Category Chemistry

                 It is commonly observed during the rainy season that water falling on roads and streets disappears after a few hours. Similarly, in summer, wet clothes dry up very soon. Do you know where this water goes? This water gets converted into vapour and goes into the atmosphere. Conversion of water into vapour is called evaporation.

                 How does evaporation take place? Every substance is made up of very small particles called molecules. These molecules are held together by strong forces of attraction called cohesive forces. These forces are opposed by the repulsive forces caused due to the motion of molecules. As long as the cohesive forces are far greater than the repulsive forces, the substance remains in the solid state. When the substance is heated up, it absorbs heat energy due to which the molecules are set in rapid motion. This motion starts counterbalancing the cohesive force, that is, this force tries to separate them apart. When the repulsive force generated by the motion of molecules equals the cohesive force, matter changes from solid to liquid state. If the liquid is continued to be heated, the molecules move still faster and when the force due to the increased motion exceeds the cohesive force, the molecules of the liquid become free and escape into the air. This is how liquid is converted into vapour. The liberation of the molecules from the liquid surface into the air is called evaporation. Evaporation takes place at all temperatures.

              This explains the drying up of clothes in the air. The rate of evaporation increases with the rise in temperature. Wet clothes put in the sun dry up faster than in shade because at higher temperature the motion of molecules becomes faster and the evaporation is also faster. It also takes place more rapidly when the air is dry. That is why clothes dry up more quickly on a dry sunny day than on a damp cloudy day.

Silicon is a non-metallic element. It is not found in nature in a pure form. Pure Silicon is a hard, dark grey coloured material. It shines like metals and is crystalline in nature. At ordinary temperature, it does not react with other elements, but at higher temperature it makes compounds with other elements.

              The earth’s crust has 28% Silicon whereas china clay contains 50% Silicon. It is also found in rocks, sand, water, bones, etc. Sand has a large quantity of Silicon.

              Silicon is extracted from the compound Silicon dioxide. When Silicon dioxide is heated in an electric furnace, oxygen is removed and Silicon is separated from Silicon dioxide.

             Most of Silicon is found in the form of Silica which is another name of Silicon dioxide, a compound of Silicon and Oxygen. Quartz, jasper, milky stone and sand – all are different forms of Silica. Silicate is another compound of Silica. Mica and asbestos are well known forms of Silicates.

            Silicates are very useful to us. They are used in making many different kinds of glasses, enamels, china clay, etc. Sodium Silicate is used in the making of soaps, prevention of the rotting of wood and eggs and in dyeing. It is also used for smoothening and for making artificial rubber.

            Chief use of Silicon in its pure form is in making photocells, transistors and Silicon chips containing micro circuits for computers and other electronic components. A compound of Silicon and carbon called Silicon carbide or carborundum is used for polishing metals. By mixing Silicon in steel, its utility is enhanced. Silicon is also used for making semiconductors which have proved very useful in our life. Silicon mixed with sand and earth is used for making bricks.

             An object is said to be hot if its temperature is more than our body temperature. If we touch such an object, heat flows from it to our body and we feel its sensation. Similarly, an object is said to be cold if its temperature is lower than our body temperature. When we touch it, heat flows from our body into that object. Do you know why it happens?

            We know that our body is made up of cells. And consequently these cells are made up of molecules. At normal body temperature these molecules are in motion. When a hot substance touches a part of our body, the fast moving molecules of that substance accelerate the motion of the molecules of the cells of the affected part of our body. And when the molecules of cells of that part stand the fast vibrations, the cells start breaking up. It is the breaking up of the cells that give the burning sensation. To repair the damaged cells, blood circulation becomes faster in the affected area. That is why the affected part becomes red.

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                 You might have seen small drops of water on grass, plants and trees shining like pearls in the early hours of morning. These water-drops are called dew. Quite often people think that dew drops, like rain-water, fall on the earth from sky during night but this is not true.

                  Dew drops are formed due to condensation of water vapours. Air around us contains water vapours which we call moisture or humidity. Hot air contains more moisture as compared to cold air. During the night when the hot air comes into contact with some cold surface, water vapour present in it condenses on the cold surface in the form od froplets. These tiny drops of water are called dew drops. The process of dew formation can be seen in a simple experiment. Take a tumbler and place it on a table. Now put some ice or ice-cooled water inside the tumbler. You will notice that after some time on the outer surface of the tumbler small drops of water have appeared. These water drops are formed by the condensation of the water vapours present in the air. Exactly in the same way when the trees, plants and grass become cold during night, the water vapours of the air condense on them in the form dew.

                  The dew formation is more when the sky is clear and less when it is cloudy. When the sky is clear and the trees and plants are cooler at nights, there is more evaporation of water and hence more dew formation. But when it is cloudy, trees and plants do not get cool in the night and hence there is less dew formation. As the sun raises high in the sky, these dew drops evaporate into air. 

              Every substance is made up of very tiny particles called molecules. A molecule is the smallest part of matter that can exist independently and exhibits all the properties of that substance. For example, if we take a molecule of sugar, it will exhibit all the properties of sugar such as taste, shape, colour etc. Molecules of different substances differ in size. Some are only few billionths of a centimetre in size while some other molecules are thousand times larger. Molecules of gases are small in size. One cubic centimetre of air contains about (25,000, 000, 000, 000, 000, 000) molecules.

             Even though a tiny piece of matter contains a very large number of molecules, there are empty spaces in between them. Molecules of matter are constantly in random motion. As the temperature increases, the motion of the molecules also increases. The hotter the substance, the faster is the motion of the molecules. Molecules of a gas move very fast in comparison to the molecules of liquids and solids. Even the molecules of ice are in motion. Isn’t it surprising then that though molecules of all substances are in motion, substances don’t appear shaky? Why? The reason is that there exists between the molecules a force of attraction which keeps them together. If this force is not present, the molecules of matter will scatter away in all directions.

             When the molecules of a solid are heated, their speed increases and the force of attraction between them decreases. If we continue adding heat, the speed of the molecules would be still faster and the solid will change into the liquid state and, finally, to the gaseous state. 

From the very ancient times man has a deep fascination for this shiny yellow coloured metal which symbolizes affluence. The statues and temples of our gods and goddesses are decorated with gold ornaments. Kings and Emperors always gave great importance to gold. Man has always shown a great affinity for it. What is so special about gold that it has always been costly and tends to become costlier?

             The increasing preciousness of a substance depends on many factors. First is its rare availability. Second factor is its utility, third is its beauty and fourth is the least reactivity with air and water. Gold is an element that possesses all the four qualities. It is a rare metal found in very small quantities. Its colour and shine are very attractive. It is not affected by heat, cold, wind and humidity. No wonder, it is a costly metal.

            Gold is a shining metal of yellow colour, found in nature in very small quantity. It is found in both free and combined states. To extract gold from its ores is a costly process. An exception was a small rock of seventy and a half kilogram found in Victoria, Australia which yielded seventy kilograms of pure gold. But such rocks are rare. Extremely small quantity of gold is obtained from gold mines. It is traceable in sea water also, but its extraction from sea is uneconomical. It also occurs as a fine grain is sand and gravel.

Continue reading “Why is gold so costly?”