Category Chemistry

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 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 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.

 

How do deep-sea divers operate?

           Since ancient times, man’s curiosity has led him to explore the dark, mysterious world of the deep seas. Diving has therefore developed to be an important sport over the years. But how do men stay under water for long periods of time?

          The first practical diving apparatus was devised by a German scientist, named Augustus Siebe in 1819. It comprised a metal helmet with a shoulder plate attached to a waterproof leather jacket. A tube running from the helmet was attached to an air pump. This was the first of many major experiments he carried out in trying to perfect a safe method of staying and working under-water. In 1830 he designed and developed a complete suit and helmet with air valves. Although many improvements have since been made, Siebe’s principles remain in universal use. 

          Deep sea divers, such as those who search shipwrecks for treasure, are divided into groups. They are skin divers who wear rubber suits that fit tightly like the skin, and divers known as ‘hard hats’ who wear heavy diving dress.

          A deep sea diver should use seven essentials: (a) An air pump for pushing air downwards to him. (b) A helmet, usually of steel, with glass windows to see. (c) A flexible waterproof suit fitting closely at wrists and ankles. (d) A length of air tubing that must be flexible, but must not collapse under the pressure of water. (e) A pair of heavy boots to keep the feet on the bottom. (f) Lead weights, hooked to chest and back, to prevent floating up to the surface. (g) A life-line to communicate with the surface by a system of jerks. One jerk may mean danger, and so on!

          Some divers also have a telephone so that they can talk to the ship. The wires for these telephones are built into the lifelines.

          Water pressure is a big problem for deep sea divers. The deeper a diver goes, more becomes the pressure of water around him. So the air pumped down must enable him to breathe properly and also balance the water pressure outside.

          In the past, deep sea divers used to breathe ordinary air, which contained nitrogen.

          This was very dangerous because when the pressure was very high, nitrogen would dissolve in the blood. When the diver surfaced, the pressure quickly returned to normal, which caused the nitrogen to bubble out of the blood. This led to a very painful illness which could even kill the diver, called as ‘Bends’ or ‘Caisson disease’. To avoid this, divers now breathe a mixture of oxygen and helium. Helium does not dissolve in the blood, so it is safer to use. But breathing helium makes divers speak with a high, squeaky voice because sound travels three times as fast as it does in air!

          In recent years, diving has not only become a popular sport, but is also useful in performing important jobs. Divers are needed for the construction and repair of bridges. They study plant and animal life beneath the surface of water. They aid in finding drowned people, and they also help in the search for buried treasure! 

 

When was the Sahara desert covered by Ice?

          Sahara is the world’s largest desert covering an area of 9 million sq km. in northern Africa. It extends from the coast of Atlantic Ocean in the west to the Red sea and Iraq. It includes parts of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Sudan and Tunisia. One third of the desert is covered by sand dunes and the rest consists of rocky uplands and stony plains. Crude oil and natural gas have been discovered beneath the Sahara and now being extracted. But there was a time when this great desert was covered by ice. Do you know when?

          The first clue of ice was discovered when geologists found evidence of glaciations in the bedrock of the Algerian desert. The approximate time of the ice covering was calculated to be about 450 million years ago. The location of the desert at that time, as research studies have found out was near the South Pole. The size, shape and position of the continents or landmasses of the earth have been constantly changing over the years. This happens due to the movement of plates in the earth’s crust. When these giant plates move they carry the continents along with them. As per the available evidences, 200 million years ago there was a supercontinent called Pangaea. It was formed when separate continental plates drifted together but later Pangaea also broke apart. But geologists are not sure about the continental locations before the formation of Pangaea. But rock studies provide some clue to the then location of Sahara. They suggest that Sahara was situated near the South Pole which eventually leads us to believe that it was covered by ice during that period of history. This period, according to geological classifications, is called the Ordovician period when North Africa was at South Pole ice-cap and the equator ran diagonally across today’s North America.