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S3-EFFECTS OF HEATING SUBSTANCES


By PragyanSharma Polavarapu

THERMAL EXPANSION OF SOLID METAL OBJECTS

Let us examine what happens when a small block of iron is heated in a flame (by holding it carefully in a hot flame with a tongs). All the billions of atoms in outer layers become very hot and give out heat radiation. The atoms are vibrating to greater distance ( i.e. perhaps by a few billionth of centimeter increase in one atom’s “amplitude”! ) . So the iron block also grows a little larger which we can observe only when checked with help of microscope . Metal rods , thin metal plates ,solid metal blocks etc when heated gradually show a little increase in length, surface area and volume respectively. As mentioned above the expansion is so small that we can measure it only by checking with help of a microscope. Brittle solids like stones and bricks etc break and disintegrate into small particles when heated strongly. When strogly heated some crystals which do not melt, disintegrate .

COMBUSTION

Burning of candle , burning wood and burning petrol in an engine—all these are chemical reactions. Heat energy is released during the process as billions upon billions of atoms react most violently . It is actually like a war scene on atomic scale. What we see as burning is a violent and high speed chemical reaction accompanied by release of “heat quanta” or “heat photons” coming from inside the billions,billions and billions of atoms. The heat energy photons or quanta are created by the electrons revolving in orbits. Oxygen is utilized in reaction and carbon dioxide and water vapour are formed and are released into air along with much heat energy.

“Combustion” in the science of heat means extensive or complete burning of a substance like dry wood, paper , dry cloth, oil-soaked cloth etc. Metals do not burn . Combustion scientifically means reaction between the various hydrocarbons and other organic chemicals (contained in substances like wood, paper, cotton ,oil-soaked cloth etc) with the oxygen contained abundantly in air . Oxygen is contained in air upto about 21%.

The reacting and resultant gases rise to a very high temperature (around 1000 degrees centigrade ) . The very hot gas atoms give out both heat and visible light and the gas mass becomes visible as tongues of fire .We call this mass of visible reacting gases as flame. The flame is the visible region of chemical reaction between gases chemically reacting at very high temperature.

Upto 200 or 300years back even big scientists had wrong notions about combustion. They believed that all combustible substances had a real substance called “Caloric” inside them and that it was weightless and flowed from hot to cold substances and is used up during combustion. At that time scientists did not know about existence of element Oxygen. They did not know that Oxygen reacts with Carbon(contained in hydrocarbons) to produce Carbon- dioxide gas and reacts with Hydrogen (also contained in hydrocarbons ) to produce pure water vapor.

They believed that the weightless substance “Caloric” existed inside every combustible substance ( i.e. even before its burning) where as modern science states that only the “heat energy” coming from outside initiates the “combustion” which is only a chemical reaction between oxygen and hydrocarbons taking place at very high temperature . Combustion is nothing but a visible high temperature chemical reaction occurring before our eyes. The area of violent chemical reaction is the visible flame.











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