Melting point is the temperature at which a given solid substance changes its solid-state to liquid. Both the phases, i.e., solid and liquid, exist in equilibrium at the melting point. Therefore, the melting point of a given solid is equal to the freezing point of a liquid. For example, water shows equilibrium at 0°C.
It is a physical property of a substance, which is utilized for various applications. The most common example of melting in everyday life is the melting of ice cubes when some sort of heat is applied.
How Melting Occurs?
Crystalline solids consist of a 3-dimensional arrangement of extremely fine particles, which are referred to as lattice structures. Lattice forces are operated within the particles of lattice structures. When this type of solid structure is heated, its constituent particles vibrate more rapidly because they absorb the kinetic energy. This vibration continues until the forces of attraction between them are no longer stronger enough to maintain the crystalline structure. This, in turn, breaks down the solid structure, and eventually, a solid start to melt. The temperature at which this process takes place is known as its melting point. In amorphous solids, particles are in a random arrangement in the crystal lattice, for example, glass, resins, and many synthetic plastics.
Melting Process
Factors affecting Melting Point
The melting point of a substance depends on the strength of attractive forces, which are operating inside a solid structure. For example, sodium chloride (NaCl) is an ionic compound, which is made up of strong ionic bonds. Thus, it melts at high temperatures, i.e., 801°C, whereas ice is a compound comprising of hydrogen bonds, whose strength is less than ionic bonds. Hence, ice melts at low temperatures, i.e., 0°C. On increasing the pressure of a solid, the melting point of a substance decreases. The presence of impurities in a solid structure also lowers its melting point, which is termed as- “Melting point depression”.
Who measures Melting Points?
Analytical Laboratories
Analytical laboratories perform various tests for the examination of materials, which are performed by various industries such as pharmaceuticals, food, electronics, and plastics to check the purity of products. One of the analytical techniques applied to the characterization of purity of products (from raw materials to finished products) is the melting point determination. It is important to carefully choose the melting point determination procedure for generating certifiable results for quality control (QC) and quality assurance (QA). It is a must for analytical QC/QA labs to calibrate their melting point instrumentation on regular basis to determine whether their instruments are in accordance with the specific requirements defined by their local, national, and international laboratories.
Examples
1. Purity Analysis of Ghee/Butter
Ghee or Butter is one of the most consumed items in India. It is essential to ensure its purity as adulterated cow ghee is mixed with vegetable oils and animal body fat. Rancid ghee is also available in the market because of its same color and texture. Here are some easy tricks, which can be performed at home, and these are based on the concept of melting to ensure its purity.
Heat Test
Take a teaspoon of ghee in a vessel and heat it. If ghee melts immediately and shows some brownish color, then it is of pure quality. However, if it takes some time to melt and shows some light yellow texture, then it is impure, i.e., it is mixed with some impurities.
Palm Test
Take a teaspoon of ghee in your palm, if it melts by itself, then it is pure, or otherwise not.
Double-Boiler Method
Ghee is also adulterated with different kinds of oils, especially coconut oil, and hence, a double boiler method is used to check whether ghee contains coconut oil or not. In this method, ghee is melted in a glass jar using a double boiler, which contains a jar inserted over a pan of simmering water, and then, put this jar in the refrigerator for some time. If ghee and coconut oil solidify in separate layers, then ghee is not pure as different layers show the presence of oil in ghee.
2. Ice Skating
In the winter season, many people find the ice fascinating. They enjoy ice skating in the snow-covered areas, but have you ever wondered what is behind the slipperiness of ice that enables them to do ice skating? The reason behind the slipperiness of ice is the idea of pressure melting and friction. When the pressure is applied on the ice, it melts the top layer of ice and creates a thin layer of water that enables it to skate. The pressure melting theory is based on the fact that the freezing temperature of the water is lower than 0°C for a particular high-pressure range.
3. Alloy Formation
Alloy formation is an example of melting. The majority of alloys are prepared by mixing different metals in the molten state, which is followed by solidification. All metals have different melting points. This creates some difficulty in alloy formation. For example, copper melts at 1083°C, while zinc melts at 419°C and boils at 907°C. So, in making brass (an alloy of copper and zinc), if we melt both metals by heating them above 1083°C, the liquid zinc will also boil away, and the vapor would oxidize in the air. To avoid this, first, we have to melt the metal with high temperature, i.e., copper, and when copper comes to a molten state, the solid zinc is added to it, and it is quickly dissolved in liquid copper before much of zinc is boiled away. Here, proper knowledge of melting points of metals, and the melting process is a must in to make different alloys, which in turn offers many applications in various industries.
Super-alloys
Super-alloys are made up of different combinations of nickel, iron, and cobalt alloys, and these alloys are mostly used in jet engines. These are produced by electron beam melting. This specialized melting process involves melting under a high vacuum using an electron beam. This melting process is costly, so its usage is limited.
Steel Melting
Steel is an alloy of iron and carbon. The knowledge of its melting point is important for its fabrication into a valuable form. The melting point of steel depends on its type. The traces of other elements are also added to improve its properties, and their percentage in them also affects their melting point. Out of the 5 main types of steel, stainless steel is used mostly for kitchen cutlery.
4. Light Bulbs
Light bulbs contain a filament, which is made up of tungsten metal. Tungsten metal is used because it has the highest melting point among metals. To produce light, the metal used in bulb filaments must be heated to extreme temperatures. Most metals usually melt before reaching such extreme temperatures. Therefore, light bulbs are manufactured with tungsten filament because it has an abnormally high melting point. The filament in a light bulb is covered in a sealed, oxygen-free chamber to avoid its combustion. The tungsten filament evaporates more rapidly as the temperature of the filament rises. As more and more atoms evaporate, the disintegration of filament takes place, and the glass becomes darker, which reduces the life of the bulb, whereas, in modern light bulbs, argon gas is used, which enhances the life of tungsten filament. In such bulbs, when tungsten atoms evaporate, it will collide with argon atoms and bounce back to the filament, where it will rejoin the solid structure. Since argon is an inert gas, there is no chance of combining this element in a combustion reaction. Although it is still a popular method of lightening inside houses, in recent times, cooled light sources, such as fluorescent lamps and LEDs, are slowly edging out the old light bulbs.
5. Sugars
Sugar is a form of carbohydrate, which contains carbon, hydrogen, and oxygen atoms. When carbohydrates are consumed by the human body, it is converted into glucose, which serves as the preferred energy source for all cells throughout the human body. There are two main forms of sugars, i.e., simple and complex. The difference lies in their chemical structure. Simple carbohydrate is composed of sugar molecules, whereas complex carbohydrate is composed of two or more sugar molecules. The four most common forms of sugar in our diet are Glucose, Fructose (Fruit Sugar), Sucrose (Table Sugar), Lactose (Diary Sugar).
Melting Sugars
The decomposing temperature range for sugars is between 184°C to 186°C. When we heat sucrose gently, it produces a phenomenon known as “apparent melting”. Sugars crystals actually do not melt but show a reaction called “inversion” which means two molecular components of sugars, i.e., glucose and fructose decomposes. This process is known as caramelization, which consists of two phases. In the first phase, the sugar structure changes as the temperature increases. This can be observed when sugar starts to ‘melt’. In the second phase, the more increase in temperature causes the elimination of water molecules, which produces a reaction called- ‘beta elimination’. After this, the substance darkens in color and tastes more like “caramel”. If excessive heat is applied, only carbon will remain, and this indicates that our caramel is burnt. When the inversion takes place, it gives us “invert sugar” that is very hygroscopic (able to absorb a high number of water molecules), which makes them ideal for the preparation of soft sweets and desserts.
6. Glass Melting
In the glass making process, raw materials are heated to a temperature ranging from 1500°C to 1700°C in the glass melting furnaces. It is transformed through a sequence of chemical reactions. Glass melting furnaces can be electric, gas-fired, or oil-fired.
7. Coin Melting
Have you ever noticed that why does the size of coins is decreasing in India? The logic behind this is the melting of coins. The government always tries to reduce the metallic value of coin than its face value because if the metallic value of coins is more than its face value, then the people will be encouraged to melt the coin and sell that metal in the open market to get profit. This can be understood by this example, suppose a person has a coin of Rupees 1, if he melts the coin and sells the metal in the open market for Rupees 2, then he would gain a profit of Rupees 1. Imagine, if all the people start this melting process, then all the coins will vanish, and it will create a challenging situation for the government. Thus, it is the reason that the government tries to reduce the metallic value than its face value so that people will not be encouraged to melt the coins.
8. Snow Melting
Heavy snow in cold climatic regions causes the freezing of soil and large deposits of snow everywhere, which often results in motor vehicle accidents and risks the lives of drivers and passengers. Thus, snow melting is essential to avoid such situations. Snow melting involves 2 types of methods, i.e., chemical method, which involves the application of chemicals, such as NaCl or CaCl2, for the removal of snow, and heating method that involves the heat pump, infrared lamp, or heating wires to melt the snow.
9. Wax Melts
Wax melts are a form of wickless candles. They are available in different shapes and sizes. It is a type of scented wax that is melted in wax warming devices (electric devices), which releases a more stronger and consistent fragrance than a candle. Wax melts releases the fragrance oil once the wax is melted. Mostly wax melts available in markets are made with either paraffin wax or soy wax. Paraffin wax should be avoided because it is made up of petroleum byproducts.
Soy Wax Melts
10. Metal that melts into your Palm
Gallium is an unexpected metal that melts in the palm of your hand. Its melting point is relatively low, i.e., 85.6°F, and just the heat of your palm is able to melt it. This property makes gallium ideal for recording high-temperature thermometers.
Gallium Metal Melting