In this chapter, you will learn
- —Understand the difference between physical and chemical changes
- —Learn what a chemical reaction is and how to represent it using a chemical equation
- —Identify the signs that indicate a chemical reaction has taken place
- —Understand different types of chemical reactions: combination, decomposition, displacement, oxidation, and reduction
- —Differentiate between exothermic and endothermic reactions
- —Recognise chemical reactions in daily life such as rusting, burning, cooking, and digestion
- —Understand the concepts of reactants, products, oxidising agents, and reducing agents
Physical Changes vs Chemical Changes
Changes around us can be classified into two types: physical changes and chemical changes.
Physical Changes:
- Only the appearance (size, shape, colour, state) of a substance changes
- No new substance is formed
- Generally reversible
- Examples: melting of ice, dissolving sugar in water, tearing paper, breaking glass
Chemical Changes:
- The internal structure of a substance changes to form a new substance
- New substance(s) with different properties are formed
- Always irreversible
- Examples: burning of wood, rusting of iron, cooking food, curdling of milk
Key Difference: In a physical change, no new substance is formed. In a chemical change, one or more new substances are always formed.
Exam Tip
CBSE frequently asks you to classify changes as physical or chemical. Remember: if a NEW substance is formed, it is a chemical change. If only appearance changes, it is physical.
Common Mistake
Students often think dissolving sugar in water is a chemical change. It is a physical change because sugar can be recovered by evaporation - no new substance is formed.
What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances (reactants) undergo a change to produce one or more new substances (products) with different properties.
Signs that a chemical reaction has taken place:
- Change in colour: e.g., black copper oxide turns brown when heated with hydrogen
- Evolution of gas: e.g., bubbles when zinc reacts with dilute acid
- Change in temperature: e.g., heat released when quicklime reacts with water
- Formation of precipitate: e.g., lime water turns milky when CO2 is passed through it
- Change in smell: e.g., food items develop a foul smell when they go bad
Exam Tip
When asked to give signs of a chemical reaction, always give at least 3-4 signs with examples. Examiners expect specific examples, not just the sign names.
Chemical Equations
A chemical equation is a symbolic representation of a chemical reaction using formulae and symbols.
General form:
Reactants → Products
Key terms:
- Reactants: Substances that undergo change in a reaction (written on the left side of the arrow)
- Products: New substances formed after the reaction (written on the right side of the arrow)
- The arrow (→) means "yields" or "produces"
- The plus sign (+) separates two or more reactants or products
Example:
C + O2 → CO2
(Carbon + Oxygen → Carbon dioxide)
Here, C and O2 are reactants; CO2 is the product.
Exam Tip
Always label reactants and products clearly in your answers. If a word equation is given, convert it to a chemical equation and vice versa.
Combination Reaction
A combination reaction (also called a synthesis reaction) is a reaction in which two or more substances combine to form a single new substance.
General form:
A + B → AB
Examples:
- Burning of carbon: C + O2 → CO2
- Formation of water: 2H2 + O2 → 2H2O
- Burning of magnesium: 2Mg + O2 → 2MgO (bright white flame)
- Quicklime with water: CaO + H2O → Ca(OH)2 (releases heat)
In each case, two or more reactants combine to form a single product.
Exam Tip
Combination reactions are the opposite of decomposition reactions. In combination, many substances form one; in decomposition, one substance forms many.
Decomposition Reaction
A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances when energy (heat, light, or electricity) is supplied.
General form:
AB → A + B
Examples:
- Heating sodium bicarbonate: 2NaHCO3 → Na2CO3 + H2O + CO2
- Heating calcium carbonate: CaCO3 → CaO + CO2
- Electrolysis of water: 2H2O → 2H2 + O2 (using electricity)
Decomposition reactions are the opposite of combination reactions. They require energy to break down the compound.
Exam Tip
Remember: Decomposition needs energy input. If the question says a substance 'breaks down on heating,' it is always a decomposition reaction.
Displacement Reaction (Single Displacement)
A single displacement reaction occurs when a more reactive element displaces a less reactive element from its compound.
General form:
A + BC → AC + B
Here, element A is more reactive than element B, so A displaces B from compound BC.
Examples:
- Iron displaces copper: Fe + CuSO4 → FeSO4 + Cu
(Iron nail dipped in copper sulphate solution - blue solution turns green, reddish-brown copper deposits on the nail) - Zinc displaces hydrogen: Zn + H2SO4 → ZnSO4 + H2
(Zinc reacts with sulphuric acid to release hydrogen gas)
Exam Tip
The iron-copper sulphate experiment is a very common exam question. Remember the colour change: blue CuSO4 turns green FeSO4.
Common Mistake
Students confuse which element displaces which. The MORE reactive element always displaces the LESS reactive one from its compound.
Oxidation and Reduction Reactions
Oxidation and reduction are two important types of chemical reactions that always occur together.
Oxidation Reaction:
- Addition of oxygen to a substance, OR
- Removal of hydrogen from a substance
Examples of Oxidation:
C + O2 → CO2 (carbon gains oxygen - oxidised)
H2S + Cl2 → 2HCl + S (H2S loses hydrogen - oxidised)
Reduction Reaction:
- Removal of oxygen from a substance, OR
- Addition of hydrogen to a substance
Examples of Reduction:
MgO + C → Mg + CO (MgO loses oxygen - reduced)
H2S + Br2 → 2HBr + S (Br2 gains hydrogen - reduced)
Important: Oxidation and reduction always occur simultaneously. When one substance is oxidised, another is reduced.
Exam Tip
Memory aid: OIL RIG - Oxidation Is Loss (of hydrogen or gain of oxygen), Reduction Is Gain (of hydrogen or loss of oxygen).
Common Mistake
Students often forget that oxidation can also mean removal of hydrogen, not just addition of oxygen. Similarly, reduction can mean addition of hydrogen.
Oxidising and Reducing Agents
In oxidation-reduction reactions, special terms are used for the substances that cause these changes:
Oxidising Agent:
- A substance that causes oxidation of another substance
- It causes addition of oxygen or removal of hydrogen
- The oxidising agent itself gets reduced in the process
- Example: In C + O2 → CO2, oxygen is the oxidising agent
Reducing Agent:
- A substance that causes reduction of another substance
- It causes removal of oxygen or addition of hydrogen
- The reducing agent itself gets oxidised in the process
- Example: In MgO + C → Mg + CO, carbon is the reducing agent
Exam Tip
Remember: The oxidising agent is always reduced, and the reducing agent is always oxidised. They sacrifice themselves for the other substance!
Exothermic and Endothermic Reactions
Chemical reactions can also be classified based on whether they release or absorb heat energy.
| Feature | Exothermic Reaction | Endothermic Reaction |
|---|---|---|
| Heat | Heat is released | Heat is absorbed |
| Temperature | Surroundings get warmer | Surroundings get cooler |
| Energy needed | May need initial energy, then releases heat | Needs continuous supply of energy |
| Examples | Burning of fuels, respiration, CaO + H2O | Decomposition of NaHCO3, photosynthesis |
Exothermic example:
C + O2 → CO2 + Heat
Endothermic example:
2NaHCO3 + Heat → Na2CO3 + H2O + CO2
Exam Tip
Exo = Exit (heat exits/releases). Endo = Enter (heat enters/absorbs). This memory trick helps remember which is which.
Common Mistake
Students sometimes think all chemical reactions release heat. Many reactions like decomposition actually absorb heat (endothermic).
Rusting of Iron
Rusting is a slow chemical reaction in which iron reacts with oxygen and moisture (water) in the air to form a reddish-brown substance called rust (iron hydroxide).
Chemical equation:
4Fe + 3O2 + 6H2O → 4Fe(OH)3
(Iron + Oxygen + Water → Iron hydroxide / Rust)
Conditions required for rusting:
- Presence of oxygen (from air)
- Presence of moisture (water)
Prevention of rusting:
- Painting - prevents contact with air and moisture
- Oiling/greasing - creates a protective layer
- Galvanisation - coating iron with a layer of zinc
- Alloying - mixing iron with other metals (e.g., stainless steel)
Rusting is an oxidation reaction because iron gains oxygen.
Exam Tip
Rusting requires BOTH oxygen and moisture. If either is absent, iron will not rust. This is a very common exam question.
Burning (Combustion)
Burning (combustion) is a chemical reaction in which a substance reacts with oxygen to produce heat and light.
Examples:
- Burning of carbon: C + O2 → CO2 + Heat + Light
- Burning of magnesium: 2Mg + O2 → 2MgO (bright white flame)
- Burning of candle wax: Wax + O2 → CO2 + H2O + Heat + Light
Important note about burning of a candle:
Burning of a candle shows both physical and chemical changes:
- Physical change: The solid wax melts into liquid wax (change of state - reversible)
- Chemical change: The wax burns in the presence of oxygen to form carbon dioxide and water (irreversible)
Burning is always an exothermic reaction because it releases heat and light.
Exam Tip
The 'burning of candle' question is a favourite in CBSE exams. Always explain BOTH the physical change (melting) and chemical change (burning).
Chemical Reactions in Daily Life
Chemical reactions are happening around us all the time. Here are important examples:
| Daily Life Example | Type of Reaction | Explanation |
|---|---|---|
| Cooking food | Chemical change | New substances with different taste, colour, and texture are formed |
| Digestion of food | Chemical change | Complex food molecules break down into simpler substances |
| Respiration | Exothermic (oxidation) | Glucose reacts with oxygen to release energy, CO2, and water |
| Curdling of milk | Chemical change | Lactobacillus bacteria convert lactose into lactic acid |
| Rusting of iron | Oxidation | Iron reacts with oxygen and moisture to form rust |
| Food spoilage | Chemical change | Enzymes cause chemical reactions that make food unsafe after a period |
Why pickles should not be stored in iron jars: Pickles contain acetic acid, which reacts with iron to form iron salts and hydrogen gas. The metal salts make the food unfit to eat. This is a chemical change (displacement reaction).
Exam Tip
Competency-based questions often ask about daily life examples. The pickle-in-iron-jar question and the 'Best Before' date question are frequently asked.
Crystallisation
Crystallisation is the process of obtaining pure crystals of a substance from its saturated solution.
Key points:
- Crystals are formed from a saturated solution
- It changes the shape, size, and state of a substance
- It does NOT change the chemical composition of the substance
- Therefore, crystallisation is a physical change, not a chemical change
Example: Obtaining crystals of copper sulphate or alum from their solutions by slow evaporation.
Exam Tip
Crystallisation is a physical change because no new substance is formed - only the shape and state change. Do not confuse it with a chemical change.