What makes a cola foam up when you put a pretzel in it?

Posted by Keshav Jain on

The cola foaming up when a pretzel is put in it is a physical interaction, not a chemical reaction. It works with other carbonated beverages too. Carbonated beverages when opened and released from pressure or supersaturated solutions of gas with more carbon dioxide dissolved in the beverage than would be possible at normal pressures.

 Left alone for two hours, the drink would slowly lose the gas and go flat. Sodium chloride particles seem to provide an especially good surface for gas to collect on, form bubbles and quickly rise to the surface and escape.


Why does boiled water lose taste?

Posted by Keshav Jain on



 



 



 



 



 



 



Naturally occurring water contains quite a few substances dissolved in it. For example, it contains gases such as oxygen and carbon dioxide, sulphates and carbonates of calcium and magnesium, and other elements such as iron depending the nature of the soil. These constitutes impart not only taste but also hardness to the water. Due to boiling, the dissolved gases are released and hardness is removed. Insoluble carbonates and hydroxides are formed which get deposited on the surfaces and the bottom of the vessel as scales. Their separation from water deprives it of its taste.


Does cold water extinguish fire faster than hot water?

Posted by Keshav Jain on



 



       



 



 



 



            Cold water extinguishes fire faster. It’s easy availability, non-toxicity, high specific heat, low    boiling point, high latent heat of evaporation, high volume expansion, water to steam, makes it suitable for fire extinguishing.



        In case hot water is used, the heat absorbed from the source (fire), by a definite quantity, to reach its boiling point will be far lesser vis-a-vis the same quantity of cold water. However, the other parameters will remain the same. Colder the water, faster would be the extinction of the fire.


Why are soap bubbles and oil layers coloured?

Posted by Keshav Jain on





 Coloured bands on soap bubbles and oil layers are caused by interference of light waves with themselves. Sunlight is a composite of seven colours (violet, indigo, blue, green, yellow, orange and red, in that order) each of which lies in a specific wavelength range. For example, violet light has a wavelength of 380 nanometers (one nanometer is a billionth of a metre) and red, above 600 nm.



Light waves falling on any thin film are partially reflected from the top and bottom surfaces of the film. A wave reflected from the bottom surface has to travel a longer distance than the one reflected from the top. This difference in distance travelled by the waves, called path difference, leads to a difference in the phases of the light waves. (The phase describes the electric and magnetic fields associated with light waves.) This phase difference depends on the thickness of the film at the point of reflection and angle of viewing. If two waves are ‘out of phase’, then they cancel each other (destructive interference) and if they are ‘in phase’ they add up (constructive interference.) Thus the regions of the spectrum (colours) which interfere constructively can be seen whereas those which interfere destructively are lost. Even if there is a slight variation in the thickness of the film, it will be coloured differently. 

How does carbon change into diamond?

Posted by Keshav Jain on

 Carbon changes into diamond and graphite in the depths of the earth under high pressure and temperature. The process might have taken millions of years. A French scientist, Moissan, verified this theory by experiment. He heated carbon (as charcoal) and iron together in an electric furnace at 3,500 degrees Celsius. Carbon dissolved in the molten iron and the mass was cooled suddenly by dipping in water. Then it was put in acid. The iron dissolved and the residue was found to contain a few very small diamonds (less than one millimeter in size) and some graphite. These results were confirmed by others too. Today diamonds are mass-produced by this method.

Artificial diamonds have the same structure, density and hardness as natural diamonds, but are very small. They are also opaque to x-rays. Hence they are used mostly for making hard-edged cutting tools and dies and for cutting and polishing natural diamonds, and not for making ornaments.


What does 43 and 53 grade cement mean?

Posted by Keshav Jain on

Grade denotes the minimum strength obtainable in a cement sample.  This is determined after making a mould of it, tom, under specified conditions on the 28th day of its moulding. The strength is expressed in megapascals (MPa).

The moulds in the shape of cubes are made by mixing cement with equal proportions of three grades of sand – coarse, medium and fine. This is cast in cubes of side about 7.07 cm so that a surface area of 50 square centimeters is obtained. Cubes cast thus are removed after 24 hours and cured in water for 28 days. Then they are subject to compression tests.



 The maximum stress in Newton/square mm which they can withstand is measured and given as their compressive strength.



If this lies between 33 and 43 N/square mm, it is termed as 33 grade cement. Similarly, if it lies between 43 and 53 N/square mm, it is 43 grade cement and if it is more than 53 N/square mm, it is 53 grade cement.  To avoid any discrepancies, always only batches of three cubes are subject to compression tests and their average is taken. Generally 53 grade cement is used for making high strength concretes.