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Chemical Reaction And Equation
1.0Master Chemical Transformations in Minutes
Unlock the secrets of matter and discover how substances rearrange themselves to form entirely new products. Learn the difference between physical and chemical transformations, master the art of balancing chemical equations using the hit-and-trial method, and explore the major types of chemical reactions that drive our world—from the rust on an old iron nail to the metabolic energy powering your body.
2.0Learning Outcomes
After completing this lesson, you will be able to:
- Distinguish between physical changes and chemical reactions using experimental evidence.
- Write down word equations and translate them into skeletal chemical formulas.
- Apply the Law of Conservation of Mass to perfectly balance complex chemical equations.
- Classify chemical reactions into five core archetypes based on atomic behavior.
- Identify oxidizing and reducing agents in everyday Redox phenomena.
- Explain the chemical mechanisms behind corrosion and rancidity, along with practical prevention methods.
3.0Introduction to the Lesson
Every day, countless changes take place around us—iron rusts, milk turns into curd, food gets digested, and fuels burn to release energy. These changes are examples of chemical reactions, where substances transform into entirely new substances with different properties. Understanding chemical reactions and writing them in the form of balanced chemical equations helps us describe these changes scientifically and accurately. In this chapter, you'll explore how reactions occur, how equations are balanced, and the different types of chemical reactions that shape the world around us.
Change is the law of nature.
There are so many situations of daily life, where we can observe various changes. Like,
(i) Conversion of water into vapours from a cup of hot tea.
(ii)Corrosion of iron articles (rusting) if exposed to a humid atmosphere.
(iii) Cooking of food.
(iv) Digestion of food in our body.
(v) Breaking of any article like glass.
(vi) Combustion of fuel in our vehicle.
Scientists classify these changes as
(1) Physical changes (2) Chemical changes
4.0Physical Changes
A change in which physical properties of a substance changes but the chemical composition does not change.
For example, freezing, melting, boiling, condensation, etc.
Characteristic Features of Physical Changes
(1) The identity of the substance is maintained.
(2) The change is generally temporary.
(3) Heat change may or may not take place.
(4) Only the physical state or some of the physical properties of the substances are changed.
5.0Chemical Changes
A change in which one or more substances change into new substances with a different chemical composition.
For example, burning of a candle, rusting of iron, combustion of fuel, etc.
Characteristic Features of Chemical Changes
(1) The identity of original substance is completely lost.
(2) The change is generally permanent.
(3) The change is generally accompanied by energy change.
(4) The change cannot be reversed generally.
6.0Chemical Reaction And Its Characteristics
The process in which a substance or substances undergo a chemical change to produce new substances, with entirely new properties are known as chemical reactions.
Characteristics of Chemical Reactions
(i) Change in state: The physical state of the substances normally changes.
For example,
Formation of solid MgO from solid Mg and gaseous O2.
2Mg(s) + O2(g) = 2MgO(s)
Magnesium Oxygen Magnesium oxide
(from air) (White powder)
(ii) Change in colour: In some of the chemical reactions, change in colour can be observed.
For example,
(a) Formation of reddish-brown rust on grey iron nails.
(b) Formation of yellow ppt. of lead iodide from colourless solution of Pb(NO3)2 and KI.
(iii) Evolution of a gas: In some cases, a gas may be evolved.
For example,
Evolution of H2 gas, in the reaction between Zn and dil HCl.
Zn + 2HCl ⎯→ ZnCl2 + H2(g)↑
(iv) Change in temperature: Most of the reactions are accompanied by temperature change i.e. increase or decrease in temperature.
For example,
(a) In the reaction between Zn and H2SO4, the flask was found to be warm. Thus, a rise in temperature has taken place (Exothermic).
(b) If a reaction between barium hydroxide (Ba(OH)2) and ammonium chloride (NH4Cl) is carried out in a test tube, it is observed that the bottom of the test tube becomes cooler (Endothermic).
Memory map
7.0Writing A Chemical Equation
Reaction between magnesium and oxygen can be described as – when a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide. This description of a chemical reaction in sentence form is quite long. It can be written in a shorter form. So, some short-hand representation of chemical reaction is followed.
It can be done in two ways:
Word Equation
A chemical equation which represents a chemical reaction briefly in words is called the word equation.
For example, the word equation can be written as:
Magnesium + Oxygen ⎯→ Magnesium oxide
Chemical Equation
A chemical equation is a short-hand method that describes a chemical reaction in terms of symbols and formulae of different reactants and products.
For example, the chemical equation can be written as
Mg + O2 → MgO
Rules For Writing A Chemical Equation
(i) The symbols and formulae of the reactants are always written on the L.H.S. (left hand side) of the arrow and a plus (+) sign is put between them.
(ii) The symbols and formulae of the products are always written on the R.H.S. (right hand side) of the arrow and a plus (+) sign is put between them.
(iii) An arrow (⎯→) sign is put between the reactants and the products, pointing from reactants towards products.
For example,
Na + H2O → NaOH + H2
A chemical equation expressed in symbols and formulae, such that the number of atoms of different elements towards the side of the reactants is not equal to the number of atoms of different elements towards the side of the products, is called skeletal equation or unbalanced equation. To make this equation meaningful, this equation is balanced according to the law of conservation of mass, then it is called a balanced chemical equation.
8.0How to balance an unbalanced chemical equation
According to the law of conservation of mass, the total mass of products must be equal to the total mass of the reactants (as mass can neither be created nor destroyed). This is possible only if the number of atoms of each element is equal on the two sides of the equation.
Balancing a chemical equation means making the number of atoms of each element equal on both sides of the equation.
(i) To understand this, let us consider the following word equation for
Zinc + Sulphuric acid ⎯→ Zinc sulphate + Hydrogen
Chemical equation for the above word equation will be,
Zn + H2SO4⎯→ ZnSO4 + H2
Let us examine the number of atoms of different elements on both the sides of the arrow.
As the number of atoms of each element is the same on both sides of the arrow, the equation can be said to be a balanced chemical equation.
(ii) Now consider another chemical equation.
Fe + O2 + H2O ⎯→ Fe2O3.H2O
On counting the number of different atoms on both the sides of the arrow, we will find that this equation is not balanced.
Let us learn about balancing a chemical equation step by step.
Step-I : Write the word equation for the given chemical reaction.
Step-II : Convert the formed word equation in the chemical equation (Skeletal chemical equation).
Step-III : Formula of each reactant and product has to be enclosed in boxes, so that during balancing, formula of substances cannot be changed.
Step-IV : Listing of the number of reactants and products is done.
Step-V: Balancing should be started with compounds which have the biggest formula.
Step-VI : Then, balancing of different elements is done one by one.
Step-VII : Finally, the equation is checked.
(iii) Let us balance some of the chemical reactions by following the above steps.
Magnesium metal reacts with hydrochloric acid to form magnesium chloride and hydrogen.
Law of conservation of mass
Step-I : Word equation
Magnesium + Hydrochloric acid ⎯→ Magnesium chloride + Hydrogen
Step-II : Chemical equation,
Mg + HCl ⎯→ MgCl2 + H2
Step-III : Enclose all formulae into boxes.
Step-IV: Count the number of atoms for all elements.
We can see that the number of Mg is the same on both sides but Cl and H atoms differ on both sides.
Step-V: As the number of atoms is deficient at the reactant side, let’s begin from here. On the reactant side HCl is the bigger formula, so we will start with it.
Step-VI: Put coefficient 2 before HCl to make chlorine equal to reactant side.
Mg + 2HCl → MgCl2 + H2
Here, we can see that H, automatically gets balanced.
Step-VII : Now, check the number of atoms of different elements on both sides of the equation. These are equal. This means that the equation is balanced.
9.0Balancing of Equations
Steam is passed over red hot iron to form Iron (II, III) oxide and hydrogen in the presence of air.
Step-I: Iron + Steam ⎯→ Iron (II, III) Oxide + Hydrogen
Step-II : Fe + H2O ⎯→ Fe3O4 + H2
Step-III:
Step-IV : Formula selected is Fe3O4 to start balancing oxygen,
(i) To balance O-atoms, multiply H2O in LHS by 4.
(ii) Now balance Fe atoms.
(iii) Balance H atoms.
Note: Mixture of ferrous oxide (FeO) and ferric oxide (Fe2O3) is also known as magnetic oxide of iron.
Step-V : On checking the number of all elements, we found that the equation is balanced now.
3Fe + 4H2O ⎯⎯→ Fe3O4 + 4H2
10.0Making Chemical Equations More Informative
On examining a balanced chemical equation, we observe that it does not give any information about the physical state of the reactant and product.
Let’s make balanced chemical equations more informative by following instructions.
(1) Writing symbols for the physical state of reactants and the products,
(s) for solid state
() for liquid state
(g) for gaseous state
(aq) for aqueous solution i.e. solution prepared in water.
(2) Sometimes a gas evolved in a reaction is shown by the symbol (↑) i.e. by an arrow pointing upwards.
Similarly, the precipitate if formed during the reaction is indicated by the symbol (↓) i.e. by an arrow pointing downwards.
The abbreviation ‘ppt’ is also used to represent the precipitate if formed.
(3) The conditions of temperature, pressure and the presence of catalyst, if any, may be represented by writing these conditions above and / or below the arrow (⎯→) between the reactants and the products.
Few examples,
CO(g) +2H2 (g)340atm→CH3OH (g)
6CO2(g) + 6H2O(g)⎯→Chlorophyll Sunlight C6CH12O6(aq) + 6O2(g)
2Na(s) + 2H2 O(g) ⎯→ 2NaOH(aq) +H2 (g) or H2
Ca(OH)2(aq) + CO2(g) ⎯→ CaCO3(↓) (or ppt) +H2 O(g)
C(s) + O2(g) ⎯→ CO2(g) + heat
N2(g) + O2(g) + heat ⎯→ 2NO(g)
11.0Important topics in Class 10 Science chemical Reactions and Equations
12.0EUREKA by ALLEN – Your Smart Companion for Class 10 Success
Designed for all Class 10 students preparing for their CBSE & State Board Exam, ALLEN EUREKA is a unique digital program utilizing very specific learning techniques to enable students to master their content. It brings together real experience with professional content to ensure that students can understand and recall difficult subject areas. In addition, EUREKA provides opportunities for students to develop strong fundamental skills and boost their confidence when doing exam-focused work through the use of interactive lessons, AI doubt-clearing tools, and elaborate practice opportunities.
13.0Supporting Study Materials
This study material, including CBSE Notes and NCERT Solutions for Chapter 1 of Class 10 Science, is crafted according to the latest NCERT guidelines. Packed with step-by-step coefficient balancing guides, color-coded precipitate reactions, and high-yield subjective questions, this guide ensures absolute structural confidence for your school assessments and board examinations.
14.030-Second Quick Revision: Chemical Reactions and Equations
- A chemical reaction is a process in which one or more substances change into new substances.
- Reactants are the substances that take part in a reaction.
- Products are the new substances formed after a reaction.
- A chemical equation represents a chemical reaction using symbols and formulae.
- A balanced chemical equation has an equal number of atoms on both sides.
- Indicators of a chemical reaction: Change in colour, Change in temperature, Evolution of gas, Formation of precipitate and Change in state
- Types of reactions: Combination Reaction, Decomposition Reaction, Displacement Reaction, Double Displacement Reaction and Redox Reaction
- Law of Conservation of Mass must be followed while balancing equations.
- Balancing ensures that matter is neither created nor destroyed.
15.0Previous Year Questions (PYQs) on Chemical Reaction And Equation
Question: Balance the following chemical equation: Fe + H₂O → Fe₃O₄ + H₂
Solution: Balanced equation:
3Fe + 4H₂O → Fe₃O₄ + 4H₂
Checking atoms:
- Fe = 3 on both sides
- H = 8 on both sides
- O = 4 on both sides
Answer:
3Fe + 4H₂O → Fe₃O₄ + 4H₂
Question: Why should a chemical equation be balanced?
Solution: According to the Law of Conservation of Mass, matter can neither be created nor destroyed during a chemical reaction. Therefore, the number of atoms of each element must be equal on both sides of the equation.
16.0Recommended Next Topics
- pH scale
- Indicators
- Salts
- Hydrocarbons