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

www.GOALias.blogspot.comCarbondioxideVinegar +Baking sodaLimewaterFig. 6.5 Set up to pass gas through lime waterCarbon dioxide (CO 2) + Lime water[Ca(OH) 2] → Calcium Carbonate(CaCO 3) + Water (H 2O)When carbon dioxide is passedthrough lime water, calcium carbonateis formed, which makes lime water milky.The turning of lime water into milky is astandard test of carbon dioxide. You willuse it in Chapter 10 to show that the airwe breathe out is rich in carbon dioxide.In Activities 6.6–6.8, you saw thatin each change one or more newsubstances were formed. In Activity 6.6,the ash was the new substance formedwhen magnesium was burnt. In Activity 6.7,the reaction of copper sulphate with ironproduced iron sulphate and copper.Both of these are new substances.Copper was deposited on the shavingblade of iron. In Activity 6.8, vinegar andbaking soda together produced carbondioxide, which turned lime water milky.Can you name the new substanceformed in this reaction?A change in which one or morenew substances are formed is called achemical change. A chemical changeis also called a chemical reaction.Chemical changes are very importantin our lives. All new substances areformed as a result of chemical changes.For example, if a metal is to be extractedfrom an ore, such as iron from iron ore,we need to carry out a series of chemicalchanges. A medicine is the end productof a chain of chemical reactions. Usefulnew materials, such as plastics anddetergents, are produced by chemicalreactions. Indeed, every new materialis discovered by studying chemicalchanges.We have seen that one or more newsubstances are produce in a chemicalchange. In addition to new products,the following may accompany a chemicalchange:• Heat, light or any other radiation(ultraviolet, for example) may be givenoff or absorbed.• Sound may be produced.• A change in smell may take place ora new smell may be given off.• A colour change may take place .• A gas may be formed.Let us look at some examples.You saw that burning of magnesiumribbon is a chemical change. Burningof coal, wood or leaves is also a chemicalchange. In fact, burning of anysubstance is a chemical change.Burning is always accompanied byproduction of heat.62SCIENCE