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1.Chemical ReactionsThis is the fourth text which will be used for practicing dictation andtranslation in the written part of the exam.The key words are emphasized in boldand there is a list of them at the end of the presentation.
2.Chemical Reactions• When a chemical reaction occurs, new substances (called products)form from the substances taking part in the reaction (called reactants).• Burning is an exothermic reaction – more heat is given out duringthe reaction than is taken in.• Oxidation occurs when a substance combines with oxygen.• Reduction occurs when a substance loses oxygen.
3.Chemical Reactions• Reversible chemical reactions can go forwards and backwards. Whenheated, nitrogen dioxide (NO2) breaks down into nitrogen monoxide(NO) and oxygen (O2), when cooled the change is reversed.• Some reactions take in more heat energy than they give out. Thesereactions are called endothermic.• Electron transfer – During the oxidation process, atoms lose electronsand are oxidized. During reduction, atoms gain electrons and arereduced.
4.Chemical Reactions• Each element has a chemical symbol to identify it, and eachcompound a chemical formula. The formula indicates how theelements in the compound are combined. A chemical equation showswhich substances react and the products that result.• An atom’s valency shows the number of chemical bonds it can form.It is the number of electrons the atom gains, loses or shares when itmakes bonds.
5.Chemical Reactions• Controlling reactions – Chemists speed up reactions by making thereacting particles collide with each other more often or with greaterenergy. Substances called catalysts speed up reactions by helpingsubstances react together. They remain unchanged by the chemicalreaction.• Increasing the concentration of a reactant speeds up a reaction.Dyeing a material is faster with a concentrated dye – there are moredye molecules to collide with the material.
6.Chemical Reactions• The surface area of a solid object is the size of its outer surface.Increasing the surface area of a reacting substance speeds up thechemical reaction.• Yeast is a fungus containing enzymes, which are biological catalysts.The enzymes in yeast make starches and sugars break down morerapidly into carbon dioxide and ethanol.
7.Key words:• Product – produkt, proizvod – Reactant - reaktant• Burning – sagorevanje• Reversible/reversed – reverzibilna, povratna• Oxidation/reduction – oksidacija/redukcija• Chemical symbol – hemijski simbol• Chemical formula – hemijska formula• Chemical equation – hemijska jednacina• Chemical bonds – hemijske veze
8.Key words• Catalyst – katalizator• Catalyzation - katalizacija• Dye – boja• Dyeing – bojenje• Surface area – ukupna spoljna povrsina• Yeast – kvasac• Starch – skrob• To break down – razgraditi