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SAI INTERNATIONAL SCHOOL
                                                      CLASS-VII
                                             SUBJECT- CHEMISTRY
                              TOPIC-PHYSICAL AND CHEMICAL CHANGES
                                      SUB TOPIC- CHEMICAL CHANGE


                                                  LESSON NOTE

               Chemical changes:-


               When two or more substance react in such a way that there is formation of one or
               more new substances, the change is called a chemical change.When a substance
               undergoes a chemical change, the chemical properties of the substance changes
               and it is transferred into a new substances it different chemical composition.

                Chemical changes are generally irreversible in nature and it is indicated by evolution
               or absorption of heat, evolution of gas, formation of precipitate etc.

               Reactant:A substance that takes part in a reaction and undergoes a change is
               known as reactant.

               Product:A substance that is formed during a reaction is called product.


               Baking a cake

               When we prepare a cake we add sodium bicarbonate or baking soda (NaHCO3) to
               the ingredients.  Baking soda decomposed to carbon dioxide and makes the cake
               fluffy and soft.  This is a chemical change as a new substance is formed and its
               property totally different from its ingredients. The change is irreversible.

               Lighting a firework

               When light a firework, a chemical reaction takes place. After the reaction carbon
               dioxide gas is evolved and ash remains as residue whose properties are totally
               different from the reactant. The reaction is irreversible in nature.


               Burning of a candle

               In burning of candle, wax is melted which is a physical change because wax melts
               due to heat can be solidified  by cooling. So no new substances formed. But in case
               of burning carbon dioxide and water vapour is formed. Their properties are different
               from candle. The change is irreversible.

               Cooking of food

               Cooking of food is an example of chemical change. During cooking raw grains and
               vegetables are converted to tasty food. Cooked food cannot be changed back to raw
               ingredients and its properties also different from the uncooked food.



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