixed to have solutions of varied freezing points. Change of volume during phase change: Because of melting the solid becomes liquid and due to freezing the liquid becomes solid again. In most of the cases, the solids have higher densities and smaller volumes than their corresponding liquid states. There are some exceptions. We observe that a piece of ice floats on water whereas a piece of solid wax sinks. The solid ice has lower density (and thus greater volume) than the water. Some other materials like cast iron, brass, antimony, bismuth etc. show similar behaviour like that of ice. Good castings (making dice) can be made using the property of the expansion of solid, like cast iron, which expands due to solidification. When the water freezes in winter, in cold countries, there is often enormous pressure exerted by ice in the water pipes. The bursting of pipes and the cracking of rocks in the mountains are often due to this. Effect of Pressure on Melting point: • For the materials which expand due to melting, the melting points of those materials become higher due to increase in pressure on them. This means, the materials now melt at higher temperatures due to increased pressure on them. • The materials which contract due to melting (or expand due to freezing), such as ice, the melting points of them come down due to increase in external pressure. This means, the materials now melt at lower temperatures due to increased pressure on them. For example, a pressure of 500 atm forces the ice to melt at − 40 C . We have often observed two pieces of ice can be joined by pressing them together. The pressure exerted upon them makes the melting point lower where two faces join (interface). The ice melts at a temperature lower than 0 0 C due to excess pressure and when the pressure is withdrawn, the melted water at that site freezes again which makes the two ice pieces unite. This phenomenon is called regelation. Regelation was discovered by Michael Faraday. Regelation: This is the phenomenon in which water refreezes to ice after it has been melted by pressure at a temperature below the freezing point of water. Examples: Fundamental Physics I by Dr. Abhijit kar Gupta (email: [email protected]) 4 • Pressure makes an ice skate, form a film of water that freezes once again after the skater has passed. • A weighted wire slowly melts through a block of ice whereas the ice refreezes behind it. This stops the ice block from breaking in half. This is known as Bottomley’s experiment. What is a freezing mixture? Freezing mixture is a mixture of substances whose freezing point is lower than that of its constituents. A commonly used freezing mixture is ice with salt (NaCl). For example, the freezing point of a 1 M NaCl solution is − 3.4 °C, approximately. Any solutions will have such a lowering or depression of freezing point. In case of saline water, the higher the concentration of salt, the greater is the freezing point depression. Any foreign substance added to the water will cause a freezing point depression. In cold countries, the water in lake or river freezes in winter. However, the sea water does not easily freeze. The freezing point of saline water in sea comes down below 0 0 C . Note: For example, normal seawater, which contains approximately 3500 parts per million salt 0 (including Na, Ca, Cl, Mg, K, SO 4 etc.), freezes at around − 2.2 C . In case of water with extreme salinity, such as very salty lake waters at Death Valley, California (approximately 300,000 parts per million salt content) may freeze at a temperature as low as − 200 C . Ice mixed with salt melts quickly. Why does salt melts ice? The following is a microscopic explanation: Ice starts melting at 0 0 C and melts at any temperature above this. Two opposite things happen at this time. Molecules on the surface of ice escape into water (melting) and the molecules of water are captured on the surface of ice (freezing). The water and ice are said to be in dynamic equilibrium in this way. Adding salt to the system disrupts this equilibrium. Salt gets easily dissolved into water. Now some of the water molecules are replaced by salt molecules. So, the number of water molecules captured by ice per second goes down and thus the rate of freezing goes down. On the other hand, the rate of melting is unchanged by the presence of the foreign molecules (salt). So, so melting occurs faster than freezing. This makes the ice melt quicker. Vaporization by Evaporation and Boiling: Vaporization is a phase change process, in general, whereby a liquid changes to gaseous phase. This may happen in the following ways: Fundamental Physics I by Dr. Abhijit kar Gupta (email: [email protected]) 5 • Evaporation: Evaporation is the process of vaporization whereby atoms or molecules from the surface of a liquid phase gain sufficient energy to enter the gaseous phase. This is the opposite process of condensation. (In case of molecules or atoms evaporating from a solid surface, it is known as sublimation. Example is camphor.) The process of evaporation is an important component of the hydrological cycle in nature. Solar energy drives evaporation of water from oceans, lakes, rivers, moisture in soil etc. to form clouds and rain.
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