NCERT NOTES Class 10 Science Chapter 1 Chemical Reaction And Equations

 

                      Science Chapter 1

                   Chapter 1

 Chemical Reactions and Equations

Chemical change: - Is a change in which one or more new substances are formed In a chemical change.

  • New substances are formed. 
  • Energy changes are involved.
  • There is a change in mass during the reaction.
  •  Permanent change takes place.

 Examples

  • Cooking of food.
  •  Rusting of iron .
  • Heating of Lead nitrate. 
  • Souring of milk.
  •  Ripening of fruit.

 Rusting of iron Is a chemical change because

  •  A new substance iron oxide is formed.
  •  The change is permanent; the article has got a rust layer (which may only peal oft). 
  • There is an increase in mess when rust forms.
  •  An energy change has taken place (which may not be visible). 

Chemical changes are also known as chemical reactions.

 Exothermic Reaction A chemical reaction which is accompanied by evolution of heat energy is know as exothermic reaction.

 Examples:

 The amount of heat (energy produced is written along with the products This indicates that heat is given out.

Endothermic Reactions A chemical reaction which is accompanied by absorption of hear energy is known as exothermic reaction.

 Examples 

 The amount of heat (energy) produced is written along with the reactants This indicates that hear is absorbed.

Chemical Formulae- The chemical formula of a substance is the symbolic representation of the actual number of atoms present In one molecule of that substance 

(a)   Formula of one molecule of water is H0. It shows the one molecule of water s made up of 2 atoms           of hydrogen and one atom of oxygen

 (b) Formula of one molecule of sulphuric acid is H:SO It shows that one molecule of sulphuric acid is           made up of 2 atoms of hydrogen, 1 Mom of Sulphur and 4 atoms of oxygen .

Chemical Equations-Representation of a chemical change in terms of symbols and formulae of the reactants and products is known as chemical equation of the reaction.

Steps for writing Chemical Equation

Step I: 

Writing skeletal Equation A word-equation shows change of reactants to products through an arrow placed between them (----)The arrow may be read as "to yield" or to form or to give" and shows the direction of the reaction .

The reactants are placed on the let hand side (LHS) of the arrow and the products on the right hand side (RHS) The different reactants as well as products are connected by a plus sign () A complete chemical equation represents the reactants, products ard their physical states symbolically and is a balanced account of a chemical transaction.

Step II:

Balancing of Chemical Equation

 An equation in which number of atoms of each element is equal on both the sides of the equal ion is known as balanced chemical equation. 

A chemical equation is balanced so that the numbers of atoms of each type involved in a chemical reaction are the same on the reactant and product sides of the equation.

Equations must always be balanced Whenever HO s present on any side, the number of hydrogen on both the sides should be an even number (2 moms of hydrogen in water) If there are 4H;O, then 4 x 2= 8 hydrogen atoms If there are 7H:0, then 7 x 2=14 hydrogen atoms Le., all are even numbers.

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