com www.GOALias.blogspot.com www.GOALias ... - fun learning

www.GOALias.blogspot.comExplosion of a firework is a chemicalchange. You know that such anexplosion produces heat, light, soundand unpleasant gases that pollute theatmosphere. That is why you are advisednot to play with fireworks.When food gets spoiled, it producesa foul smell. Shall we call this change achemical change?You must have noticed that a slice ofan apple acquires a brown colour if it isnot consumed immediately. If you havenot seen this change in colour, cut afresh slice of apple and keep it away forsome time. Repeat the same activity witha slice of potato or brinjal. The changeof colour in these cases is due to theformation of new substances. Are notthese changes chemical changes?In Chapter 5, you neutralised an acidwith a base. Is neutralisation a chemicalchange?A protective shieldYou must have heard of the ozone layerin our atmosphere. It protects us fromthe harmful ultraviolet radiation whichcome from the sun. Ozone absorbs thisradiation and breaks down to oxygen.Oxygen is different from ozone. Canwe call the breaking down of ozone achemical change?If ultraviolet radiation were notabsorbed by ozone, it would reach theearth’s surface and cause harm to usand other life forms. Ozone acts as anatural shield against this radiation.We learnt in Chapter 1 thatplants produce their food bya process called photosynthesis.Can we call photosynthesisa chemical change?Paheli said that even digestionis a chemical change.6.3 R6.3 RUSTINGOF IRONLet us get back to rusting. This is onechange that affects iron articles andslowly destroys them. Since iron is usedin making bridges, ships, cars, truckbodies and many other articles, themonetary loss due to rusting is huge.The process of rusting can berepresented by the following equation:Iron (Fe) + Oxygen (O 2, from the air)+ water (H 2O) → rust (iron oxide Fe 2O 3)For rusting, the presence of bothoxygen and water (or water vapour)is essential.In fact, if the content of moisture inair is high, which means if it is morehumid, rusting becomes faster.So, how do we prevent rusting?Prevent iron articles from coming incontact with oxygen, or water, or both.One simple way is to apply a coat ofpaint or grease. In fact, these coatsshould be applied regularly to preventrusting. Another way is to deposit a layerof a metal like chromium or zinc on iron.PHYSICALAND CHEMICALCHANGES6363