1. Introduction to Chemical Reactions

A chemical reaction is a process in which one or more substances (called reactants) are transformed into new substances (called products) with different chemical properties. During a chemical change, electrons may be lost, gained, or shared, leading to the formation of new compounds.

Example: Rusting of iron, burning of wood, cooking rice, curd formation from milk, and photosynthesis.

A chemical equation is the symbolic representation of a chemical reaction, using symbols and formulas to show reactants and products.

Example:

2H2+O2→2H2O

Reactants: H2 and O2​

Product: H2O

2. Types of Chemical Reactions

Chemical reactions are classified based on how reactants change to form products:

a) Combination (Synthesis) Reaction

Definition: A reaction in which two or more reactants combine to form a single product.

General form: A+B→C

Examples:

2Na+Cl2→2NaCl (Sodium + Chlorine → Sodium chloride)

C+O2→CO2​ (Carbon + Oxygen → Carbon dioxide)

N2+3H2→2NH3​ (Nitrogen + Hydrogen → Ammonia)

2Mg+O2→2MgO (Magnesium + Oxygen → Magnesium oxide)

b) Decomposition Reaction

Definition: A reaction in which a single reactant breaks down into two or more products. It usually requires heat, light, or a catalyst.

General form: AB→A+B

Examples:

CaCO3→CaO+CO2​ (Calcium carbonate → Calcium oxide + Carbon dioxide)

2NaOH→Na2O+H2O

2KClO3→2KCl+3O2

c) Displacement Reaction

Definition: A reaction in which an element or radical in a compound is replaced by another element or radical.

Types:

i) Single Displacement Reaction:

One element displaces another from a compound.

General form: A+BC→AC+B

Examples:

Zn+H2SO4→ZnSO4+H2

Cu+2AgNO3→2Ag+Cu(NO3)2

ii) Double Displacement Reaction:

Mutual exchange of ions between two compounds to form two new compounds.

General form: AB+CD→AD+CB

Examples:

NaCl+AgNO3→AgCl+NaNO3

BaCl2+Na2SO4→BaSO4+2NaCl

d) Acid-Base Reaction (Neutralization)

Definition: A reaction in which an acid reacts with a base to form salt and water.

This is a type of double displacement reaction.

Examples:

HCl+NaOH→NaCl+H2O

H2SO4+2NaOH→Na2SO4+2H2OH

CaO+2HCl→CaCl2+H2O

3. Rate of Chemical Reactions

Rate of reaction refers to the speed at which reactants turn into products per unit time.

Reactions can be fast (e.g., acid-base reactions) or slow (e.g., rusting of iron).

At the start of a reaction, the concentration of reactants is highest and that of products is zero. Over time, products increase and reactants decrease.

4. Factors Affecting Reaction Rate

Several factors affect the speed of a chemical reaction:

a) Catalyst

A catalyst is a substance that increases or decreases the reaction rate without being permanently changed.

Positive catalyst: Speeds up reaction (e.g., MnO₂ in H₂O₂ decomposition)

Negative catalyst: Slows down reaction (e.g., H₃PO₄ in some reactions)

b) Temperature

Increasing temperature usually increases reaction rate because particles move faster and collide more frequently.

c) Surface Area

Higher surface area of reactants increases the rate of reaction because more particles are exposed to collisions.

d) Pressure (for gases)

Increasing pressure increases the reaction rate for gaseous reactions because particles are closer and collide more often.

e) Light

Some reactions (like photosynthesis) require light to proceed and may occur faster in its presence.

5. Observations and Activities

The chapter emphasizes practical activities such as:

Mixing acids and bases (to observe neutralization)

Reaction of zinc with HCl (displacement)

Effect of heat, light, and surface area on reaction rate

Copper plating on iron nail (displacement reaction)

These experiments help students understand concepts practically and reinforce theoretical knowledge.

6. Summary Table of Reaction Types

Reaction TypeGeneral FormExampleCombinationA + B → C2Na + Cl₂ → 2NaClDecompositionAB → A + B2KClO₃ → 2KCl + 3O₂Single DisplacementA + BC → AC + BZn + H₂SO₄ → ZnSO₄ + H₂Double DisplacementAB + CD → AD + CBNaCl + AgNO₃ → AgCl + NaNO₃Acid-BaseAcid + Base → Salt + WaterHCl + NaOH → NaCl + H₂O

 

 Physical Change & Chemical Change

Physical Change — Definition

A physical change is a change in form, shape, size, or state of a substance without forming a new substance.

Examples

Melting of ice

Dissolving sugar in water

Breaking glass

Chemical Change — Definition

A chemical change is a change in which one or more new substances are formed having different properties from the original substances.

Examples

Rusting of iron

Burning of wood

Milk turning into curd

Cooking food

2. Difference Between Physical & Chemical Change

Physical ChangeChemical ChangeNo new substance is formedNew substance is formedMostly reversibleMostly irreversibleChange in physical stateChange in chemical compositionEnergy change is smallEnergy change is large

 3. Examples of Chemical Changes in Daily Life

ProcessType of ChangeReasonRusting of ironChemicalNew substance (iron oxide) formedMilk → curdChemicalAction of bacteriaCooking riceChemicalNew food substance formedBurning woodChemicalForms charcoal, ash, CO₂Burning matchstickChemicalCombustion occursPhotosynthesisChemicalForms glucose & oxygen

 

 11. Differences (Important)

Single vs Double Displacement

SingleDoubleOne element displaces anotherExchange of ions between compoundsA + BC → AC + BAB + CD → AD + CBExample: Zn + HClExample: NaCl + AgNO₃

Combination vs Decomposition

CombinationDecompositionA + B → ABAB → A + BMany → oneOne → many

 12. Sample Exercise Answers

Give reason

i. Hydrogen peroxide is stored in brown bottles
Because light decomposes it.

ii. Acid–base reaction is neutralization
Because acid + base forms neutral salt + water.

iii. Rate of reaction differs
Because reactions have different activation energies, reactants, conditions.

 13. Important Reaction: Copper Sulphate & Iron

Type: Single displacement
Iron displaces copper.

Balanced reaction:

Fe+CuSO4→FeSO4+Cu

 14. Magnesium Ribbon Burning

Substance ‘A’ = Magnesium oxide (MgO)

Reaction:

2Mg+O2​→2MgO 

 Complete Chapter Summary 

Chemical reactions occur when reactants convert into products with new properties. They can be combination, decomposition, displacement, double displacement, or acid-base reactions. Reaction speed depends on temperature, catalyst, pressure, surface area, and light. Chemical equations represent reactions using symbols and must be balanced. Many natural and daily changes—from rusting to photosynthesis—are examples of chemical reactions.