class x chapter -1 CHEMICAL REACTIONS AND EQUATIONS part -1​ notes

Introduction of Chemical Reaction 

In our daily life, we encounter many changes in our surroundings, though we do not notice them, such as souring of milk, rusting of iron, stretching of a rubber band, bread becoming toast, melting of wax, liting of a match, etc. All these changes that take place  around us are physical change or chemical change. 

Changes undergo in nature : 

• There are two types of changes undergo in our surroundings. These are as follows:~ 

     ( I )  Physical Changes 

    ( II ) Chemical Changes 

I )  Physical changes 

• Physical changes are the changes that change the physical Properties(traits as shape, size, colour, state etc.)  of the substance, without making any change in their internal structure. 
•   In other word, Physical change refers to a change in which the  molecules are rearranged but their internal composition remains same. 

Characteristics :  

• Change in physical properties of the substance, i.e. shape, size, colour etc. 
• No new chemical substances are formed. 
• Reversible 
• Example -Tearing of paper, melting/freezing of water, cutting of trees, etc. 

(II) CHEMICAL CHANGE 

• Chemical Change is a process in which the substance transforms into a new substance, having different chemical composition or chemical formula. 

     Characteristics:  

• Change in chemical properties and composition of the substance. 
• New product is formed. 
• Irreversible 
• Example -Burning of wood/trees/paper, rusting of iron, setting of curd, etc. 

Chemical reaction: 

• It Is the process of The formation of new chemical substance is the result of interaction of two or more chemical substance. 
• The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc. 
• A chemical reaction is a process in which one or more substances, also called reactants, are converted to one or more different substances, known as products. Substances are either chemical elements or compounds. 
• Characteristics of Chemical reactions 

• The conversion of reactants into products in a chemical reaction is often accompanied by some features which can be easily observed easily. 
• The important characteristics of chemical reactions are: 

• Evolution of a gas 
• Formation of a precipitate 
• Change in colour 
• Change in temperature 
• Change in state 

Evolution of a Gas: 

• Some chemical reactions are characterized by the evolution of a gas. 
  The chemical reaction between zinc and dilute sulphuric acid is characterized by the evolution of hydrogen gas. 
  Chemical reaction :  

     H2SO4      +Zn          ---         ZnSo4     +   H2 

• Products : zinc-sulphate and hydrogen gas 
  Identification: beaker becomes hot and hydrogen gas is produced which makes match stick burn with pale blue flame and popping sound. 

Formation of a precipitate 

• Some chemical reactions are characterized by the formation of precipitate. A precipitate is a ‘solid product’ which separates out from the solution during a chemical reaction. 
  The chemical rexn between potassium iodide and lead nitrate is characterized by the formation of a yellow precipitate of lead iodide. 

chemical reaction :  

                 BaI2      +   PbCl2    →     BaCl2+   PbI2  (yellow ppt) 

Products : barium chloride white color and yellow color precipitate of lead iodide. 

Change in colour: 

Some chemical reactions are characterized by a change in color. 

The chemical reaction between citric acid and purple colored potassium permanganate solution is characterized by a change in color from purple to colorless. 

Change in temperature: 

Some chemical reactions are characterized by a change in temperature. 

The chemical reaction between quicklime and water to form slaked lime is characterized by a change in temperature. 

Change in state 

• Some chemical reactions are characterized by a change in state. 
• When wax is burned (in the form of wax candle), then water and carbon dioxide are formed. 
• Now, wax is a liquid whereas carbon dioxide is a gas. This means that during the combustion reaction of wax, the physical state changes from solid to liquid and gas. 

writing chemical equation 

• need to know the symbols for the elements  
• Writing reactants on Left Side of the Equation 
• Writing Products on Right Side of the Equation 
• Direction of Reaction: Arrow 

                  CH4        +     O2                      →     CO2       +      H2O    

Chemical equations: 

Short –hand method of representing a chemical reaction      OR 

The method of representing a chemical reaction with the help of symbols and formula of the substances involved in it is known as a chemical equation. 

There are two ways of representing a chemical reaction as follows : 

I). In term of words (called word equation) 

• Example- Zinc metal reacts with dilute Sulphuric acid to form zinc sulphate and hydrogen gas. 

         Zinc        +       Sulphuric acid sulphate   →        Zinc sulphate   + hydrogen gas 

II). In term of symbol and formulae (called chemical equation)- 

 Putting the symbols and formula of all the substances in the above word equation, we get the following chemical equation:                                 Zn + H2SO4            →        ZnSO4 + H2                        

• The substances which combine or react are known as reactants. 
• The new substances produced in a reaction are known as products. 

Balanced and unbalanced chemical equations: 

A balanced chemical equation has the number of atoms of each element equal on both sides. 

 OR  A balanced chemical equation occurs when the number of the atoms involved in the reactants side is equal to the number of atoms in the products side. . 

• Example: Zn + H2SO4 → ZnSO4 + H2
• Unbalanced chemical equation - 

      If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in the product, then the chemical equation is called Unbalanced Chemical Equation. 

Example: Fe + H2O → Fe3O4 + H2