Category Science & Technology

How do we measure the hardness of materials?

            Hardness is a characteristic property of the solid objects. It is measured by the resistance which the body offers to anything which tends to scratch it. The hardness of the various materials is measured either on the ‘Mohs’ scale or the ‘Knoop’ scale.

            The Mohs’ scale, first devised in 1822 by Friedrich Mohs, measures resistance to indentation as judged by the material that will scratch another. Mohs’ scale is numbered from 1 to 10, that is, it gives ten grades of hardness. In this scale diamond is the hardest material and talc is the softest. Diamond has a hardness of 10 Mohs and talc has a hardness of 1 Mohs.

            Mohs’ scale, which assigns numbers to natural minerals, has been widely accepted and is used by mineralogists. This test, however, is not quantitative. For example, the hardness of sapphire is 9 on the Mohs’ scale; it does not mean that sapphire is 10% softer than diamond.

            The mineralogists carry a box containing pieces of the above minerals for testing samples in the field. For example, if they find a mineral that can be scratched by feldspar but not by appetite, its hardness lies between 5 and 6 on the hardness scale.

            To measure hardness in the Knoop scale, an elongated diamond-shaped indenting device is employed to measure the indentation it makes in a given test material. By this method, the hardness of extremely brittle materials including glass and even diamond can be measured without damaging either the indenter or the test piece. The size of the indentation is taken as a measure of the material’s hardness.

 

What are the different abrasives?

          An abrasive is a substance used for grinding, cutting, scroping or polishing the materials. There are two types of abrasives: natural and artificial abrasives. Natural abrasives include quartz, sandstone, pumice, diamond and corundum; artificial abrasives include rouge, whiting and carborundum.

          Abrasives are available mainly in two forms: paper and grinding wheels. The abrasive paper is made by coating ordinary paper with glue and adding the abrasive material to it. The sandpaper, emery paper, and carborundum paper are made in this way. To make a grinding wheel, abrasive material such as quartz is mixed with clay and water. This mixture is then pressed into the desired size and shape and fired in a furnace. The heat inside the furnance makes a strong bond among the materials put inside the furnance. 

          The fineness or coarseness of the particles used in an abrasive material is described in terms of its ‘grit number’. The abrasive materials with a grit number of 60 are much finer than those with a grit number of 30.

The hardness of an abrasive is also an important factor. It is measured on the Mohs’ scale. The Mohs’ scale ranges from 1 to 10. An abrasive is chosen according to the material to be ground. It should be harder than the material that is to be polished.

          The most widely used abrasives are fused aluminium oxide and silicon carbide. The aluminium oxide is known as alumina. It is used to grind and polish metals like steel, wrought iron and hard bronze. The silicon carbide is known as carborundum. It is made by fusing sand and coke in an electric furnace. Carborundum is used to grind and polish brass, copper, aluminium, stone, glass and ceramics.

          Many varieties of quartz are also important abrasives. Pumice, a volcanic rock, when ground to a fine powder, can be used in scouring powder and soaps. Crystalline iron oxide is used to polish jewellery and glass. It is known as rouge because of its red colour.

          The synthetic diamonds, diamond powders and diamond pastes are also used as abrasives. They are used to make drill bits and cutting wheels. Tungsten carbide is used in the machine tool industry for drilling, cutting and polishing metals. Boron carbide is another important abrasive. It is valuable because it is almost as hard as diamond. It is also used in nuclear reactor as a moderator and also as an abrasive. 

 

What is a clinometer?

            A clinometer is an instrument used by surveyors and civil engineers to measure the angle of inclination of slopes and hills by referring to a plumb bob or spirit level. It is also used to measure the height of an object if the distance from the observer to the object is known. A clinometer is also called Abney level.

            It consists of a sighting tube surmounted on a graduated vertical arc with an attached spirit level. A 45° mirror inside the tube enables the observer to see the bubbles at the same time the observer sights a point or a graduated rod with a horizontal wire. The surveyor first makes sure that the bubble is in the middle of the spirit level tube. This indicates that the clinometer is being held in a horizontal position. He or she looks through the eye hole and adjust the vertical arc to coincide with the sighted point. The vertical angle is indicated on the arc.

            In the ‘plumb-bob’ clinometer, a plumb bob or a weight attached to a piece of string hangs from the sighting device. As the device is tilted upward to view the top of the hill, the string moves across a protractor like face and indicates the angle of inclination.

            The clinometer is mainly used to measure the angles of slope. If set on a slope it can be used to give the angle of inclination of the surface. With the arc set at 0°, it can be used at a hand level.

 

What is electroplating?

               Electroplating is a process of metal coating through electrolysis. Electrolysis is passing of an electric current through an electrolyte solution. In other words, it is the process to cover a metal with a thin coating of another metal either for protection against corrosion or for beautification of house hold items. The electroplating may also be used to impart certain other properties to a metal surface, such as hardness, wear resistance and anti-frictional, electrical, magnetic or optional properties. Do you know how metals are electroplated?

               Electroplating is done in large vats containing a solution of some suitable salt of the metal to be coated. Bars or plates of metal are used as anode, and are arranged inside the vats. This metal body, called the work piece, makes the cathode. When the electric current is passed through the solution, by connecting the positive terminal of the battery to the anode and negative terminal to the cathode, the metal ions from the solution go towards the cathode and get deposited on the work piece and form a thin layer of metal on it. The metal from the anode goes on dissolving in the solution and finally gets deposited on the work piece.

               To ensure an even deposit, the work piece may be slowly rotated inside the vat. The surface to the work piece must be clean and free from grease, dirt or oxide films. These days the metals that are electroplated include silver, gold, nickel, copper and chromium. For silver plating, double cyanides of potassium and silver are used. The silver plating is usually done on brass table-wares such as spoons, forks and other utensils. It is also done on ornaments. The gold baths also contain double cyanides of gold and potassium. This plating is also done on ornaments. The nickel plating baths involve double sulphates of nickel and ammonium. The copper bath contains a solution of copper sulphate with small quantities of sulphuric acid. The chromium plating is done by using the solutions of chromic acid and chromic sulphate with small quantities of chromium carbonate usually used on machine parts.

               The other metals which are electroplated commercially include cadmium, cobalt, platinum, rhodium, tin, zinc, etc. In certain cases two or more metals are plated simultaneously as alloy coatings, e.g. copper-zinc, copper-tin, lead-tin, lead-tin-copper, tin-nickel and nickel-cobalt.

How do satellites stay up in space?

            A satellite is a body that moves in orbit around a larger body. The moon is a natural satellite of Earth because it orbits around the Earth. All the planets, except Mercury and Venus, have natural satellites.

            Today we also have artificial satellites. These ‘artificial’ satellites are man-made and launched into the space by powerful rockets. They orbit around Earth performing certain specified tasks.

            A satellite orbiting the Earth is like a chest-nut being whirled on the end of a piece of string. The centrifugal force drives it outwards, but Earth’s gravity keeps it from moving away. These two forces balance it and the satellite continues to orbit around the earth. It moves without any resistance since there is no air in the space. It will keep on orbiting forever, unless the upper atmosphere of Earth drags on the satellite and slows it down. The satellites move in elliptical orbits and not in circles. The nearest point to Earth is called the perigee and the farthest, the apogee. Manned space craft’s are temporary satellites during a space mission, but most artificial satellites are unmanned. The geostationary satellites positioned at a height of about 36,000 km. have the advantage that they have a stable position in the space in respect to any point on Earth.

            Hundreds of artificial satellites have been launched since Sputnik I which was the first satellite to be launched by the Soviet Union on October 4, 1957. Artificial satellites have become an integral part of our day to day life. They serve many useful purposes in different fields — communication, weather forecasting, geological survey, oceanography, astronomical experiments and observations etc. They also help in navigation and air traffic control. The satellites can be either unipurpose or multipurpose, depending on their service in either one specific area or more than one area. The Indian National Satellite (INSAT) series is a multipurpose one used for domestic telecommunications, meteorological observations, radio and TV broadcasting etc. 

How is sulphuric acid manufactured?

          Sulphuric acid is called the king of acids because of its importance as an industrial chemical. It is used in the manufacture of fertilizers, dyes, drugs, explosives, paints, synthetic fibres and detergents. It is also used in the manufacture of other acids such as hydrochloric acid and nitric acid. Different metals are pickled in sulphuric acid to clean them. It is also used in refining sugar and petroleum and to produce a vast range of chemicals. Do you know how this acid is manufactured?

          There are two methods used to manufacture sulphuric acid. One is known as Lead Chamber Process which dates back to about 200 years. The other is known as Contact Process. The former is less efficient and complex than the latter; still it is of considerable commercial importance. In Lead Chamber Process, first sulphur dioxide is obtained by burning sulphur or roasting pyrites. Then the sulphur dioxide thus obtained is oxidized by oxides of nitrogen to get sulphur trioxide which reacts with steam to produce sulphuric acid.

          Sulphuric acid is commercially manufactured by contact process. In this method the sulphur dioxide gas is mixed with air and heated with a catalyst. The catalyst is either the metal platinum or a compound called vanadium pentaoxide. The catalyst helps to quicken the reaction. The sulphur dioxide combines with the oxygen in the air to form sulphur trioxide. When sulphur trioxide is dissolved in water, it forms sulphuric acid.

          Pure sulphuric acid is a heavy, oily, colourless liquid. It is very reactive and attacks most of the metals to form salts called sulphates. It quickly absorbs water and is often used as a drying agent.

          While handling sulphuric acid, one should add sulphuric acid to water and not vice versa. If water is added to sulphuric acid, the heat produced causes water to boil. This makes the hot acid spit dangerously.