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Class Seven Science Text Book Chapter 6

Chapter 6: Physical and Chemical Change

Activity

To show that the reaction of limestone with water is an exothermic reaction.

Materials required: Limestone, a 400 ml beaker and water.
Method: Take 100 ml of water in the beaker and add a few small pieces of limestone in it.

Observation: It is observed that when limestone is added to water, it reacts with water and water becomes hot and after a few seconds it starts boiling.

Conclusion: After the reaction, in the form of product, we obtain Calcium Hydroxide or slaked lime and CO2 with evolving of heat energy.
Result: The heat is evolved during reaction. So, it is an exothermic reaction.
(ii) Endothermic Reaction: The reactions in which heat is absorbed by the reactants to form a product are termed as endothermic reactions. For example, The process of photosynthesis is an endothermic reaction.
In this reaction, sunlight is required as energy.

3. Change in Colour: In a chemical reaction in which colour changes after reaction as a product. When iron nails react with copper sulphate that is blue in colour, after the reaction the blue colour will change into green and the colour of iron nail from black to reddish-brown.

4. Evolution of gas: Evolution of gas is one of the indicators of the chemical reaction taking place when sodium reacts with water. The reaction is very vigorous and liberates hydrogen gas and forms sodium hydroxide.

Types of Chemical Reaction


Certain patterns are observed in chemical reactions. Chemical reactions are grouped according to these patterns. This makes the study of reactions easier. There are many types of chemical reactions.

1.Combination Reactions

A + B————-> AB
(a) Reactions in which two or more elements combine :
(i) Nitrogen reacts with hydrogen to form ammonia.

(ii) Magnesium reacts with oxygen on heating to form magnesium oxide.

Activity

To prepare magnesium oxide by burning magnesium ribbon in the air.
Materials required: Magnesium ribbon, bunsen burner, tongs.
Method: Take a fresh magnesium ribbon and burn it in the presence of open air over the burner. It burns with a dazzling flame to form
magnesium oxide as a white powder and turns red litmus paper blue.

Formation of Magnesium oxide by burning
Magnesium ribbon

Observation: It indicates that magnesium oxide is basic in nature.
Result: The experiment shows, it is an example of a combination reaction.

2.Decomposition reaction:

A decomposition reaction is one in which a compound breaks up into two or more simple substances. Decomposition reactions are generally caused by heat, light, electricity and so on.

(1) When Calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.

(2) Silver Iodide decomposes in the presence of light to form silver.

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