Category Science

             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. 

              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?”