Have you ever wondered why the water in an open vessel eventually evaporates or drys out? Matter around us exists in four general phases (or states). Ice, for example, is the solid phase of {H}_{2}{O}. Add energy, and you would add motion to the rigid molecular structure to form a liquid phase of {H}_{2}{O}. Add more energy, and the liquid would change to the gaseous phase, water vapors. Add some more energy, and the molecule breaks down into ions and electrons, giving the plasma phase. In general terms, the phase of the matter depends upon the kinetic energy of the system (an ensemble or collection of molecules under consideration). In thermodynamics, there are few different ways to define the kinetic energy of a system. One of them is temperature, the measure of the average kinetic energy of each molecule in the system. Another measure of the kinetic energy of a system is internal energy (or thermal energy), represented by the letter U. It is the measure of the kinetic energy of all the molecules in the system added together. The amount of thermal energy added to or removed from a system is what we call “heat.” There is a common misconception about heat and temperature among people around the world. Strictly speaking, heat refers only to the energy in transit. Once the heat has transferred, we say that the internal energy U of the object has increased, not that it contains more heat, and therefore, has more temperature. In thermodynamics, heat is the energy, denoted by Q, that is transferred between the systems when they are at different temperatures. Now, two things happen to a system when the heat is added to or removed from it—either the temperature changes or the phase. For instance, if we have an ice block of 1 kg at standard atmospheric pressure and -10 ºC temperature, on adding heat to it, the temperature will absolutely rise. At a certain point, when the temperature hits 0 ºC, the increase in temperature will stop for some time because the heat that is being added will now change the phase of the ice to liquid water. Later, when the ice is completely melted, a further increase in heat will raise the temperature again until it reaches 100 ºC. At that point, the water will again go into a phase change, from liquid to gas. The quantity of heat absorbed or released when a system changes its physical state, with its temperature being constant, is defined as Latent Heat. It is categorized into two types:
- The Latent heat of fusion is the amount of heat required to change a system from solid to liquid phase.
- The Latent heat of vaporization is the amount of heat required to change a system from liquid to gaseous phase.
Moreover, the amount of heat required to change a system’s temperature or phase also depends on the mass of the system. Therefore, in a quantitative analysis of a thermodynamical system, specific latent heat is preferred over latent heat. The term “specific” expresses the latent heat in terms of energy per unit mass, denoted by the symbol L, with units Joules per gram ({J}{g}^{-1}). The equation for specific latent heat is:
{L} = \frac{Q}{m}where:
- L is the specific latent heat
- Q is the heat absorbed or released
- m is the mass of a substance
Sensible Heat
Sensible heat is a term used in contrast to latent heat. In any thermodynamical system, sensible heat is the energy required to change the temperature of a system with no phase change. Technically, the exchange of heat changes the temperature of the body or system and some macroscopic variables of the body or system, but it leaves certain other macroscopic variables of the body or system such as volume or pressure unchanged. It is the heat that can be “sensed” as a change in an object’s temperature. Both sensible and latent heats are observed in many processes while transporting energy in nature. Latent heat is associated with changes of state, measured at a constant temperature, especially the phase changes of atmospheric water vapor, mostly vaporization, and condensation, whereas sensible heat directly affects the temperature of the atmosphere. Energy moves through the atmosphere using both latent heat and sensible heat acting on the atmosphere to drive the movement of air molecules that creates wind and vertical motions.
1. Sweating Causes Cooling
Have you ever wondered why we sweat when our environment is hot or when we exercise? It may look filthy, but sweating is a life-saving bodily function that helps regulate your body temperature. Also called perspiration, sweating is the release of a salt-based fluid from your sweat glands. Without sweating, the body cannot regulate its temperature, which can lead to overheating or even heat stroke. Why does sweating have a cooling effect? Well, when the weather is hot or your body temperature rises due to exercise or fever, sweat is released through ducts in your skin. It contains 90% of water. The sweat drops present on your body utilizes the heat of the body for the phase transition from liquid to vapor. This results in a cooling effect (called evaporative cooling) that helps to maintain body temperature and cools the body down when it gets too hot. For water, the value of specific latent heat is 540 calories/gram or 2.26 x 106 joules/kilogram. So, if you can produce one liter of sweat, which is equal to 1000 g or 1 kg (density of water is 1 g/ml or 1 kg/l) in one hour, then 540,000 calories of heat can be removed from your body. This is an extreme example of the maximum amount of sweat that a person can make.
2. Earthen Pot
An earthen pot is a water-storage vessel that is used all over the Indian subcontinent to keep water cool during summers. It has been in use since ancient times and can be found in houses of every class. They are made by the combination of two types of mud clay: the first is taken from the surface of the earth and the second after digging more than 10 feet deeper into the earth. The walls of an earthen pot are porous, which leads to seepage of water via capillary action. This is the reason why the outside wall of an earthen pot mostly remains wet. Latent heat of evaporation cools the pot in the same way it cools your skin by perspiration. If the pot is at a lower temperature than the water in it, heat energy will be transferred to the pot from the water by conduction. This heat energy will cause a change in the phase of water present outside the wall of the earthen pot. One may ask how, if the air present around is hotter than the water, can heat flow from the water into the hot air? The answer to this is the thermodynamical equation of specific latent heat. As mentioned above in the formula, specific latent heat is inversely proportional to the mass. Since the mass of the water present inside the pot is way more than the water present outside, the specific latent heat required to vaporize the water inside is also high.
3. Dumplings (Momo)
Many of us enjoy dumplings for their moist-juicy texture and nutrition values. It is a common dish in most of central Asia, which is prepared by steaming. It is a method of cooking that works on moist heat. Unlike boiling food submerged in water, with steaming the food is kept separate from the boiling water but comes into direct contact with the hot steam. You only need two simple pieces of equipment to steam food on the stovetop: a pot and a steamer basket. The pot is filled with a small amount of liquid that is brought to a simmer; the item to be cooked is placed in a basket suspended above the liquid, and the pot is then covered. The hot steam circulates through the pot and cooks the food very quickly. This technique is known as “compartment steaming.” The bamboo steamers or banana leaves used in Asian cuisine are an example of a compartment steamer. this method of cooking is full of benefits. Steaming is a gentle cooking method that ensures the greatest possible preservation of vitamins, minerals, and trace elements of food. It does not alter their appearance or flavor, however, the freshness of a product is even enhanced by this natural cooking. It is also a cooking method that eliminates dietary fats, acids, and toxins to the point that organic eater recommends this kind of cooking for non-organic food, cooking in water vapor resulting in draining any pesticides in lower parts of the container.
4. Ice Cream
Ice cream is one of the many food items that do not need any introduction. There is no better way to beat the heat during a hot summer day than to enjoy your favorite flavored ice cream. At first sight, ice cream appears to be a simple frozen mixture of milk, cream, and sugar. Despite its seeming simplicity, ice cream is a prime example of some fairly complex chemistry that first meets the eye. Well, scientifically speaking, it is a frozen mixture of water, fat (dairy or vegetable), milk proteins, sugars, salt, and air. When you mix all these ingredients together in certain proportions, you get a mixture that is equal parts an emulsion, a dispersion, and a foam. The different textures we like in different varieties of ice cream are the result of different levels of ice crystallization in ice cream. This is achieved by adding salt to the mixture. The action of the salt on the ice causes it to (partially) melt by absorbing the latent heat and bringing the mixture below the freezing point of pure water. Generally speaking, slow freezing—slow transport of latent heat—causes fewer nuclei to form per a given volume and time, which allows more space for each crystal to grow in and results in larger crystals.
5. Drinking Bird Toy
A drinking bird, also known as an insatiable bird, is a wonderful toy, based on heat engine principles, that mimics the motion of a bird flexing back and forth while drinking water from a vessel. To someone who does not have much knowledge of thermodynamics, a drinking bird may seem like a perpetual motion machine that can go forever without any external force. But perpetual motion machines are impossible due to the law of conservation of energy. As a thermodynamic system does work, it loses heat. Whereas, when the work is done on the system, it gains heat. In general, heat can be converted into mechanical work, and vice-versa. The drinking bird is filled with a fluid, that has a low boiling point, so it can change from liquid to gas and back, with just a slight change in temperature. The bottom of the bird’s belly is filled with such liquid, with some vapors present in its head. When the head of the bird is dipped in the water, it gets wet. It keeps the head bobbing back and forth for a while, and as the water evaporates, latent heat of vaporization causes condensation of the vapors present in the head. This, in turn, reduces the pressure, by creating a partial vacuum, in the head. Due to this pressure differential, the liquid starts rising in the tube making the head heavier and tip over. As the bird pivots forward, at some point, the tube in the bottom is no longer completely submerged into the liquid, and the vapors start rising instead of liquid. This allows the liquid to flow back into the bottom and set the whole cycle back in motion.
6. Regelation
Figure: The wire gradually passes through the ice without cutting it in half.
Throwing snowballs at each other during the winter season surely seems like fun, but have you ever wondered how the ice sticks together when compressed? When we compress the snow with our hands, we cause a slight melting of the ice crystals; when pressure is removed, refreezing occurs, and it binds the snow together. This phenomenon of melting under pressure and freezing again when the pressure is removed is known as regalation. To understand the role of latent heat in regelation, let’s discuss an experimental approach. Consider a block of ice resting on two supports, and a thin copper wire with heavyweights at each end hanging over it (Figure). Eventually, depending on the size of the block, the wire cuts right through it and falls to the floor, leaving the ice still in a solid block. The latent heat required for the melting comes, in the first instance, from the copper wire. As soon as the water passes above the wire, it is no longer under pressure, and therefore, it refreezes. In doing so, it gives out latent heat, and this heat is conducted down through the wire to provide heat for further melting of the ice beneath. It must be realized that the thermal conductivity of a material is an important factor in the process. An iron wire of smaller thermal conductivity cuts through much more slowly. A thin string of very low conductivity will not pass through at all. For the same reason, making snowballs in an extremely cold environment is very difficult, as the pressure we apply is not enough to melt the snow.
7. Pressure Cooker
Pressure cookers hold a special place in the kitchen, particularly when it comes to energy efficiency and speed of cooking. Do you know why does a pressure cooker cooks faster than an ordinarily covered pot? At some level, a pot of steaming water and a pressure cooker, both are pretty similar. The energy involved in both the processes is latent heat energy; however, in the case of the pressure cooker, there is an unconventional interplay of latent heat is involved in cooking. When the temperature reaches 100 ºC in an ordinary vessel, the water starts boiling, but the temperature will not increase further till all the water is converted into steam. However, in the case of the pressure cooker, its top is fully closed, so steam can not escape from the beaker, and the pressure in the beaker will start to increase. Due to the increase in vapor pressure over the water surface, the remaining water molecule cannot escape, and it cannot be converted into steam. Therefore, the heat given to the pressure cooker increases the temperature now rises the temperature of the water, which is further utilized as a latent heat by the food itself. This increases the speed of cooking, roughly speaking, up to twice as fast. The purpose of the whistle on a pressure cooker is to act as a pressure relief valve that lets steam escape when it exceeds a specified pressure and to notify when to reduce the heat.
8. Geysers
A geyser is a rare kind of hot spring that is under pressure and erupts, sending jets of water and steam into the air. In simpler terms, a geyser is a periodically erupting pressure cooker. It consists of a long, narrow, vertical hole into which underground water streams discharge. The column of water is heated to temperatures exceeding 100 ºC by the volcanic heat. This happens because the deep verticle column exerts pressure on the water, thereby increasing its boiling point. The narrowness of the shaft shuts off the convection currents, which allow the deeper portions to become considerably hotter than the water present above. Boiling begins near the bottom, where the rising bubble pushes out the column of the water above, which leads to an eruption. As the water gushes out, the pressure on the remaining water is reduced. It then boils rapidly and erupts with great force.
9. Ice-Skating Rinks
Ice skating rinks are popularly known as the arena to play ice hockey. While normal ice skating rinks can exist as frozen naturally occurring water bodies, the ones used for playing ice hockey are specially designed. They are made by laying a thick plastic sheet over a leveled surface. A mesh of specially made polymer tubings is then laid over these sheets. These tubes are then filled with an anti-freezing liquid called glycol. The whole network of these tubes acts as a heat radiator when connected to a super chiller. The glycol remains liquid while freezing down the water present in its surroundings into ice. Carefully flooding with layers and layers of water, a 5 cm thick bed of ice is made to support the weight of the hockey players. This simple yet incredible engineering technique is used in making ice rinks for several sports, even in the Olympics.