1. What is a Chemical Reaction?
A Chemical Reaction is a process in which one or more substances (called reactants) undergo a chemical transformation to form one or more completely new substances (called products) with entirely different chemical properties. During a chemical reaction, the identity and properties of the initial substances are permanently altered.
Chemical Reaction: A process that involves the rearrangement of molecular or ionic structures, wherein old chemical bonds between reactant atoms break and new chemical bonds form to yield distinct products.
Everyday Examples of Chemical Reactions:
- Milk left at room temperature during summers turns sour or curds form.
- An iron nail, pan, or girder gets rusted when exposed to a humid atmosphere.
- Grapes ferment into wine.
- Food is digested in our gastrointestinal tract.
- We respire (cellular respiration where glucose oxidizes to produce energy).
4 Characteristics / Indicators of a Chemical Reaction:
A chemical reaction is confirmed whenever one or more of the following observable changes take place:
- Change in State: E.g., combustion of wax or hydrogen gas producing liquid water.
- Change in Colour: E.g., reddish-brown rust coating forming over shiny grey iron.
- Evolution of a Gas: E.g., production of hydrogen gas when metals react with acids.
- Change in Temperature: E.g., significant heat evolution when quicklime reacts with water.
Activity 1.3: Reaction of Zinc Granules with Dilute H₂SO₄ & Hydrogen ‘Pop’ Test
2. Chemical Equations & Balancing (Hit-and-Trial Method)
A symbolic and shorthand representation of a chemical reaction using chemical formulas and state symbols is called a Chemical Equation.
- Reactants: Substances that undergo chemical change (written on the left-hand side, LHS).
- Products: New substances formed as a result of the reaction (written on the right-hand side, RHS).
Balanced Chemical Equation: A chemical equation in which the total number of atoms of each element is equal on both sides of the reaction arrow (LHS = RHS).
According to the Law of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction. Hence, the total mass of the products must equal the total mass of the reactants. This requires equal numbers of atoms for each element on both sides.
Step-by-Step Balancing: Reaction of Iron with Steam
Unbalanced Skeleton Equation:
Step 1: Draw boxes around each chemical formula. Do not change anything inside the boxes.
Step 2: List the number of atoms of each element on both sides:
• Fe: LHS = 1, RHS = 3
• H: LHS = 2, RHS = 2
• O: LHS = 1, RHS = 4
Step 3: Start balancing with the element having the highest number of atoms (Oxygen). Put coefficient 4 in front of $\text{H}_2\text{O}$ on LHS:
$$\text{Fe} + 4\text{H}_2\text{O} \longrightarrow \text{Fe}_3\text{O}_4 + \text{H}_2$$
Step 4: Balance Hydrogen atoms. There are 8 H atoms on LHS ($4 \times 2$), so put coefficient 4 before $\text{H}_2$ on RHS:
$$\text{Fe} + 4\text{H}_2\text{O} \longrightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$$
Step 5: Balance Iron (Fe) atoms. Put coefficient 3 before Fe on LHS:
$$3\text{Fe} + 4\text{H}_2\text{O} \longrightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$$
Step 6: Add physical state notations: solid (s), liquid (l), gas (g), aqueous solution (aq).
Never change subscripts in a formula while balancing (e.g., never write $\text{H}_2\text{O}_4$ instead of $4\text{H}_2\text{O}$). Only modify coefficients placed in front of formulas.
3. Types of Chemical Reactions
(A) Combination Reaction
A reaction in which two or more reactants combine to form a single product is known as a combination reaction.
Combination Reaction: $$A + B \longrightarrow AB$$ Two or more elements or compounds unite chemically to yield only one new product.
Slaked lime solution [$\text{Ca(OH)}_2$] is applied to walls. It reacts slowly with atmospheric carbon dioxide ($\text{CO}_2$) over 2 to 3 days to form a thin, shiny layer of calcium carbonate ($\text{CaCO}_3$) on the walls, imparting a brilliant white finish: $$\text{Ca(OH)}_2\text{ (aq)} + \text{CO}_2\text{ (g)} \longrightarrow \text{CaCO}_3\text{ (s)} + \text{H}_2\text{O (l)}$$ (Note: Marble, limestone, and chalk all share the exact chemical formula $\text{CaCO}_3$).
Exothermic Reactions:
Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
- 1. Combustion of Natural Gas: $$\text{CH}_4\text{ (g)} + 2\text{O}_2\text{ (g)} \longrightarrow \text{CO}_2\text{ (g)} + 2\text{H}_2\text{O (g)} + \text{Energy}$$
- 2. Respiration is an Exothermic Process: During digestion, carbohydrates break down into glucose. In cells, glucose combines with oxygen to release vital energy: $$\text{C}_6\text{H}_{12}\text{O}_6\text{ (aq)} + 6\text{O}_2\text{ (g)} \longrightarrow 6\text{CO}_2\text{ (g)} + 6\text{H}_2\text{O (l)} + \text{Energy (ATP)}$$
- 3. Decomposition of vegetable matter into compost is also an exothermic reaction.
(B) Decomposition Reaction
A reaction in which a single reactant breaks down to yield two or more simpler products is called a decomposition reaction. It is the direct opposite of a combination reaction.
Decomposition Reaction: $$AB \xrightarrow{\text{Energy}} A + B$$ A single compound splits into multiple simpler entities upon the input of energy (heat, light, or electricity).
1. Thermal Decomposition: Carried out by heating
Activity 1.5: Thermal Decomposition of Ferrous Sulphate & Emission of SO₂ / SO₃ Gases
2. Electrolytic Decomposition (Electrolysis): Carried out by electricity
Activity 1.7: Electrolysis of Water (2:1 Volume Ratio of H₂ at Cathode to O₂ at Anode)
NCR & PAO: Negative is Cathode = Reductive / H₂ gas (2 parts) | Positive is Anode = Oxidative / O₂ gas (1 part)
3. Photolytic Decomposition (Photolysis): Carried out by sunlight
Endothermic Reactions:
Reactions that absorb energy in the form of heat, light, or electricity to break reactant bonds are known as endothermic reactions. All decomposition reactions are prime examples of endothermic processes.
(C) Displacement Reaction
A chemical reaction in which a more reactive element displaces a less reactive element from its aqueous salt solution is called a displacement reaction.
Displacement Reaction: $$A + BC \longrightarrow AC + B$$ (Where element A is more reactive than element B based on the Reactivity Series).
Activity 1.9: Displacement of Copper by Iron Nail in Copper Sulphate Solution
(D) Double Displacement & Precipitation Reaction
Reactions in which there is an exchange of ions between reactants to form two new compounds are called double displacement reactions.
Double Displacement & Precipitation: $$AB + CD \longrightarrow AD + CB$$ Mutual exchange of cations and anions, typically yielding one insoluble solid (precipitate).
(E) Oxidation, Reduction & Redox Reactions
Redox Reaction: A reaction in which one reactant gets oxidized (gains oxygen or loses hydrogen/electrons) while the other reactant simultaneously gets reduced (loses oxygen or gains hydrogen/electrons).
• Oxidizing Agent = $\text{CuO}$ | Reducing Agent = $\text{H}_2$
Activity 1.11: Oxidation of Copper Powder to Black Copper(II) Oxide
4. Everyday Effects of Oxidation: Corrosion and Rancidity
1. Corrosion
When a metal is attacked by substances around it such as moisture, oxygen, and acids, it is said to corrode, and the process is called Corrosion.
- Rusting of Iron: Formation of reddish-brown flaky hydrated ferric oxide ($\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$).
- Tarnishing of Silver: Black coating of silver sulphide ($\text{Ag}_2\text{S}$) upon exposure to traces of $\text{H}_2\text{S}$ in air.
- Corrosion of Copper: Green coating of basic copper carbonate [$\text{CuCO}_3\cdot\text{Cu(OH)}_2$] from moist $\text{CO}_2$.
5 Prevention Methods: Painting, greasing/oiling, galvanization (coating with zinc), electroplating (tin/chromium), alloying (stainless steel).
2. Rancidity
When fats and oils are oxidized, they become rancid and their smell and taste change. This phenomenon is termed Rancidity.
4 Prevention Methods: Flushing snack/chip bags with Nitrogen ($\text{N}_2$) gas, adding antioxidants (BHA, BHT), storing in airtight containers, and refrigeration.
| Parameter | Exothermic Reaction | Endothermic Reaction |
|---|---|---|
| Energy Transformation | Heat energy is released into the surrounding environment. | Heat energy is absorbed from the surrounding environment. |
| Temperature Effect | Temperature of reaction mixture and vessel rises (feels warm). | Temperature of reaction mixture drops (feels cold). |
| Bond Energetics | Energy released during bond formation is greater than bond breaking. | Energy required to break existing bonds is greater than bond formation. |
| Enthalpy Change (ΔH) | Negative (ΔH < 0) | Positive (ΔH > 0) |
| Real-World Examples | Combustion of methane, cellular respiration, slaking of quicklime. | Photosynthesis in plants, thermal decomposition of limestone. |
| Representative Equation | $$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{Heat}$$ | $$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$$ |
| Parameter | Displacement Reaction | Double Displacement Reaction |
|---|---|---|
| Fundamental Definition | A more reactive element displaces a less reactive element from its aqueous salt solution. | Mutual exchange of ions occurs between two compounds to form two new compounds. |
| Nature of Reactants | Typically one free element and one aqueous compound. | Both reactants are typically aqueous ionic compounds. |
| Reactivity Dependency | Directly governed by the Metal Reactivity Series. | Independent of reactivity series; driven by ion precipitation and solubility products. |
| Precipitate Formation | Solid metal deposit coats on the reacting metal surface. | Frequently forms a distinct insoluble precipitate (solid suspension). |
| General Equation Form | $$\text{A} + \text{BC} \longrightarrow \text{AC} + \text{B}$$ | $$\text{AB} + \text{CD} \longrightarrow \text{AD} + \text{CB}$$ |
| CBSE Benchmark Example | $$\text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)}$$ | $$\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\downarrow + 2\text{NaCl}$$ |
| Criteria | Oxidation | Reduction |
|---|---|---|
| Oxygen Criterion | Addition of oxygen to a substance. | Removal of oxygen from a substance. |
| Hydrogen Criterion | Removal of hydrogen from a substance. | Addition of hydrogen to a substance. |
| Electronic Concept (OIL RIG) | Loss of electrons by an atom or ion (Oxidation Is Loss). | Gain of electrons by an atom or ion (Reduction Is Gain). |
| Oxidation State | Increase in oxidation state. | Decrease in oxidation state. |
| Agent Classification | Substance that gets oxidised acts as the **Reducing Agent**. | Substance that gets reduced acts as the **Oxidising Agent**. |
| Chemical Example | $$2\text{Cu} + \text{O}_2 \rightarrow 2\text{CuO}$$ (Cu oxidised) | $$\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}$$ (CuO reduced) |
| Metal | Symbol | Reactivity Level | Displacement Rules & Behavior |
|---|---|---|---|
| Potassium | K | Most Reactive (Top) | Violent reaction with cold water, instantly displaces hydrogen. |
| Sodium | Na | Extremely Reactive | Vigorous exothermic reaction with cold water; stored in kerosene. |
| Calcium | Ca | Highly Reactive | Reacts with water; floats due to hydrogen bubbles sticking to it. |
| Magnesium | Mg | Moderately Reactive | Reacts with hot boiling water and steam. |
| Aluminium | Al | Moderately Reactive | Reacts with steam; forms protective Al₂O₃ oxide layer. |
| Zinc | Zn | Moderately Reactive | Displaces Fe, Pb, Cu from their aqueous solutions. |
| Iron | Fe | Moderately Reactive | Displaces Cu from blue CuSO₄ yielding pale green FeSO₄. |
| Lead | Pb | Low Reactivity | Slow reaction with dilute acids. |
| Hydrogen (Non-metal ref) | H | Standard Reference (0.00 V) | Metals above H displace H₂ from dilute acids; metals below do not. |
| Copper | Cu | Very Low Reactivity | Does not displace H₂ from dilute HCl/H₂SO₄; displaced by Fe and Zn. |
| Silver | Ag | Noble Metal | Corrosion resistant; readily displaced by Cu in AgNO₃ solution. |
| Gold | Au | Least Reactive (Bottom) | Unreactive with oxygen, moisture, or dilute acids; zero corrosion. |
Chapter Resource Hub — Class 10 Science
Access related study assets of Chapter 1 in one click.
Revision Notes / संक्षिप्त नोट्स
Fast track revision summaries and memory maps.
Interactive MCQs / बहुविकल्पीय प्रश्न
Self-practice quiz to test chapter concepts.
Important Questions & Answers
Board pattern short/long questions solved.
Previous Year Questions (PYQs)
Last 10 years solved paper segments.