NCERT 10th Class Science Chapter Chemical Reaction And Equation


    We observe many chemical changes taking place in our daily life. The milk turns sour if kept for a long time at room temperature, milk changes to curd, rusting of iron, digestion of food in our body are examples of chemical changes.
    In such changes, the nature and the properties of the substances change and we say a  chemical reaction has taken place.
    A chemical reaction is represented by a chemical equation which is a convenient way of  describing a chemical reaction with the help of symbols of elements and formulae of chemical compounds.
    In this chapter, we shall discuss about chemical formulae, chemical equations, balancing  of chemical equations and types of chemical reactions.
²    Physical change :– A change in which the physical properties of the substance changes but the chemical composition does not change. The substance is restored to its original state as soon as the cause of change is withdrawn.
²    Chemical change :– In a chemical change, at least one of the reacting substance changes into a new substances with a different composition. The new substances can not be changed back to the original substance even if the cause of change is withdrawn.

Difference between physical & chemical change


Chemical reaction

    The processes, in which a substance or substances undergo a chemical change to produce new substance         or substances, with entire new properties, are known as chemical reaction.
    The nature and identity of products totally changes from the reactants. 

Some important characteristics of chemical reactions are:
(i)    Change in state: The physical state of the substances normally changes.
    e.g    (a) Formation of solid MgO from solid Mg and gaseous O2.
        (b) Formation of solid Pbl(ppt) from liquid solutions of PbNO3 and Kl.
        (c) Formation of H2 gas from the reaction of solid Zn with liquid H2SO4.

(ii)    Change in colour : In some of the chemical reactions change in colour can be observed.
    e.g.    (a) Formation of brown rust on black iron nails.
        (b) Formation of yellow ppt. of lead iodide from colourless solution of PbNO3 and Kl.

(iii)    Evolution on a gas: In some cases, a gas may be evolved. 
    e.g.     (a) Evolution of H2 gas, in the reaction between Zn and dil HCL 
        (b) Evolution of CO2 gas, during burning of any fuel, which contains carbon.

(iv)    Change in temperature: Most of the reactions are accompanied by temperature change. i.e. 
increase or decrease in temperature. 
    e.g.     (a) In the reaction between Zn and H2S
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