2023-24 Class 10 Science Chapter 1 Chemical Reactions and Equations Notes PDF

Class 10 Science Chapter 1 Chemical Reactions and Equations Notes PDF in English free

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Class 10 Science Chapter 1 Chemical Reactions and Equations in Notes in English medium

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Class 10 Science Notes : Chapter 1 Chemical Reactions and Equations

Introduction 

• Everything around us changes with time For ex. a small plant that grows and changes into tree after a certain period of time.

• These changes are of two types: Physical change and chemical change.

• Chemical Reaction: It is a process in which one or more substance transformed into new substance then such process is called chemical reaction.

• Or in simple way, we can also say that it is rearrangement of atoms.
Ex.  A + B → C + D
Ex.  Burning  of Magnesium ribbon in air.

Experiment : (1) We take a dry and clean piece of Mg ribbon and rub it with sand paper so that layer of MgO can be removed.
(2) Now, hold it with pair of tongs and burn it in lamp or flame. Keeping a china dish below it.
Observation:
(i) Mg burns with a dazzling light.
(ii) white coloured powder MgO is Collected in China dish.

• Identification of Chemical Reaction:

(Characteristics)

1. Colour- Whenever colour of substance get changed in a reaction it indicates that reaction is chemical reaction.

For ex. When iron needles are dipped into blue coloured CuSO₄ (copper sulphate) solution its (needle) colour changes from blue to
brown.

2. Change in temperature: Whenever temp. is increased or decreased the reaction is called chemical reaction.

For ex. When dilute HCL (hydrochloric acid) is added to Zinc granules in flask then the flask get heat up which indicates temperature is increased due to release of heat.
Zn + 2HCl →  ZnCl₂ + H₂

3. Evolution of gas: When gas is released or formed in a reaction then it is called a chemical reaction.

For ex. When dilute HCl (Hydrochloric acid) is added to Zinc granules in flask; a gas called H₂ gas is released.
Zn + 2HCL → ZnCl₂ + H₂

4. Formation of precipitate: When insoluble substance (precipitate) is formed in a chemical reaction then it is also a type of chemical reaction.

For ex. When lead nitrate [Pb(NO₃)₂] solution is mixed with potassium iodide solution then yellow coloured insoluble substance lead iodide (PbI₂) is formed.
Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

5. Change in state: When physical state of substance get changed and new product is formed then such reaction is referred as chemical reaction.

For ex. When Mg ribbon is burn in air it converts into MgO powder.
Mg + O₂ → MgO

Part- 02 Chemical Equation

Chemical Equation

♦ Chemical Equation
The easiest way to represent a chemical reaction is called chemical equation.

♦ Component of chemical equation 

• Reactant
• Product
• Direction of reaction

♦ Reactant : Those substances which react in reaction are called reactants.
In simple way; these are raw materials of a chemical reaction.
♦ Products : Those substances which are formed in a chemical reaction are called products.
♦ Arrow (→) represents direction of reaction.
Example :

In above example Mg and O₂ both are reactants and MgO is referred as products.
Note : Generally reactants are written in L.H.S (Left hand side) and products are written in R.H.S.( Right hand side)
♦ Chemical equation is of two types:
• Word equation
• Symbol equation

♦ Word equation : A word equation links together the names of the reactant with those of products.
For example : When magnesium ribbon burns in oxygen to form a white powder of magnesium oxide may be written as follows:

♦ Symbol equation : A symbol equation is brief representation of chemical reaction in terms of symbols and formula.
For example : Hydrogen gas react with oxygen gas to form water.
2H2+O2→2H2O
♦ Balanced chemical equation : The chemical equation in which no. of atoms are same in both L.H.S. and R.H.S. is known as balanced chemical equation.
For example : 2Mg+O2⟶2MgO
♦ Important Point-:
• A balanced chemical equation must obey the law of conservation of Mass. This means that the total mass of reactant and the products participating in an equation must be the same.

♦ Unbalanced chemical equation : The chemical equation in which no. of atoms are unequal in L.H.S. and R.H.S. then such equation is called unbalanced chemical equation.

For example : H2+O2⟶H2O
In this reaction on L.H.S. no. of oxygen atoms are 2 while in R.H.S. it is 1.

Balancing of Chemical Equation

• Balancing of chemical equation : Number of each element should be equal on both sides of equation.
Hence equations are balanced by Hit and Trial method.

• Hit and Trial method : There are following steps are there in hit and trial method.
a. Write the skeletal equation for the reaction using symbols and chemical formula and count the number of atoms of each element on both side of the equation.
b. Select the biggest formula and balance the elements of this compound by hit and trial and then balance other elements by one.

c. Check the correctness of atoms of each element on both  sides of  the equation.
d. Note that formula of compound should not be changed.
Let we balance the following example :
For example1 :Mg+HCl→MgCl2+H2

Step-1 : Let us count the number of atoms of all the elements in the reactant and products on both side of equation.

Step-2 : Now, select ‘H’ to balance it.

Now, new equation :
Mg+2HCl→MgCl2+H2

Step-3 : Check remaining elements

Hence L.H.S.= R.H.S.
Equation is balanced.
Mg+2HCl→MgCl2+H2
For example 2 : To balance this equation
H2+Cl2→HCl

• Hit and trial method

Step-1: To count no. of atoms of elements in both L.H.S. and  R.H.S.

Step-2 : Now balance ‘H’

Hence, rewrite new equation
H2+Cl2→2HCl

Step-3 : Check remaining atoms

Hence it is balanced now
H2+Cl2→2HCl
For example 3 : To balance this equation
Al+O2→Al2O3

• Hit and trial method

Step-1 : To count no. of atoms of elements in both L.H.S. and  R.H.S.

Step-2: Now balance ‘O’

Hence, rewrite new equation
Al+3O2→2Al2O3

Step-3 : Check remaining atoms

Hence, rewrite new equation
4Al+3O2→2Al2O3
Hence it is balanced equation now.

Writing symbol of physical state:

• To denote physical state of compounds some symbols are used.

Example: Solid: (S)
Liquid: (l)
Gas: (g)
Aqueous Solution : (aq.) 
Gas evolved:  ( ↑)
Precipitate formed: ( ↓)  (PPT)
Heat:  

• Types of chemical reaction

1. Reversible reaction: Those reaction which occur in both direction which means reactant can be converted into product and vice versa.

• “⇌“ this symbol is used for direction.
Example: H2(g)+I2(g)⇌2HI(g)
N2(g)+3H2(g)⇌2NH3(g)                             

2. Irreversible reaction: The reaction which only occur forward direction means reactant completely converts into product. Such reaction is called irreversible reaction.
Example: 2Na(s)+2H2O(I)→2NaOH(aq.)+H2(g)
Zn(s)+H2SO4(dil.)→2ZnSO4(aq.)+H2(g)  

3. Exothermic reaction: The reaction in which heat is released is called exothermic reaction.
Example:
1. CaO+H2O→Ca(OH)2+ heat 
2. C6H12O6+6O2→6CO2+6H2O+ energy    
Above mentioned equation is the equation of respiration. Since process of respiration is an exothermic reaction
because the inhaled o₂ is used by our cells for the breakdown of glucose and other organic substrate and also release energy.
Burning of  natural gas:
CH4(g)+2O2(g)→CO2(g)+2H2O(g)
Decomposition of vegetable matter into compost.

4. Endothermic reaction: The chemical reaction in which heat is absorbed or given is called endothermic reaction.
Example 

Combination reaction: The chemical reaction in which two reactants combine to form a single product is called combination reaction.

Example: 
1. CaO(s)+H2O(I)⟶Ca(OH)2(aq)
_{(Quick Lime)                      (Slaked Lime)}

2. Burning of Coal:
�(�)+�2(�)⟶��2(�)

3. Formation of ammonia:
N2(g)+3H2(g)⟶2NH3(g)

4. Formation of water: 
2H2(g)+O2(g)⟶2H2O(g)

Note: Quick lime (CaO) react with H₂O to Form Ca(OH)₂ which is called lime water.
Solution of CaO is used for white washing.

Decomposition reaction: The reaction in which a single reactant splits or breakdown into two or
more than two substance is called decomposition reaction.

• All decomposition reactions requires energy.
• It is of three types

1. Thermal decomposition
2. Electrical decomposition
3. Photo decomposition

♦ Thermal decomposition: When decomposition is carried out by heating then such decomposition is called thermal decomposition.
Example:
1.
 

2. 

3.

♦ Electrical decomposition: When decomposition is carried by electric current then such decomposition is called electrical decomposition.
Example: 

• Experiment :
• 1. Take a plastic mug. Drill two holes at its base and fit rubber in these holes. Insert carbon electrodes in these rubber stoppers as shown in Figure.
• 2. Connect these electrodes to a 6 volt battery.
• 3. Fill the mug with water such that the electrodes are immersed. Add a few drops of dilute sulphuric acid to the water.
• 4. Take two test tubes filled with water and invert them over the two carbon electrodes.
• 5. Switch on the current and leave the apparatus undisturbed for some time.
• 6. You will observe the formation of bubbles at both the electrode. These bubbles displace water in the test tubes.
• 7. Once the test tubes are filled with the respective gases, remove them carefully.
• 8. Test these gases one by one by bringing a burning candle close to the mouth of the test tubes.

Observation : The level of water in both the test tubes is different.

Result : The electrical decomposition of water results in the formation of H₂ and O₂ gas.
Conclusion : 2H2O→2H2+O2 In this reaction ratio of hydrogen to oxygen is 2:1. Hence amount of gas evolved at anode is twice than the gas that is evolved at Cathode.

♦Photo decomposition: When decomposition is carried out by sunlight then such decomposition is called photo decomposition.

Example:
1. 2����(�)⟶ Sunlight 2��(�)+��2(�)
2. 2AgBr(s)⟶ Sunlight 2Ag(s)+Br2(g)

Above reactions are used in black and white photography.

♦ Displacement reaction- The reaction in which more reactive element takes the place of less reactive element ; is called displacement reaction.
Example:  

• Generally all displacement reactions are exothermic in nature.
• Displacement reaction can be easily understood with the help of reactivity series.

♦ Double Displacement reaction: The chemical reaction in which two reactant exchange their ions to form two new compounds is called double displacement reaction.
Example: ����3(��)+����(��)→����(��)+����3(��)

♦ Precipitation reaction: When aqueous solution of ionic compounds react by exchanging their ions to form two new compounds is insoluble salt then such reaction is called precipitation reaction.
Example:

♦ Oxidation- It may be defined as the addition of oxygen to a substance or removal of hydrogen from a substance in a chemical reaction.

For Ex. When brown coloured copper powder is heated in a China dish in presence of air then it becomes coated with black copper oxide because oxygen is added to copper hence oxidation takes place.

♦ Reduction- It may be defined as the addition of hydrogen to a substance or removal of oxygen from a substance in a chemical reaction.

For Ex. If hydrogen gas is passed over to copper oxide powder it get converted into Cu which means that oxygen is removed from CuO.

♦ Redox Reaction- A chemical reaction in which both oxidation and reduction take place simultaneously such reaction is known as redox reaction.

For Ex.
 

♦ Oxidising agent- The substance which either gives oxygen or removes hydrogen in an oxidation reaction, is known as oxidising agent.

♦ Reducing agent- The substance which either gives ‘H’ or removes oxygen in a reduction reaction, is known as reducing agent.

♦ Corrosion – The chemical process of slow eating up of the surfaces of certain metals when kept in open for a long time.

For Ex. In case of iron, corrosion is called rusting. Rust is a soft, flaky (non-sticky) brown substance which is formed by the chemical action of moist air (containing CO₂ and H₂O) on iron. It is basically an oxidation reaction and formula of rust is Fe₂O₃.xH₂O. It is very slow in nature and once started keeps on occurring. Thus, corrosion of iron is continuous process which ultimately destroys or eats up the whole iron object.

Methods used to prevent Rusting –

• The surface is coated with paint or some chemicals, e.g., biphenol.
• The surface is protected by applying a thin film of oil or grease.
• The metal is electroplated with metals like tin, nickel, zinc, chromium, aluminium, etc.

• ♦ Rancidity- Oxidation Rancidity also has damaging effects on food and eatables. When food containing fat and oil is left as such for a long time, it becomes stale. The stale food often develops bad taste and smell. This is very common in case of curd or cheese particularly in summer. Actually, the oils and fats are slowly oxidised to certain bad smelling compounds. These are of volatile nature and release foul smell. This is known as rancidity.

Rancidity may be defined as : ”The slow oxidation of oils and fats present in food materials in some bad smelling compounds.”

Following measures can be adopted to prevent or slow down rancidity :

• Food materials are often packed in air tight containers. Oxygen has no access to them and thus, oxidation resulting in rancidity is prevented. However, these days, preference is given to vacuum packing.
• Refrigeration of food also slows down rancidity because the temperature inside refrigerator is very low and direct contact with air or oxygen is avoided.
• In bags containing potato chips and other similar stuff, the air is quite often replaced by nitrogen This checks their oxidation as well as rancidity.

Questions and Answers

PAGE. NO. 6

Q.1 Why should magnesium ribbon be cleaned before burning in air?
Ans. Magnesium ribbon is cleaned before burning so that coating of impurity (such as oxide) formed on its surface is removed and it becomes pure magnesium.

Q.2 Write the balanced equations for the followings chemical reactions.
a. Hydrogen + Chloride   hydrogen chloride
b. Barium chloride + aluminium sulphate barium sulphate + aluminium chloride
c. Sodium + Water   sodium hydroxide + hydrogen
Ans. (a) H_2(g) + Cl_2(g)  →  2HCl_(g)
(b) 3BaCl_2(aq.)­­ + Al₂­(SO­₄)_3(aq.) → 3BaSO_4(s) + 2AlCl_3(aq.)
(c) 2Na_(s) + 2H₂O  →  2NaOH_(aq.) + H_2(g)

Q.3 Write a balanced chemical equation with state symbols for the following reactions:
a. Solutions of Barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

b. Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Ans. (a) BaCl­_2(aq.) + Na₂SO_4(aq.)  BaSO_4(s)  +  2NaCl_(aq.)
(b) NaOH_(aq.) + HCl_(aq.) NaCl_(aq.) + H₂O_(l) 

PAGE. NO. 10

Q.1 A solution of the substance X is used for white washing.
i. Name the substance X and write its formula.
ii. Write the reaction of the substance X with water.
Ans. (i) The substance X is calcium oxide (also called quick lime). Its formula is  CaO. 
(ii) CaO_(s) + H₂O_(l)  →  Ca(OH)_2(aq.)
      (X)                           Calcium hydroxide

Q.2 Why is double the amount of gas collected in one of the test tubes in electrolysis of water than the amount collected in the other? Name the gas.
Ans. On electrolysis, water decomposes according to the reaction,

Thus, water decomposes to give hydrogen and oxygen in the ratio 2:1 by volume. Hence, the volume of hydrogen collected will be twice the volume of oxygen collected.

PAGE. NO. 13

Q.1 Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Ans. The reaction between copper (II) sulphate and iron is represented as
CuSO­_4(aq.) + Fe_(s)  →  FeSO­_4(aq.) + Cu_(s)
^{Blue                                     Light green        Brown}
In this displacement reaction, a more reactive element iron displace another less reactive element from its compound copper sulphate. The brown copper metal gets deposited on the iron nails. The colour of the solution changes from blue to light green due to the formation of ferrous sulphate (having Fe­­­­^{2+ ion}).

Q.2 Give an example of a double displacement reaction.
Ans. In the reaction of silver nitrate and sodium chloride, silver exchanges its nitrate ions with chloride ions of sodium chloride and forms a precipitate of silver chloride. The chemical reaction is
AgNO­_3(aq.) + NaCl_(aq.)  →  AgCl_(s) + NaNO_3(aq.)

Q.3 Identify the substance that are oxidized and the substance that are reduced in the following reactions:
i. 4Na_(s) + O_2(g) → 2Na₂O_(s)
ii. CuO_(s) + H­_2(g) → Cu_(s) + H₂O_(l)
Ans.
(i) 
Na is oxidised and O₂ is reduced.

(ii) 

H₂ is oxidised and CuO is reduced.

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