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Chapter 1 Chemical Reactions and Equations

Chemical Reactions and Equations 

1.  Chemical reactions- The transformation of chemical substance into a new chemical substance by making and breaking of bonds between different atoms is known as Chemical Reaction.
2.  Signs of a chemical reaction- These factors denote that a chemical reaction has taken place- change of state of substance, change of color of substance, evolution of heat, absorption of heat, evolution of gas and evolution of light.
3.  Chemical Equation: The representation of chemical reaction by means of symbols of substances in the form of formulae is called chemical equation. E.g. - H2 + O2 ⇒ H2O
4.  Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both left and right sides of the reaction.
      *Note- According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.
5.  Types of Chemical Reactions-
                      I.  Combination- When two elements or one element and one compound or two compounds combines to give one single product.
                     II.  Decomposition- Splitting of a compound into two or more simple products.
                    III.  Displacement- It takes place when a more reactive metal displaces a less reactive metal.
                    IV.  Double displacement- Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.
                     V.  Precipitation- The insoluble compound called precipitate forms in this reaction.
                    VI.  Exothermic- Reactions which produce energy are called exothermic reaction. Most of the decomposition reactions are exothermic.
                   VII.  Endothermic- Reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic.
                  VIII.  Oxidation: Gain of oxygen or removal of hydrogen or metallic element from a compound is known as oxidation.
                     IX.  Reduction: Addition of hydrogen or removal of oxygen from a compound is called reduction.
                       X.  Redox- A chemical reactions where oxidation and reduction both take place simultaneously are also known as redox reaction.
                             Eg - NaOH + HCl ⇒ NaCl + H2O
6.  Rusting- When iron reacts with oxygen and moisture forms a red substance called rust.
7.  Rancidity- Oils and fats when get oxidized on exposure to air show a change in taste and smell.
8.  Corrosion- Metals when attacked by oxygen, water, acids, gases, present in air changes its surface which is called corrosion.

















Question 1
Why should a magnesium ribbon be cleaned before burning in air ?
Answer:
Magnesium gets covered with a layer of magnesium oxide when kept in air for a long time. This layer hinders the burning of magnesium. Hence, it is to be cleaned before burning.
Question 2
Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer:
(i) H2 + Cl2 → 2HCl
(ii) 3 BaCl2 + Al2(SO4)3 → BaSO4 + 2 AlCl3
(iii) 2Na + 2H2O → 2NaOH + H2
Question 3
Write a balanced chemical equation with state symbols for the following reactions :
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answer:
(i) BaCl2 (aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl(aq) → NaCl(aq) + H2O(l


Question 1
A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Answer:
(i) The substance whose solution in water is used for white washing is calcium oxide (or quick lime). Its formula is CaO.
NCERT Solutions for Class 10 Science Chapter 1 intext questions P10 q1
Question 2
Why is the amount of gas collected in one of the test tubes in text book Activity 1.7 (i.e., electrolysis of water) double of the amount collected in the other? Name this gas. [CBSE 2015 (Delhi)]
Answer:
In Activity 1.7, water is electrolysed to give H2 gas at one electrode and O2 gas at the other electrode.
2H2O(l) → 2H2(g) + O2(g)
Thus two molecules of water on electrolysis give two molecules of hydrogen gas and one molecule of oxygen gas or in other words the amount of hydrogen gas collected would be double than that of oxygen gas.
Question 1
Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
OR
An iron nail is dipped in the solution of copper sulphate for about 30 minutes. State the change in colour observed. Give reason for the change. [CBSE 2015 (Delhi)]
Answer:
When an iron nail is dipped in copper sulphate solution, the displacement reaction takes place. The colour of copper sulphate solution fades due to the formation of light green solution of iron sulphate.
NCERT Solutions for Class 10 Science Chapter 1 intext questions P13 q1
Question 2
Give an example of a double displacement reaction other than the one given in Activity 1.10 (NCERT Text Book).
Answer:
Sodium hydroxide and hydrochloric acid react to form sodium chloride and water.
NCERT Solutions for Class 10 Science Chapter 1 intext questions P13 q2
Question 3
Identify the substances that are oxidised and the substances which are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO (s) + H2(g) → Cu (s) + H2O(l)
Answer:
(i) Substances oxidised is Na as it gains oxygen and oxygen is reduced.
(ii) Substances reduced is Cu as hydrogen is oxidised as it gains oxygen.



Question 1.
Why should magnesium ribbon be cleaned before burning in air?
Answer.
Magnesium is a reactive metal. When it is exposed to atmosphere for longer time, the oxygen present in the atmosphere combines and MgO is formed on the metal surface. This oxide layer does not burn in the flame so magnesium ribbon has to be cleaned with sandpaper before burning in air.
Question 2.
Write the balanced equations for the following chemical reactions:
(a) Hydrogen + Chlorine ➝ Hydrogen chloride
(b) Barium chloride + Aluminium sulphate ➝ Barium sulphate + Aluminium chloride
(c) Sodium + Water ➝ Sodium hydroxide + Hydrogen
Answer.
The balanced equations are written as:
(a) H2 + Cl2 ➝ 2HCl
(b) 3BaCl2 + Al2(SO4)3 ➝ 3BaSO4 + 2AlCl3
(c) 2Na + 2H2O ➝ 2NaOH + H2

Question 3.
Write the balanced equations 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 and water.
Answer.
The symbol equations in balanced form for the reactions are:
(a) BaCl2(aq) + Na2SO4(aq) ➝ BaS04(s)+ 2NaCl(aq)
(b) NaOH(aq) + HCl(aq) ➝ NaCl(aq) + H2O(l)
Question 4.
A solution of the substance ‘X’ is used for white washing.
(a) Name the substance ‘X’ and write its formula.
(b) Write the reaction of the substance ‘X’ with water.
Answer.
(a) The substance ‘X’ is calcium oxide (also called quick lime). Its formula is CaO.
2018-10-26 17_19_58-Chap 1 4 b
Question 5.
Why is the amount of gas collected in one of the test tubes in Activity 7 double of the amount collected in the other? Name the gas.
Answer.
It is so because in water, the hydrogen is two times more than oxygen by mass and volume. The equation is expressed as:
2018-10-26 17_19_58-Chap 1 5
Here the two gases have volume in ratio of 2 : 1. The gas is hydrogen.
Question 6.
When you mix solutions of lead(II) nitrate and potassium iodide.
(a) What is the colour of the precipitate formed? Name the compound involved.
(b) Write a balanced chemical equation for the reaction.
(c) What type of a reaction is this?
Answer.
(a) The precipitate is yellow in colour. The compound is lead(II) iodide with chemical formula PbI2.
(b) Pb(NO3)2(aq) + 2KI(aq) ➝ Pbl2(s) + 2KNO3(aq)
(c) It is a double displacement reaction.
Question 7.
Why does the colour of copper sulphate change when an iron nail is dipped in it?
Answer.
Iron nail acquires a brown coating of copper as a result of displacement reaction. The solution becomes light green.
2018-10-26 17_19_58-Chap 1 7
Question 8.
Give one example of the double displacement reaction.
Answer.
2018-10-26 17_19_58-Chap 1 8 1
Question 9.
Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(a) 4Na(s) + O2(g) ➝ 2Na2O(s)
(b) CuO(s) + H2(g) ➝ Cu(s) + H2O(l)
Answer.
(a) In this reaction, sodium (Na) is oxidised to sodium oxide (Na2O). This means that oxygen (O2) gets reduced.
(b) In this reaction, hydrogen (H2) is oxidised to form water (H2O) while copper(II) oxide (CuO) is reduced to copper (Cu).
Question 10.
Magnesium ribbon burns with a dazzling flame in air (or oxygen) and changes to a white substance magnesium oxide. Is magnesium being oxidised or reduced in this reaction?
Answer.
The balanced chemical equation is:
2Mg(s) + O2(g) ➝ 2MgO(s)
Yes, magnesium (Mg) gets oxidised to form magnesium oxide (MgO) in this reaction.
is same as the total mass of products. It implies mass can neither be created nor be destroyed during a chemical reaction.
Chapter End Questions
Question 1.
Which of the following statements about the reaction given below are incorrect?
2PbO(s) + C(s) ➝ 2Pb(s) + CO2(g)
(i) Lead is getting reduced.
(ii) Carbqn dioxide is getting oxidised.
(iii) Carbon is getting oxidised.
(iv) Lead oxide is getting reduced.
(a) (i) and (ii)
(b) (i) and (iii)
(c) (i), (ii) and (iii)
(d) all are incorrect
Answer.
(a) (i) and (ii)
Question 2.
Fe2O3 + 2Al ➝ Al2O3 + 2Fe
The above reaction is an example of:
(a) combination reaction
(b) double displacement,reaction
(c) decomposition reaction
(d) displacement reaction
Answer.
(d) displacement reaction

Question 3.
What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer.
(a) Hydrogen gas and iron chloride are produced.
Question 4.
What is a balanced chemical equation? Why should chemical equations be balanced? [2013]
Answer.
A chemical equation is said to be balanced if:
(a) the atoms of different elements on both sides of the equations are equal.
(b) the equation is molecular, i.e. the gases if involved in the equation must be in the molecular form (e.g. H2, O2, N2, Cl2, etc.).
Necessity to balance chemical equations: The chemical equations have to be balanced to fulfil the requirement of law of conservation of mass. According to the law, the total mass of reactants
Question 5.
Transfer the following into chemical equations and balance them:
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer.
(a) The symbol equation for the reaction is:
H2 + N2 ➝ NH3
The balancing of equation is done in the following steps:
Step I: Let us count the number of atoms of all the elements of the reactants and the products on both sides of the equation.
ElementsNo. of atoms of reactants (LHS)No. of atoms of products (RHS)
H23
N21
Step II: In order to equate the number of H atoms on both sides, put coefficient 3 before H2 on the reactant side and coefficient 2 before NH3 on the product side.
3H2 + N2 ➝ 2NH3
Step III: On counting, the number of N atoms on both sides of the equation are same. This means that the equation is balanced.
Step IV: The balanced equation can be written in physical state as:
3H2(g) + N2(g) ➝ 2NH3(g)
(b) The symbolic equation for the reaction is:
H2S + O2 ➝ H2O + SO2
The balancing of equation is done in the following steps:
Step I: Let us count the number of atoms of all the elements on both sides of the equation.
ElementsNo. of atoms of reactants (LHS)No. of atoms of products (RHS)
H22
S11
O23
Step II: In order to equate the number of O atoms, put coefficient 3 before O2 on the reactant side and coefficient 2 before SO2 on the product side.
H2S + 3O2 ➝ H2O + 2SO2
Step III: 0 atoms are still not balanced. To achieve this, put coefficient 2 before H2O on the product side.
H2S + 3O2 ➝ 2H2O + 2S02
Step IV: To balance S atoms, put coefficient 2 before H2S on the reactant side.
2H2S + 3O2 ➝ 2H2O + 2SO2
Step V: On inspection, the number of atoms of all the elements on both sides of the equation are equal. Therefore, the equation is balanced.
Step VI: The balanced equation can be written in physical state as:
2H2S(g) + 3O2(g) ➝ 2H2O(g) + 2SO2(g)
(c) The symbolic equatipn for the reaction is:
BaCl2 + Al2(SO4)3 ➝ AlCl3 + BaSO4
The balancing of equation is done in the following steps:
Step I: Let us count the number of atoms of all the elements on both sides of the equation.
ElementsNo. of atoms of reactants (LHS)No. of atoms of products (RHS)
Ba11
Al21
Cl23
S31
O124
Step II: In order to equate the number of Al atoms, put coefficient 2 before AlCl3 on the product side.
BaCl2 + Al2(SO4)3 ➝ 2AlCl3 + BaSO4
Step III: In order to balance Cl atoms, put coefficient 3 before BaCl2 on the reactant side.
3BaCl2 + Al2(SO4)3 ➝ 2AlCl3 + BaSO4
Step IV: To balance Ba atoms, put coefficient 3 before BaSO4 on the product side.
3BaCl2 + Al2(SO4)3 ➝ 2AlCl3 +3BaSO4
Step V: On inspection, the number of S and 0 atoms on both sides of the equation are also found to be equal. Thus, the equation is in balanced form.
Step VI: The balanced equation can be written in physical state as:
3BaCl2(aq) + Al2(SO4)3(aq) ➝ 2AlCl3(aq) + 3BaSO4(s)
(d) The symbolic equation for the reaction is:
K + H2O ➝ KOH + H2
The balancing of equation is done in the following steps:
Step I: Let us count the number of atoms of all the elements on both sides.
ElementsNo. of atoms of reactants (LHS)No. of atoms of products (RHS)
K11
H23
O11
Step II: To balance the number of H atoms, put coefficient 2 before KOH on the product side and 2 before H2O on the reactant side.
K + 2H2O ➝ 2KOH + H2
Step III: To balance the number of K atoms in the above equation, put coefficient 2 before K atom on the reactant side.
2K + 2H2O ➝ 2KOH + H2
Step IV: On inspection, the number of atoms of all the elements are found to be equal on both sides of the equation. It is finally balanced.
Step V: The balanced equation can be written in physical state as:
2K(s) + 2H2O(l) ➝ 2KOH(aq) + H2(g)
Question 6.
Balance the following chemical equations:
(a) HNO3 + Ca(OH)2 ➝ Ca(NO3)2 + H2O
(b) NaOH + H2SO4 ➝ Na2SO4 + H2O
(c) NaCl + AgNO3 ➝ AgCl + NaNO3
(d) BaCl2 + H2SO4 ➝ BaSO4 + HCl
Answer.
(a) 2HNO3 + Ca(OH)2 ➝ Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 ➝ Na2SO4 + 2H2O
(c) The symbolic equation as given for the reaction is already balanced.
NaCl + AgNO3 ➝ AgCl + NaNO3
(d) BaCl2 + H2SO4 ➝ BaSO4 + 2HCl
Question 7.
Write the balanced equations for the following reactions:
(a) Calcium hydroxide + Carbon dioxide ➝ Calcium carbonate + Water
(b) Aluminium + Copper chloride ➝ Aluminium chloride + Copper
(c) Barium chloride + Potassium sulphate ➝ Barium sulphate + Potassium chloride
(d) Zinc + Silver nitrate ➝ Zinc nitrate + Silver
Answer.
All these equations are in word form whereas the balanced equations are written in symbolic form.
(a) Ca(OH)2 + CO2 ➝ CaCO3 + H2O
(b) 2Al + 3CuCl2 ➝ 2AlCl3 + 3Cu
(c) BaCl2 + K2SO4 ➝ BaSO4 + 2KCl
(d) Zn + 2AgNO3 ➝ Zn(N03)2 + 2Ag
Question 8.
Write the balanced chemical equations for the following reactions and identify the type of reaction:
(a) Potassium bromide(aq) + Barium iodide(aq) ➝ Potassium iodide(aq) + Barium bromide(aq)
(b) Zinc carbonate(s) ➝ Zinc oxide(s) + Carbon dioxide (g)
(c) Hydrogen(g) + Chlorine (g) ➝ Hydrogen chloride (g)
(d) Magnesium(s) + Hydrochloric acid(aq) ➝ Magnesium chloride(aq) + Hydrogen(g)
Answer.
(a) 2KBr(aq) + BaI2(aq) ➝ 2KI(aq) + BaBr2(aq)
The reaction is known as double dis-placement reaction.
(b) ZnCO3(s) ➝ ZnO(s) + CO2(g)
The reaction is known as decomposition reaction.
(c) H2(g) + Cl2(g) ➝ 2HCl (g)
The reaction is known as combination reaction.
(d) Mg(s) + 2HCl(aq) ➝ MgCl2(aq) + H2(g)
The reaction is known as displacement reaction.
Question 9.
What does one mean by exothermic and endothermic reactions? Give examples.
Answer.
A chemical reaction in which certain amount of heat energy is evolved and resulting contents get heated up is called exothermic reaction.
Examples
2H2(g) + O2(g) ➝ 2H2O(l) + heat
HCl(aq) + NaOH(aq) ➝ NaCl(aq) + H2O(l) + heat
A chemical reaction is said to be endothermic when certain amount of heat energy is absorbed.
Examples
N2(g) + 02(g) + heat ➝ 2NO
C(s) + H2O(g) + heat ➝ CO(g) + H2(g)
Question 10.
Why is respiration considered an exothermic reaction?
Answer.
Respiration is a biochemical reaction which releases energy during combustion of digested food (glucose) in the cell producing carbon dioxide and water.
C6H12O6(S) + 6O2(g) ➝ 6CO2(g) + 6H2O(l) + ATP
Question 11.
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer.
Decomposition reactions are called the opposite of combination reactions because of the following:
  • In decomposition reactions, larger molecules break to give simple molecules while in combination reactions, two or more smaller
  • elements dr compounds combine to form new compounds.
  • Decomposition reactions are endothermic while most of the combination reactions are exothermic. For example,
Combination reaction
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Decomposition reaction
CaCO3(s) ➝ CaO(s) + CO2(g)
2Pb(NO3)2(s) ➝ 2PbO(s) + 4NO2(g) + O2(g)
Question 12.
Write one equation each for decomposition reaction where energy is supplied in the form of heat, light and electricity.
Answer.
2018-10-26 17_19_58-Chap 1 12
Question 13.
What is the difference between displacement and double displacement reactions? Write equations for these reactions. [2011]
Answer.
In a displacement reaction, one element takes the place of another in a compound. For example,
Fe(s) + CuSO4(aq) ➝ FeSO4(aq) + Cu(s)
In a double displacement reaction, one com-ponent each of both the reactants get exchanged to form the products. For example,
HCl(aq) + NaOH(aq) ➝ NaCl(aq) + H2O(l)
Question 14.
In the refining of silver, the recovery of silver from silver nitrafe solution involved displacement by copper metal. Write down the reactions involved.
Answer.
The chemical equation for the displacement reaction is:
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Question 15.
What do you mean by precipitation reaction? Explain giving examples.
Answer.
In a precipitation reaction, one of the products formed during the reaction does not dissolve in solution and gets settled on the surface of the container (beaker or tube). It is known as a precipitate.
Examples
2018-10-26 17_19_58-Chap 1 15
Question 16.
Explain the following in terms of gain or loss of oxygen with two examples of each:
(a) oxidation
(b) reduction
Answer.
(a) Oxidation involves the gain of oxygen by a substance in a chemical reaction.
Examples
2018-10-26 17_19_58-Chap 1 16 1
(b) Reduction involves the loss of oxygen from a substance in a chemical reaction.
Examples
2018-10-26 17_19_58-Chap 1 16 2
Question 17.
A shining brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black-coloured compound formed. [2013]
Answer.
The element ‘X’ is copper and the black- coloured compound formed is copper(II) oxide.
2018-10-26 17_19_58-Chap 1 17 1
Question 18.
Why do you apply paint on iron articles?
Answer.
Iron is a reactive metal. In the presence of humid air (O2 + H2O), it rusts to form a layer of Fe2O3.xH2O.
To prevent it from rusting, paint is applied which forms a protective layer over iron and prevents direct contact of air and water.
Question 19.
Oil and fat containing food items are flushed with nitrogen. Why?
Answer.
Oil and fat containing food items or eatables get rancid due to oxidation by air or oxygen. In case the container or bag is flushed with nitrogen, then oxidation or rancidity will be checked.
Question 20.
Explain the following terms with one example of each.
(a) Corrosion
(b) Rusting
Answer.
(a) Corrosion Ijt is a chemical process of decay of metal when they are exposed to moisture.
(b) Rusting Corrosion of iron is called rusting. It is a slow oxidation reaction which takes place in the presence of moisture.
2018-10-26 17_19_58-Chap 1 20
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What is a chemical reaction Class 10?

Chemical Reaction: 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.
In a chemical reaction, a new substance is formed which is completely different in properties from the original substance, so in a chemical reaction, a chemical change takes place.
Only a rearrangement of atoms takes place in a chemical reaction.
  • The substances which take part in a chemical reaction are called reactants.
  • The new substances produced as a result of a chemical reaction are called products.
Example: The burning of magnesium in the air to form magnesium oxide is an example of a chemical reaction.
2Mg(s) + O2(g) \underrightarrow { \triangle } 2MgO(s)
Before burning in air, the magnesium ribbon is cleaned by rubbing with sandpaper.
This is done to remove the protective layer of basic magnesium carbonate from the surface of the magnesium ribbon.
Reactant: Substances which take part in a chemical reaction are called reactants.
Example: Mg and O2.
Product: New substance formed after a chemical reaction is called a product.
Example: MgO.
Characteristics of Chemical Reactions :
(i) Evolution of gas: The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ↑
(ii) Change in Colour: The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised by a change in colour from purple to colourless.
The chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution is characterized by a change in colour from orange to green.
(iii) Change in state of substance: The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (because the wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas, carbon dioxide produced by the combustion of wax is a gas). There are some chemical reactions which can show more than one characteristics.
(iv) Change in temperature: The chemical reaction between quick lime water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).
The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature (which is a rise in temperature).
(v) Formation of precipitate: The chemical reaction between sulphuric acid and barium chloride solution is characterised by the formation of a white precipitate of barium sulphate.
BaCl2(aq) + H2SO4(aq) → BaSO4(s) (ppt) + 2HCl(aq)

What is a chemical Equation Class 10?

Chemical Equation: Representation of chemical reaction using symbols and formulae of the substances is called Chemical Equation.
Example: A + B → C + D
In this equation, A and B are called reactants and C and D are called the products. The arrow shows the direction of the chemical reaction. Condition, if any, is written generally above the arrow.
When hydrogen reacts with oxygen, it gives water. This reaction can be represented by the following chemical equation:
Hydrogen + Oxygen → Water
H2 + O2 → H2O
In the first equation, words are used and in second, symbols of substances are used to write the chemical equation. For convenience, the symbol of substance is used to represent chemical equations.
A chemical equation is a way to represent the chemical reaction in a concise and informative way.
A chemical equation can be divided into two types: Balanced Chemical Equation and Unbalanced Chemical Equation.
(a) Balanced Chemical Equation: A balanced chemical equation has the number of atoms of each element equal on both sides.
Example: Zn + H2SO4 → ZnSO4 + H2
In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a Balanced Chemical Equation.
According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.
(b) 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
In this example, a number of atoms of elements are not equal on two sides of the reaction. For example; on the left-hand side only one iron atom is present, while three iron atoms are present on the right-hand side. Therefore, it is an unbalanced chemical equation.
Balancing a Chemical Equation: To balance the given or any chemical equation, follow these steps:
Fe + H2O → Fe3O4 + H2
Write the number of atoms of elements present in reactants and in products in a table as shown here.
Name of atomNo. of atoms in the reactantNo. of atoms in the product
Iron13
Hydrogen22
Oxygen14
Balance the atom which is maximum in number on either side of a chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen, one needs to multiply the oxygen on the LHS by 4, so that, the number of oxygen atoms becomes equal on both sides.
Fe + 4 × H2O → Fe3O4 + H2
Now, the number of hydrogen atoms becomes 8 on the LHS, which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 × H2O → Fe3O4 + 4 × H2
After that, the number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.
3 × Fe + 4 × H2O → Fe3O4 + 4 × H2
Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.
Name of atomNo. of atoms in the reactantNo. of atoms in the product
Iron33
Hydrogen88
Oxygen44
After balancing, the above equation can be written as follows:
3Fe + 4H2O → Fe3O4 + 4H2.
To Make Equations More Informative:
Writing the symbols of physical states of substances in a chemical equation:
By writing the physical states of substances, a chemical equation becomes more informative.
  • Gaseous state is represented by symbol (g).
  • Liquid state is represented by symbol (l).
  • Solid state is written by symbol (s).
  • Aqueous solution is written by symbol (aq).
  • Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of the physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.

What are the types of a chemical reaction Class 10?

Types of Chemical Reactions: Combination Reaction, Decomposition Reaction, Displacement Reaction, Double Displacement Reaction, Neutralization Reactions, Exothermic – Endothermic Reactions and Oxidation-Reduction Reactions.
Types of Chemical Reactions:
Chemical reactions can be classified in following types:
(i) Combination Reaction: Reactions in which two or more reactants combine to form one product are called Combination Reactions.
A general combination reaction can be represented by the chemical equation given here:
A + B → AB
Examples:
When magnesium is burnt in the air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.
Mg(s) + O2(g) → 2MgO(s)
Magnesium + Oxygen → Magnesium Oxide
When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.
C (s) + O2(g) → CO2(g)
Carbon + Oxygen → Carbon dioxide
(ii) Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or elements are known as Decomposition Reaction. A decomposition reaction is just the opposite of combination reaction.
A general decomposition reaction can be represented as follows :
AB → A + B
Examples:
When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.
CaCO3(s) \underrightarrow { heat } CaO(s) + CO2(g)
Calcium carbonate → Calcium oxide + Carbon dioxide
When ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3(s) \underrightarrow { \triangle } Fe2O3(s) + 3H2O(l)
Thermal Decomposition: The decomposition of a substance on heating is known as Thermal Decomposition.
Example: 2Pb(NO3)2(s) \underrightarrow { heat } 2PbO(s) + 4NO2(g) + O2(g)
Electrolytic Decomposition: Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as Electrolytic Decomposition. This is also known as Electrolysis.
Example: When electricity is passed in water, it decomposes into hydrogen and oxygen.
2H2O(l) \xrightarrow [ Electrolysis ]{ Electric\quad Current } 2H2(g) + O2(g)
Photolysis or Photo Decomposition Reaction: Reactions in which a compound decomposes because of sunlight are known as Photolysis or Photo Decomposition Reaction.
Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl(s) (white) \underrightarrow { Sunlight } 2Ag(s) (grey) + Cl2(g)
Photographic paper has a coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal.
(iii) Displacement Reaction: The chemical reactions in which a more reactive element displaces a less reactive element from a compound is known as Displacement Reactions. Displacement reactions are also known as Substitution Reaction or Single Displacement/ replacement reactions.
A general displacement reaction can be represented by using a chemical equation as follows :
A + BC → AC + B
Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taking place.
Examples:
When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
(iv) Double Displacement Reaction: Reactions in which ions are exchanged between two reactants forming new compounds are called Double Displacement Reactions.
AB + CD → AC + BD
Examples:
When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) (Precipitate) + 2NaCl(aq)
When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Note: Double Displacement Reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.
Precipitation Reaction: The reaction in which precipitate is formed by the mixing of the aqueous solution of two salts is called Precipitation Reaction.
Example:
Chemical Reactions and Equations Class 10 Notes Science Chapter 1 1
Neutralization Reaction: The reaction in which an acid reacts with a base to form salt and water by an exchange of ions is called Neutralization Reaction.
Example:
Chemical Reactions and Equations Class 10 Notes Science Chapter 1 2
(v) Oxidation and Reduction Reactions:
Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as Oxidation.
Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be Oxidized.
Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called Reduction.
The compound or element which goes under reduction in called to be Reduced.
Oxidation and Reduction take place together.
Oxidizing agent:
  • The substance which gives oxygen for oxidation is called an Oxidizing agent.
  • The substance which removes hydrogen is also called an Oxidizing agent.
Reducing agent:
  • The substance which gives hydrogen for reduction is called a Reducing agent.
  • The substance which removes oxygen is also called a Reducing agent.
The reaction in which oxidation and reduction both take place simultaneously is called Redox reaction.
When copper oxide is heated with hydrogen, then copper metal and hydrogen are formed.
CuO + H2 → Cu + H2O
(i) In this reaction, CuO is changing into Cu. Oxygen is being removed from copper oxide. Removal of oxygen from a substance is called Reduction, so copper oxide is being reduced to copper.
(ii) In this reaction, H2 is changing to H2O. Oxygen is being added to hydrogen. Addition of oxygen to a substance is called Oxidation, so hydrogen is being oxidised to water.
  • The substance which gets oxidised is the reducing agent.
  • The substance which gets reduced is the oxidizing agent.
(vi) Exothermic and Endothermic Reactions:
Exothermic Reaction: Reaction which produces energy is called Exothermic Reaction. Most of the decomposition reactions are exothermic.
Example:
Respiration is a decomposition reaction in which energy is released.
Chemical Reactions and Equations Class 10 Notes Science Chapter 1 3
When quick lime (CaO) is added to water, it releases energy.
Chemical Reactions and Equations Class 10 Notes Science Chapter 1 4
Endothermic Reaction: A chemical reaction in which heat energy is absorbed is called Endothermic Reaction.
Example: Decomposition of calcium carbonate.
Chemical Reactions and Equations Class 10 Notes Science Chapter 1 5
Effects of Oxidation Reactions in Everyday life: Corrosion and Rancidity.
Corrosion: The process of slow conversion of metals into their undesirable compounds due to their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called Corrosion.
Example: Rusting of iron.
Rusting: Iron when reacts with oxygen and moisture forms red substance which is called Rust.
Chemical Reactions and Equations Class 10 Notes Science Chapter 1 6
The rusting of iron is a redox reaction.
Corrosion (rusting) weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships etc. and cuts short their life.
Methods to Prevent Rusting
  • By painting.
  • By greasing and oiling.
  • By galvanisation.
Corrosion of Copper: Copper objects lose their lustre and shine after some time because the surface of these objects acquires a green coating of basic copper carbonate, CuCO3.Cu(OH)2 when exposed to air.
Chemical Reactions and Equations Class 10 Notes Science Chapter 1 7
Corrosion of Silver Metal: The surface of silver metal gets tarnished (becomes dull) on exposure to air, due to the formation of a coating of black silver sulphide(Ag2S) on its surface by the action of H2S gas present in the air.
Chemical Reactions and Equations Class 10 Notes Science Chapter 1 8
Rancidity: The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is called Rancidity. It is caused due to the oxidation of fat and oil present in food materials.
Methods to prevent rancidity:
  • By adding anti-oxidant.
  • Vacuum packing.
  • Replacing air by nitrogen.
  • Refrigeration of foodstuff.
1. Chemical Reaction: During chemical reactions, the chemical composition of substances changes or new substances are formed.
2. Chemical Equation: Chemical reactions can be written in chemical equation form which should always be balanced.
3. Types of Chemical Reactions:
Combination reaction: A single product is formed from two or more reactants.
2Mg + O2 → 2MgO
Decomposition reaction: A single reactant breaks down to yield two or more products.
  • Thermal decomposition: 2Pb(NO2)2 → 2PbO + 4NO2 + O2
  • Electrolysis: 2H20 → 2H2 + O2
  • Photochemical reaction: 2AgBr → 2Ag + Br2
Displacement reaction: One element is displaced by another element.
Zn + CuSO4 → ZnSO4 + Cu
Double displacement reaction: Exchange of ions between reactants.
AgNO3 + NaCl → AgCl + NaNO3
Redox reaction: Both oxidation and reduction take place simultaneously.
CuO + H2 → Cu + H2O
Exothermic reaction: A chemical reaction in which heat energy is evolved.
C + O2 → CO2 (g) + heat
Endothermic reaction: A chemical reaction in which heat energy is absorbed.
ZnCO3 + Heat → ZnO + CO2
Redox reaction: Chemical reaction in which both oxidation and reduction take place simultaneously.
4. Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen.
5. Reduction: Reaction that shows the loss of oxygen or gain of hydrogen.
ZnO + C → Zn + CO
ZnO is reduced to Zn—reduction. C is oxidized to CO—Oxidation.
6. Effects of Oxidation Reactions in Our Daily Life:
  • Corrosion: It is an undesirable change that occurs in metals when they are attacked by moisture, air, acids and bases.
    Example, Corrosion (rusting) of Iron: Fe2O3. nH2O (Hydrated iron oxide)
  • Rancidity: Undesirable change that takes place in oil containing food items due to the oxidation of fatty acids.
    Preventive methods of rancidity: Adding antioxidants to the food materials, storing food in the airtight container, flushing out air with nitrogen gas and refrigeration.

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