Category Physics

Most of the gases can be prepared in the laboratory but there are some which are found in nature only. Helium is one such gas.

                It is an inert gas. It is odourless and tasteless. Helium has some special characteristics due to which it has proved very useful for us. It is second lightest gas next to hydrogen but it has a speciality that, unlike hydrogen, it is non inflammable.

               This gas was discovered in 1868 independently by English scientists Sir Joseph Norman Lockyer and French Chemist, Pierre Janssen. While studying the solar spectrum to find the elements present in the sun, they came across some lines which were not observed earlier. These lines suggested the presence of a new element in the sun. The element was named as Helium after the Greek word ‘helios’ meaning ‘the sun’. Thereafter, scientists made efforts to know if helium was present in the earth’s atmosphere. They detected very small quantities of helium in the atmosphere. Atmospheric air contains only one part in 1, 86,000 parts, i.e., 0.0005239 percent.

               On account of its lightness and non-inflammability, it is used in weather balloons. Helium is also used to ease the breathing difficulties of asthmatic patients. Deep sea divers are given a mixture of oxygen and helium for breathing so that after coming back, they do not suffer with bends. Helium is also used to weld aluminium. A mixture of helium and neon is used to produce laser beams. Helium can be liquefied at – 268.9 degree centigrade. Liquid helium is used in low temperature work.

               There are some places in America like Texas, New Mexico, and Kansas etc. where the amount of helium present in the atmosphere goes upto 8%. It is also present in atmosphere in Canada, Africa and the Sahara desert. America is the world’s richest source of this gas and is therefore the biggest seller of helium to other countries. In earlier times this gas was very costly, but now it has become quite cheaper.

                 Without oxygen there would have been no life on this earth. Without it no living being – trees, plants or animals can survive. Hence it will not be a misnomer to call it a life-giver. Do you know what is it and why is it absolutely essential for us?

                Oxygen is a gaseous element. This was discovered by two scientists independently. Carl Scheele, a Swedish chemist, discovered oxygen in 1772 and Joseph Priestley, discovered it in 1774 in England. It is a colourless, tasteless and odourless gas. In the atmosphere the air contains 21% oxygen, 78% nitrogen and rest one percent other gases. In the earth’s crust, it is found in the form of oxides of metals, the proportion being up to 50%. At – 182.9°C under right pressure it can be liquefied. In the liquid state its colour becomes light blue. At – 218.4°C, it can be converted into solid state.

               In the laboratory, this is obtained by heating the mixture of potassium chlorate and manganese dioxide. From the atmosphere, it is obtained by the fractional distillation of air. The air is compressed to two hundred times the normal atmospheric pressure and passed through a narrow hole into a chamber. As the pressure is suddenly dropped, the air is liquefied and collected in the chamber. The nitrogen gas is separated from this by a special method and oxygen is obtained in the liquid state.

              All living beings need oxygen for breathing. Plants also ‘breathe’ oxygen. They absorb oxygen during the night. During the day they give off oxygen by photosynthesis.

              By burning oxygen gas along with the acetylene gas, a high temperature flame is produced. This flame is used for welding and cutting metals. Liquid oxygen is also used as a fuel in space vehicles.

Continue reading “How is oxygen useful to us?”

                 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.

Continue reading “Why do hot things burn us?”

                 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.