GRADE 7 LIVING SCIENCE • CHAPTER 7
Physical & Chemical Changes
COMPLETE EXAM STUDY GUIDE
All Pages Covered • All Questions Answered • All Equations Included
� Complete Contents of This Study Guide
✅ Physical & Chemical Changes — Definitions & Properties
✅ Rusting of Iron & Prevention of Rusting
✅ Characteristics of Chemical Reactions (5 types)
✅ Types of Chemical Reactions (6 types with equations)
✅ Oxidation, Reduction, Oxidizing & Reducing Agents
✅ Separating a Solid — Evaporation & Crystallization
✅ New Words / Now You Know / Our Heritage
✅ ALL Activities Answered (Activity 1–6)
✅ ALL Oral Class Discussion Questions Answered
✅ FULL Exercise Answers: MCQ, Very Short, Short, Long, HOTS
SECTION 1 — Physical & Chemical Changes (Pg 83)
1.1 Physical Change
� Definition
In a physical change, a substance undergoes changes only in its physical properties such as
shape, size, colour and state, and NO new substances are formed.
Properties:
•
•
•
•
No new substance is formed — most important point
Only physical properties change: shape, size, colour, state
Some are reversible; some are irreversible
Chemical composition stays the same
Textbook Examples:
•
Freezing of water | Melting of wax | Tearing of paper | Boiling of water to form steam
1.2 Chemical Change
� Definition
A chemical change occurs when two substances react chemically to form new substances with
different chemical properties. A chemical change is also called a CHEMICAL REACTION.
Original substances = REACTANTS. New substances formed = PRODUCTS.
Key Points:
•
•
•
•
New substances with DIFFERENT properties are always formed
Molecules of reactants undergo changes to form molecules of products
Substances undergo chemical transformation
Usually irreversible
Textbook Examples:
•
Burning of fuel | Cooking of food | Rusting of iron | Photosynthesis | Digestion of food
Feature
Physical Change
Chemical Change
New Substance?
No new substance
New substance always formed
Reversibility
Usually reversible
Usually irreversible
Molecules
Molecules do not change
Molecules change completely
Composition
Same chemical composition
Composition changes
Examples
Melting, boiling, cutting
Burning, rusting, cooking
Energy change
Small
Significant release or absorption
SECTION 2 — Rusting of Iron (Pg 84)
2.1 What is Rusting?
Iron objects (nails, gates) when left exposed to moist air are covered with a reddish-brown powdery
substance called iron oxide — commonly called RUST. Rusting is a CHEMICAL change because a
new substance (iron oxide) is formed and the change is irreversible.
Rusting occurs in the PRESENCE OF BOTH OXYGEN AND WATER. The more humid the air, the
faster rusting occurs. Rust corrodes the iron, leading to loss of material. Example: rusting of ships is a
big problem in shipping industry.
Iron + Oxygen + Water → Rust
4Fe + 3O₂ + H₂ O → Iron Oxide (Rust)
⚠️ Why Rusting is a Chemical Change
• A new substance (iron oxide/rust) is formed
• Colour changes: silvery grey → reddish-brown
• Irreversible — rust cannot be turned back into iron
• Rusting = example of SLOW OXIDATION
• Both oxygen AND water must be present
2.2 Prevention of Rusting
Rusting is prevented by NOT allowing iron to come in contact with moisture.
Method
How It Works
Example
Oil / Grease / Paint
Creates a barrier — moisture cannot
reach iron surface
Painting iron gates, oiling hinges
Zinc / Chromium
coating
Less reactive metal layer acts as
protection
Chrome plating of tools
Galvanization ⭐
Depositing a layer of ZINC on iron —
most important method!
Water pipes in homes made of
galvanized iron
SECTION 3 — Characteristics of Chemical Reactions
(Pg 84–86)
When a chemical reaction takes place, it is accompanied by one or more of the following
characteristics:
3.1 Evolution of a Gas (Effervescence)
A gas may evolve during a chemical reaction if one of the reactants is in liquid/solution form. Evolution
of gas is confirmed by appearance of EFFERVESCENCE (bubbles).
• When calcium carbonate reacts with dilute hydrochloric acid, carbon dioxide gas is given off
• The gas can be tested by passing it through lime water — lime water turns MILKY
Calcium Carbonate + Hydrochloric Acid → Calcium Chloride + Water + Carbon Dioxide
CaCO₃ + 2HCl → CaCl₂ + H₂ O + CO₂ ↑
•
When zinc reacts with dilute sulphuric acid, HYDROGEN gas is given off. Tested with a burning
matchstick — hydrogen burns with a POP sound
Zinc + Sulphuric Acid → Zinc Sulphate + Hydrogen
Zn + H₂ SO₄ → ZnSO₄ + H₂ ↑
•
When sodium nitrate is strongly heated, OXYGEN gas is given off. Tested by bringing a glowing
splint near it — the splint bursts into flame
Sodium Nitrate → Sodium Nitrite + Oxygen
2NaNO₃ → 2NaNO₂ + O₂ ↑
� CO₂ Test — Lime Water
• CO₂ reacts with lime water [Ca(OH)₂ ] to form calcium carbonate
• Calcium carbonate is white and not much soluble in water
• It forms a suspension giving lime water a MILKY appearance
• This milky appearance is the standard test for carbon dioxide
Ca(OH)₂ + CO₂ → CaCO₃ ↓ + H₂ O
• ALSO: CO₂ does NOT support combustion — a lighted candle gets extinguished in CO₂
3.2 Change of Colour
One of the products formed during a chemical reaction may have a different colour from that of the
reactants.
• Cut apple / brinjal turns brown when exposed to air — due to reaction between air and enzymes
(Activity 1)
• Rusting: iron (grey) → rust (reddish-brown)
• Green-coloured copper carbonate heated strongly → black residue of copper oxide (CuO) +
CO₂
Copper Carbonate → Copper Oxide (black) + Carbon Dioxide
CuCO₃ → CuO + CO₂ ↑
•
White crystals of lead nitrate heated strongly → yellow lead monoxide + reddish-brown nitrogen
dioxide + oxygen
Lead Nitrate → Lead Monoxide + Nitrogen Dioxide + Oxygen
2Pb(NO₃ )₂ → 2PbO + 4NO₂ ↑ + O₂ ↑
3.3 Formation of a Precipitate
When two chemicals in solution form react, an insoluble substance forms — it settles down as a
PRECIPITATE. Shown by a downward arrow (↓) in the equation.
• Iron chloride + Ammonium hydroxide → brown precipitate of iron hydroxide
Iron Chloride + Ammonium Hydroxide → Iron Hydroxide (brown ppt) + Ammonium Chloride
FeCl₃ + 3NH₄ OH → Fe(OH)₃ ↓ + 3NH₄ Cl
•
Hydrogen sulphide + Copper sulphate → black precipitate of copper sulphide
Copper Sulphate + Hydrogen Sulphide → Copper Sulphide (black ppt) + Sulphuric Acid
CuSO₄ + H₂ S → CuS↓ + H₂ SO₄
•
Sodium chloride + Silver nitrate → white precipitate of silver chloride
Sodium Chloride + Silver Nitrate → Silver Chloride (white ppt) + Sodium Nitrate
NaCl + AgNO₃ → AgCl↓ + NaNO₃
3.4 Changes in Energy — Exothermic vs Endothermic
Exothermic Reactions: Reactions accompanied with RELEASE of heat. Heat written on RIGHTHAND side of equation ('+heat' on the right). All burning reactions are exothermic.
• Carbon burns in air: C + O₂ → CO₂ + heat
• Water added to quicklime (CaO): CaO + H₂ O → Ca(OH)₂ + heat (becomes very hot — slaked
lime formed)
Endothermic Reactions: Reactions accompanied with ABSORPTION of heat. Heat written on LEFTHAND side of equation ('– heat' or 'heat' on the left).
• Carbon + Sulphur heated → carbon disulphide (heat absorbed)
Endothermic — heat on left side
C + 2S → CS₂
•
(heat absorbed)
Nitrogen + Oxygen at high temperature → nitric oxide (heat absorbed)
Nitrogen + Oxygen → Nitric Oxide
N₂ + O₂ → 2NO (heat absorbed)
Energy in other forms such as LIGHT and SOUND is also evolved in some reactions — e.g. explosion
of a firecracker (combination of combustion reactions).
3.5 Change of State
In some chemical reactions, there is a change in the physical state of reactants and products. To show
change of state: s (solid), l (liquid), g (gas), aq (aqueous solution) written after the symbols.
• Electrolysis of water (electric current + dilute sulphuric acid) → hydrogen gas + oxygen gas
Water → Hydrogen gas + Oxygen gas
2H₂ O(l) → 2H₂ (g)↑ + O₂ (g)↑
•
Hydrogen burns in oxygen → water formed (l)
Hydrogen + Oxygen → Water
2H₂ (g) + O₂ (g) → 2H₂ O(l)
SECTION 4 — Types of Chemical Reactions (Pg 87–
88)
Certain patterns are observed in chemical reactions. Chemical reactions are grouped according to
these patterns to make study easier.
4.1 Combination Reaction ⭐
� Definition
When a compound is formed from its elements, or from simpler substances, the reaction is called
a COMBINATION REACTION. Pattern: A + B → AB
•
Magnesium reacts with oxygen on heating → magnesium oxide (burns with a dazzling white
flame)
Magnesium + Oxygen → Magnesium Oxide
2Mg + O₂ → 2MgO
•
Magnesium oxide dissolves in water → magnesium hydroxide (forms a base)
Magnesium Oxide + Water → Magnesium Hydroxide
2MgO + 2H₂ O → 2Mg(OH)₂
•
Nitrogen reacts with hydrogen → ammonia
Nitrogen + Hydrogen → Ammonia
N₂ + 3H₂ → 2NH₃
4.2 Decomposition Reaction ⭐
� Definition
Decomposition reaction is one in which a compound BREAKS UP into two or more substances.
Caused by heating or passing electricity. Pattern: AB → A + B
•
Calcium hydroxide heated → calcium oxide + steam
Calcium Hydroxide → Calcium Oxide + Water
Ca(OH)₂ → CaO + H₂ O(steam)
•
Copper carbonate heated → copper oxide + carbon dioxide
Copper Carbonate → Copper Oxide + Carbon Dioxide
CuCO₃ → CuO + CO₂ ↑
•
Water electrolysed → hydrogen + oxygen
Water → Hydrogen + Oxygen (electrolysis)
2H₂ O(l) → 2H₂ (g)↑ + O₂ (g)↑
4.3 Single Displacement Reaction ⭐
� Definition
In this type, one element replaces another element from a compound. A more reactive element
replaces a less reactive element. Pattern: A + BC → AC + B
•
Iron (more reactive than copper) added to copper sulphate solution → iron sulphate + copper
deposited on iron
Copper Sulphate + Iron → Iron Sulphate + Copper
CuSO₄ + Fe → FeSO₄ + Cu
•
Copper wire dipped in silver nitrate solution → copper nitrate + silver deposited; solution turns
bluish
Silver Nitrate + Copper → Copper Nitrate + Silver
2AgNO₃ + Cu → Cu(NO₃ )₂ + 2Ag
4.4 Double Displacement Reaction ⭐
� Definition
In a double displacement reaction, two compounds react by EXCHANGING their elements or
groups. Pattern: AB + CD → AD + CB
Two types:
(a) Precipitation Reactions: Two compounds in solution react to form an insoluble solid (precipitate).
• Sodium chloride + Silver nitrate → silver chloride (white ppt) + sodium nitrate
Precipitation Reaction
NaCl + AgNO₃ → AgCl↓ + NaNO₃
•
Sodium carbonate + Copper sulphate → copper carbonate (ppt) + sodium sulphate
Sodium Carbonate + Copper Sulphate → Copper Carbonate ppt
Na₂ CO₃ + CuSO₄ → Na₂ SO₄ + CuCO₃ ↓
(b) Neutralisation Reactions: An acid reacts with a base to form salt + water. HA + BOH → AB +
H₂ O
• Sulphuric acid + Sodium hydroxide → sodium sulphate (salt) + water
Sulphuric Acid + Sodium Hydroxide → Sodium Sulphate + Water
H₂ SO₄ + 2NaOH → Na₂ SO₄ + 2H₂ O
•
Hydrochloric acid + Potassium hydroxide → potassium chloride + water
Hydrochloric Acid + Potassium Hydroxide → Potassium Chloride + Water
HCl + KOH → KCl + H₂ O
SECTION 5 — Oxidation & Reduction (Pg 88–89)
5.1 Oxidation
� Definition
OXIDATION is a chemical reaction that involves ADDITION OF OXYGEN or REMOVAL OF
HYDROGEN from a substance.
•
Carbon reacts with oxygen → carbon dioxide. Carbon is OXIDIZED by gaining oxygen.
Carbon gains oxygen → oxidized
C + O₂ → CO₂
•
Hydrogen sulphide reacts with chlorine → sulphur + HCl. H₂ S is OXIDIZED by loss of
hydrogen.
Hydrogen Sulphide loses hydrogen → oxidized
H₂ S + Cl₂ → S + 2HCl
5.2 Reduction
� Definition
REDUCTION is a chemical reaction that involves ADDITION OF HYDROGEN or REMOVAL OF
OXYGEN from a substance.
•
Hydrogen passed over heated black copper oxide → copper metal + water. CuO is REDUCED
by loss of oxygen.
Copper Oxide loses oxygen → reduced
H₂ + CuO → Cu + H₂ O
•
Chlorine reacts with hydrogen → hydrogen chloride. Chlorine is REDUCED by gaining
hydrogen.
Chlorine gains hydrogen → reduced
Cl₂ + H₂ → 2HCl
5.3 Oxidizing Agent vs Reducing Agent
Oxidizing Agent
Oxidizes other substances by PROVIDING
OXYGEN to them or REMOVING HYDROGEN
from them
Reducing Agent
Reduces other substances by REMOVING
OXYGEN from them or SUPPLYING
HYDROGEN to them
Example: In C + O₂ → CO₂ , OXYGEN is the
oxidizing agent (supplies oxygen to carbon)
Example: In H₂ + CuO → Cu + H₂ O,
HYDROGEN is the reducing agent (removes
oxygen from CuO)
Example: In H₂ S + Cl₂ → S + 2HCl, CHLORINE
is the oxidizing agent (removes hydrogen from
H₂ S)
Example: In Cl₂ + H₂ → 2HCl, HYDROGEN is
the reducing agent (supplies hydrogen to
chlorine)
SECTION 6 — Separating a Solid from a Solution
(Pg 89–91)
6.1 Evaporation
You can separate a solid dissolved in water by completely evaporating the water (provided the solid
has a higher boiling point than water). The solid is left behind as a residue.
• Salt is obtained from sea water by collecting sea water in shallow ponds and letting the water
evaporate in the heat of the sun
• The solid (salt) is left behind
Activity 5 — Separating Common Salt
• Dissolve a teaspoonful of salt in 50 mL water in a beaker
• Heat the salt solution — as water evaporates, the amount of solution decreases
• Let the water evaporate completely
• Taste the white residue left in the beaker → it is SALT
• This demonstrates that dissolving salt in water is a PHYSICAL change (salt recovered!)
6.2 Crystallization
Sugar grains viewed through a magnifying lens all have the same basic shape — a cube. These cubic
structures are called CRYSTALS. In a crystal, atoms or molecules are arranged in a regular pattern.
You can obtain crystals of common substances such as salt or alum from a solution of these
substances in water by a process called CRYSTALLIZATION.
• Crystals of different substances have DIFFERENT shapes
Activity 6 — Growing Alum Crystals
• Heat water in a beaker, remove from fire before boiling
• Add alum slowly and keep stirring till no more dissolves — saturated solution
• Filter the solution using fine cloth to remove undissolved alum
• As solution cools → it becomes supersaturated (more alum than can remain dissolved)
• Very unstable → crystallizes easily — leave undisturbed overnight
• Some crystals of solid alum form at bottom of cup when solution cools down
• Again filter → crystals on cloth act as 'seeds' to start new crystal growth
• Tie a nylon thread around the largest, best-formed crystal and suspend it in alum water
• After a few days → a good-sized crystal forms!
Supersaturated Solution: A solution that has more solid dissolved in it than it can normally hold. Very
unstable — crystallizes easily.
SECTION 7 — New Words, Now You Know & Our
Heritage (Pg 91)
7.1 New Words (Glossary)
Term
Definition
Exothermic
Reactions
Reactions during which heat is released
Endothermic
Reactions
Reactions in which heat is absorbed
Oxidizing Agents
Substances which oxidize other substances by providing oxygen to them or
by removing hydrogen from them
Reducing Agents
Substances which reduce other substances by removing oxygen from them
or by supplying hydrogen to them
Crystal
A solid in which atoms or molecules are arranged in a regular pattern
Effervescence
Appearance of bubbles due to evolution of gas during a chemical reaction
Precipitate
An insoluble solid formed when two solutions react
Galvanization
Process of depositing a layer of zinc on iron to prevent rusting
Crystallization
Process of obtaining crystals of a substance from its solution in water
Supersaturated
Solution
A solution that has more solid dissolved than it can normally hold; very
unstable
Reactants
Original substances participating in a chemical reaction
Products
New substances formed in a chemical reaction
7.2 Now You Know (Summary Points from Textbook)
•
•
•
•
•
In a physical change, a substance undergoes changes only in its physical properties such as
shape, size, colour and state, and no new substances are formed.
In a chemical change, two substances react chemically to form new substances with different
properties.
Rusting of iron occurs in the presence of oxygen and water.
Chemical reactions are characterized by one or more of the following: evolution of gas, change
of colour, formation of precipitate, energy changes and change of state.
Heat is released during exothermic reactions. Heat is absorbed during endothermic reactions.
7.3 Our Heritage — Iron Pillar (Important for Exam!)
⚠️ Our Heritage
The famous Iron Pillar near Qutb Minar in Delhi and other iron implements show that ancient
Indians (especially from the time of the Guptas, 300–500 CE) knew how to produce iron that did
NOT corrode.
This is due to the presence of HIGHER THAN NORMAL AMOUNTS OF PHOSPHORUS in the
iron.
This enabled the formation of a rust-resistant coating on the iron surface on exposure to air.
The coating prevented further corrosion.
SECTION 8 — All Oral Class Discussion Questions
ANSWERED
Oral Questions Set 1 (Pg 84) — Physical vs Chemical Change
Q: In which of these do the molecules of substances undergo changes — physical change
or chemical change?
Ans: In a CHEMICAL CHANGE, molecules of substances undergo changes. During a chemical
reaction, the molecules of the reactants change to form molecules of the products (chemical
transformation). In a physical change, only the physical form changes — the molecules
themselves do NOT change.
Q: What kind of change does this represent? Reactants → Products
Ans: This represents a CHEMICAL CHANGE (chemical reaction). Reactants are original
substances; products are the new substances formed. The arrow shows the direction of the
chemical reaction.
Q: What kind of change is: a. melting of wax?
Ans: Melting of wax is a PHYSICAL CHANGE. Wax changes from solid to liquid — only state
changes. Chemical composition remains the same. The change is REVERSIBLE — liquid wax
solidifies back on cooling. No new substance is formed.
Q: What kind of change is: b. burning of wax?
Ans: Burning of wax is a CHEMICAL CHANGE. Wax reacts with oxygen to produce CO₂ and
H₂ O — completely NEW substances. The change is IRREVERSIBLE. Heat and light are
produced (exothermic). NOTE: A burning candle shows BOTH — melting of wax (physical) AND
burning of wax (chemical).
Oral Questions Set 2 (Pg 88) — Types of Reactions
Q: Two chemicals in solution form react, forming an insoluble solid that settles down.
What is the settled solid known as?
Ans: The insoluble solid that settles down is called a PRECIPITATE. Formation of a precipitate is
a characteristic of chemical reactions and is shown by a downward arrow (↓) in chemical
equations.
Q: In which reaction will heat be released — exothermic reaction or endothermic reaction?
Ans: Heat will be RELEASED in an EXOTHERMIC reaction. The word 'exo' means out — heat
goes out to surroundings. '+heat' is written on the right-hand side of the equation. (Example:
burning of fuel, rusting of iron.)
Q: Nitrogen + oxygen → nitrogen dioxide: What type of reaction is this?
Ans: This is an ENDOTHERMIC reaction. Nitrogen and oxygen are heated at high temperature to
form nitric oxide (NO). Heat is ABSORBED in this reaction, so it is endothermic. Heat is written on
the LEFT-hand side (or '–heat' shown).
Q: What is the characteristic of the reaction: MN + PQ → MQ + PN? What is the general
name given to such reactions?
Ans: In this reaction, two compounds exchange their elements/groups: MN and PQ exchange to
give MQ and PN. This is a DOUBLE DISPLACEMENT reaction. The two types of double
displacement reactions are: (1) Precipitation reactions and (2) Neutralisation reactions.
Q: Which is true for a substance that is oxidized — oxygen is added to it, hydrogen is
removed from it, or both are true?
Ans: BOTH are true! A substance is said to be oxidized if: (a) oxygen is ADDED to it, OR (b)
hydrogen is REMOVED from it. Both these processes represent oxidation.
Oral Questions Set 3 (Pg 91) — Crystallization
Q: The atoms or molecules of a solid are always arranged in a regular pattern. Do you
agree? Give reasons.
Ans: YES, I agree — but ONLY for crystalline solids. In crystals, atoms or molecules are arranged
in a regular, repeating, orderly pattern. This gives crystals their characteristic geometric shapes.
For example, sugar and salt crystals have distinct shapes. However, amorphous solids (like
glass) do not have a regular pattern.
Q: You are given a solution of a solid and a liquid. How can you obtain the solid from it?
Ans: You can obtain the solid from the solution by the process of EVAPORATION. Heat the
solution so that the liquid evaporates completely. The solid is left behind as a residue. Example:
heating salt water leaves behind salt crystals.
Q: What is the process of obtaining crystals of a substance from its solution in water
called?
Ans: The process of obtaining crystals of a substance from its solution in water is called
CRYSTALLIZATION. The solution is first made saturated/supersaturated, and then as it cools,
crystals of the solid form. Example: growing alum crystals from an alum solution.
SECTION 9 — Exercise Answers (Pg 92–94)
A. Multiple Choice Questions (MCQ)
1. ✔ Answer: a — A substance with different chemical properties is formed
In a chemical change, new substances with DIFFERENT chemical properties are formed. This is the
defining feature.
2. ✔ Answer: d — Breaking of a glass tumbler
Breaking glass changes only shape — no new substance forms, so it's a physical change.
IRREVERSIBLE because you cannot un-break glass. Rusting of iron (a) is a chemical change.
3. ✔ Answer: b — Formation of precipitate
An arrow pointing DOWNWARDS (↓) in a chemical equation shows the formation of a PRECIPITATE
(an insoluble solid that settles down).
4. ✔ Answer: a — Exothermic reaction
A chemical reaction in which heat is GIVEN OUT (released) to the surroundings is called an
EXOTHERMIC reaction. '+heat' on the right-hand side.
5. ✔ Answer: b — One only
In a combination reaction, two or more substances combine to form ONE single product. Pattern: A +
B → AB
6. ✔ Answer: a — A more reactive element replaces a less reactive element
In displacement reactions, the rule is: a MORE REACTIVE element always replaces a LESS
REACTIVE element from its compound.
7. ✔ Answer: d — Double displacement reaction
A neutralisation reaction is a special type of DOUBLE DISPLACEMENT reaction where an acid reacts
with a base to form salt and water.
8. ✔ Answer: c — Oxygen or removal of hydrogen
Oxidation involves: addition of OXYGEN OR removal of HYDROGEN from a substance. Both qualify
as oxidation.
9. ✔ Answer: b — Decomposition reaction
In a decomposition reaction (AB → A + B), there is only ONE reactant (the compound that breaks
apart). The compound is the only reactant.
10. ✔ Answer: a — A will replace B from a salt of B
If A is more reactive than B, then A will displace/replace B from B's salt. This is the rule of single
displacement reactions.
B. Very Short Answer Questions
Q: In which of these changes do substances undergo chemical transformation — physical
or chemical?
Ans: Chemical change. In a chemical reaction, molecules of reactants undergo changes to form
molecules of the products — this is chemical transformation.
Q: Rust is a mixture and does not have a chemical formula. True or false?
Ans: FALSE. Rust is a chemical compound — iron oxide (Fe₂ O₃ or Fe₂ O₃ ·xH₂ O). It has a
definite chemical formula and is NOT a mixture.
Q: The process of depositing zinc on iron to prevent rusting is called _____.
Ans: GALVANIZATION. Water pipes used in homes are made of galvanized iron.
Q: Which gas is given off when calcium carbonate is heated?
Ans: CARBON DIOXIDE (CO₂ ). CaCO₃ → CuO + CO₂ ↑ (decomposition reaction). The CO₂
turns lime water milky.
Q: When two soluble substances in solution react to give an insoluble substance, a _____
is formed.
Ans: PRECIPITATE. An insoluble solid that settles down, shown by a downward arrow (↓) in
equations.
Q: Heat is absorbed in an _____ reaction.
Ans: ENDOTHERMIC reaction. 'Endo' means within — heat goes into the reaction from the
surroundings.
Q: If an equation has '+heat' written on its right-hand side, what type of reaction is it?
Ans: EXOTHERMIC reaction. Heat on the right-hand side means heat is a product, i.e., heat is
released/given out.
Q: Identify what kind of reaction each of the following is: a. X + Y → XY b. XY → X + Y c.
AB + XY → AY + XB d. AB + X → AX + B e. AB + CD → AD + BC f. Acid + base → salt +
water
Ans: a. Combination reaction | b. Decomposition reaction | c. Double displacement reaction |
d. Single displacement reaction | e. Double displacement reaction | f. Neutralisation reaction
Q: An oxidizing agent provides _____ or removes _____ from a substance.
Ans: An oxidizing agent provides OXYGEN or removes HYDROGEN from a substance.
Q: What method will you use to get a pure liquid if it has dissolved impurities present in it?
Ans: EVAPORATION (or distillation). The liquid is evaporated/boiled and the pure vapour is
collected and condensed. The dissolved impurities are left behind as residue.
Q: Salt is obtained from sea water by crystallization. True or false?
Ans: TRUE. Sea water is collected in shallow ponds. Water evaporates in sunlight → saturated
salt solution → salt crystallizes out. The process involves both evaporation and crystallization.
Q: A supersaturated solution of a solid in liquid crystallizes easily. True or false?
Ans: TRUE. A supersaturated solution has MORE solid dissolved than it can normally hold. It is
very unstable and crystallizes easily when disturbed or cooled.
C. Short Answer Questions
Q: Why are water pipes galvanized?
Ans: Water pipes are galvanized to PREVENT RUSTING. Iron reacts with oxygen and water
(moisture) to form rust. When water flows through iron pipes, both oxygen and moisture are
always present, accelerating rusting. Galvanization (depositing a layer of zinc on iron) creates a
protective barrier that prevents iron from coming in contact with oxygen and water, thus
preventing rust.
Q: When water is frozen, it forms ice that has different properties. Why then do we say
freezing is a physical change?
Ans: Freezing of water is a physical change because: (1) No new substance is formed — both
water and ice have the same chemical composition (H₂ O), only the state changes (liquid →
solid). (2) The change is REVERSIBLE — ice can be melted back to water. (3) Only the physical
properties change (shape, state), not the chemical composition. The 'different properties' of ice
(hardness, density) are physical, not chemical.
Q: Give one example each of a chemical reaction showing: a. evolution of a gas b. change
of colour c. formation of precipitate d. change of state from liquid to gas e. change of
state from gas to liquid
Ans: a. Evolution of gas: CaCO₃ + 2HCl → CaCl₂ + H₂ O + CO₂ ↑ (CO₂ gas evolved) b.
Change of colour: CuCO₃ heated → CuO (green carbonate becomes black oxide) c. Formation
of precipitate: NaCl + AgNO₃ → AgCl↓ + NaNO₃ (white silver chloride ppt) d. Liquid to gas:
2H₂ O(l) → 2H₂ (g)↑ + O₂ (g)↑ (electrolysis of water) e. Gas to liquid: 2H₂ (g) + O₂ (g) → 2H₂ O(l)
(hydrogen burns in oxygen to form liquid water)
Q: Distinguish between endothermic reactions and exothermic reactions.
Ans: Exothermic Reactions: Reactions in which heat is RELEASED to the surroundings. The
surroundings become warmer. Heat is written on the RIGHT-HAND side of the equation as
'+heat'. Examples: burning of fuel, rusting of iron, neutralisation. | Endothermic Reactions:
Reactions in which heat is ABSORBED from the surroundings. The surroundings become cooler.
Heat is written on the LEFT-HAND side (or '−heat'). Examples: photosynthesis, carbon + sulphur
→ CS₂ , nitrogen + oxygen → NO.
Q: What is a combination reaction? Give an example.
Ans: A combination reaction is one in which a compound is formed from its elements or from
simpler substances. Two or more substances combine to form a SINGLE product. Pattern: A + B
→ AB. Example: Magnesium burns in oxygen: 2Mg + O₂ → 2MgO. Here magnesium and oxygen
(two elements) combine to form magnesium oxide (one compound). It also burns with a dazzling
white flame (exothermic).
Q: When is a substance said to be reduced in a chemical reaction?
Ans: A substance is said to be REDUCED in a chemical reaction when: (a) OXYGEN is removed
from it, OR (b) HYDROGEN is added to it. Example: In H₂ + CuO → Cu + H₂ O, copper oxide
(CuO) loses oxygen → copper oxide is REDUCED to copper. The substance that causes
reduction is called the REDUCING AGENT.
Q: What is crystallization?
Ans: Crystallization is the process of obtaining crystals of a substance from its solution in water.
The substance is first dissolved in water to make a saturated/supersaturated solution. On cooling
or on allowing the solution to evaporate slowly, the dissolved solid comes out of solution as
crystals. Crystals are solids in which atoms or molecules are arranged in a regular, repeating
pattern. Different substances form crystals of different shapes.
Q: What is a supersaturated solution?
Ans: A supersaturated solution is a solution that contains MORE dissolved solid than it can
normally hold at that temperature. It is very unstable. Even a slight disturbance causes the
excess solid to crystallize out. A supersaturated solution is prepared by dissolving a substance in
hot water and then carefully cooling the solution without disturbing it.
D. Long Answer Questions
Q: Discuss the methods by which rusting of iron can be prevented.
Ans: Rusting of iron (formation of iron oxide) occurs in the presence of oxygen AND water. It can
be prevented by the following methods: (1) Painting or varnishing: Applying paint or varnish
creates a barrier that prevents contact with moisture and oxygen. However, if the coating is
damaged, rusting starts again. (2) Applying oil or grease: Coating with oil or grease prevents
contact with moisture. Used for tools and machinery. (3) Coating with zinc or chromium: Coating
iron with a more protective metal creates a barrier. Chrome plating is an example. (4)
Galvanization: Depositing a layer of ZINC on iron. This is the most effective and common method.
Water pipes used in homes are made of galvanized iron.
Q: Which gas is given off when dilute hydrochloric acid is added to calcium carbonate?
How will you test the gas? Write the chemical equations for both reactions.
Ans: Gas given off: CARBON DIOXIDE (CO₂ ). Equation 1 (producing CO₂ ): CaCO₃ + 2HCl →
CaCl₂ + H₂ O + CO₂ ↑ (Calcium carbonate + Hydrochloric acid → Calcium chloride + Water +
Carbon dioxide). Testing the gas: Pass the CO₂ through lime water [Ca(OH)₂ ]. Equation 2
(testing CO₂ with lime water): Ca(OH)₂ + CO₂ → CaCO₃ ↓ + H₂ O. The lime water turns MILKY
due to the formation of white, insoluble calcium carbonate — this confirms the gas is CO₂ .
Additional test: CO₂ does not support combustion — a lighted candle gets extinguished.
Q: Explain the steps involved in preparing the base magnesium hydroxide from the metal
magnesium.
Ans: Step 1 — Combination reaction (burning magnesium): Take a small piece of magnesium
ribbon. Clean its tip with sandpaper (to remove oxide layer). Hold it with tongs and bring it near a
flame. Magnesium burns with a dazzling white flame: 2Mg + O₂ → 2MgO. A powdery white ash
— magnesium oxide (MgO) — is formed. Step 2 — Dissolving in water: Collect the ash (MgO)
and mix with a small amount of water. Magnesium oxide dissolves in water to form magnesium
hydroxide: 2MgO + 2H₂ O → 2Mg(OH)₂ . Test with litmus paper — solution turns red litmus blue,
confirming it is a BASE.
Q: A decomposition reaction is the opposite of a combination reaction. Explain with an
example.
Ans: Combination Reaction: Two or more substances COMBINE to form a single product.
Pattern: A + B → AB. Example: 2Mg + O₂ → 2MgO (two substances combine into one).
Decomposition Reaction: A single compound BREAKS UP into two or more substances. Pattern:
AB → A + B. Example: CuCO₃ → CuO + CO₂ ↑ (one substance breaks into two). They are exact
opposites — combination builds up; decomposition breaks down. Decomposition is normally
caused by heating or by passing electricity through the compound.
Q: Under which two conditions do decomposition reactions normally take place? Give one
example of each.
Ans: Decomposition reactions take place under two conditions: (1) Heating (Thermal
Decomposition): A compound breaks up when heated. Example: Calcium hydroxide → Calcium
oxide + Steam: Ca(OH)₂ → CaO + H₂ O. Also: Copper carbonate → Copper oxide + CO₂ :
CuCO₃ → CuO + CO₂ ↑. (2) Passing electricity (Electrolysis): A compound breaks up when an
electric current is passed through it. Example: Electrolysis of water (with dilute sulphuric acid):
2H₂ O(l) → 2H₂ (g)↑ + O₂ (g)↑.
Q: What is a displacement reaction? Which rule governs displacement of an element in a
compound by another element? Explain with an example.
Ans: A displacement reaction is one in which one element REPLACES another element from a
compound. Pattern: A + BC → AC + B. Rule governing displacement: A MORE REACTIVE
element always replaces a LESS REACTIVE element from its compound. A less reactive element
CANNOT replace a more reactive one. Example: Iron (more reactive than copper) displaces
copper from copper sulphate solution: CuSO₄ + Fe → FeSO₄ + Cu. Iron sulphate is formed and
copper metal is deposited on the iron piece. Another example: Copper displaces silver from silver
nitrate: 2AgNO₃ + Cu → Cu(NO₃ )₂ + 2Ag (solution turns bluish — copper nitrate formed; silver
deposited).
Q: What is a double displacement reaction? How many types of double displacement
reactions do you know of? Give one example of each.
Ans: A double displacement reaction is one in which two compounds react by EXCHANGING
their elements or groups. Pattern: AB + CD → AD + CB. Two types: (1) Precipitation Reactions:
Two compounds in solution react to form an insoluble solid (precipitate). Example: NaCl +
AgNO₃ → AgCl↓ + NaNO₃ . White precipitate of silver chloride forms. (2) Neutralisation
Reactions: An acid reacts with a base to form salt + water. Example: H₂ SO₄ + 2NaOH →
Na₂ SO₄ + 2H₂ O. Sulphuric acid + sodium hydroxide → sodium sulphate (salt) + water.
Q: Explain the terms — oxidation and oxidizing agent.
Ans: Oxidation: A chemical reaction that involves ADDITION OF OXYGEN or REMOVAL OF
HYDROGEN from a substance. If a substance gains oxygen, it is oxidized. If a substance loses
hydrogen, it is oxidized. Example 1: C + O₂ → CO₂ (carbon is oxidized by gaining oxygen).
Example 2: H₂ S + Cl₂ → S + 2HCl (H₂ S is oxidized by losing hydrogen). Oxidizing Agent: A
substance that OXIDIZES other substances by: (a) PROVIDING oxygen to them, OR (b)
REMOVING hydrogen from them. In C + O₂ → CO₂ , OXYGEN is the oxidizing agent (it provides
oxygen to carbon). In H₂ S + Cl₂ → S + 2HCl, CHLORINE is the oxidizing agent (it removes
hydrogen from H₂ S).
SECTION 10 — HOTS Questions (Think and Answer)
(Pg 94)
Q: In which city do you expect rusting to be a greater problem — in Delhi or in Mumbai?
Why?
Ans: Rusting would be a GREATER PROBLEM in MUMBAI. Rusting requires BOTH oxygen AND
water (moisture). Mumbai is a coastal city with very HIGH HUMIDITY (moisture in air) throughout
the year due to its proximity to the sea. Delhi has a much drier climate (lower humidity), especially
in winter and summer. Since Mumbai has far more moisture in the air, iron objects rust much
faster there. The salt in sea air near Mumbai can also speed up the rusting process.
Q: When a candle burns, both physical and chemical changes occur. What are these
changes?
Ans: Physical Change: When a candle burns, the wax near the flame MELTS — it changes from
solid to liquid state. This is a physical change because: the chemical composition of wax does not
change; only the state (solid → liquid) changes; it is reversible — liquid wax can solidify again.
Chemical Change: The melted wax then BURNS in the presence of oxygen. This is a chemical
change because: wax reacts with O₂ to produce CO₂ and H₂ O (water vapour) — these are
NEW substances; heat and light are released (exothermic); the change is irreversible. So the
SAME burning candle shows BOTH types of change simultaneously!
Q: LPG (liquefied petroleum gas) in a gas cylinder is in the form of a liquid. Which physical
and chemical changes occur when it comes out of the cylinder and burns?
Ans: Physical Change: When LPG comes out of the cylinder, it changes from LIQUID to GAS
state (liquid → gas = vaporization/evaporation). This is a physical change — no new substance
formed, just a change of state. Chemical Change: When the LPG (hydrocarbon gas) burns in
oxygen, it undergoes COMBUSTION (a chemical change). New substances — CO₂ and H₂ O —
are formed. Heat and light are released (exothermic reaction). The change is irreversible.
Q: When a soda water bottle is opened, a gas is released. Does a chemical reaction occur?
Explain.
Ans: When a soda water bottle is opened, CO₂ gas (dissolved under pressure) is released. This
is a PHYSICAL change, NOT a chemical reaction. The CO₂ was dissolved in water under high
pressure. When the bottle is opened, pressure is released and CO₂ comes out of solution. No
new substance is formed — CO₂ was already there, just dissolved. The gas escaping (change of
state from dissolved to gaseous) is physical. However, the original DISSOLVING of CO₂ in water
to make carbonic acid (H₂ CO₃ ) was a mild chemical change.
Q: Why should we not put stainless steel spoons in pickle jars?
Ans: Pickles contain ACIDS (like acetic acid in vinegar or lactic acid). Stainless steel, while
corrosion-resistant, can still react with strong acids over time. More importantly, the acid can react
with the metal components in stainless steel (especially chromium and nickel) — a chemical
change — which could: (1) release metallic ions into the food making it harmful, (2) corrode the
spoon over time, (3) change the taste of the pickle. This is a chemical reaction between acid and
metal — similar to displacement reactions where acids react with metals.
SECTION 11 — Quick Revision Summary ⭐ MOST
IMPORTANT
⚠️ CHAPTER AT A GLANCE — Everything You Need to Know
PHYSICAL CHANGE: No new substance | Physical properties change | Usually reversible | E.g.
melting, cutting, boiling, dissolving
CHEMICAL CHANGE: New substances formed | Chemical composition changes | Usually
irreversible | E.g. burning, rusting, cooking, photosynthesis
RUSTING: Iron + O₂ + H₂ O → Iron Oxide | Slow oxidation | Prevented by galvanization, paint,
oil
CHARACTERISTICS OF CHEMICAL REACTIONS: (1) Evolution of gas | (2) Change of colour |
(3) Formation of precipitate | (4) Change in energy (exothermic/endothermic) | (5) Change of
state
TYPES OF REACTIONS:
1. Combination: A + B → AB (e.g., 2Mg + O₂ → 2MgO)
2. Decomposition: AB → A + B (e.g., CaCO₃ → CuO + CO₂ )
3. Single Displacement: A + BC → AC + B (e.g., CuSO₄ + Fe → FeSO₄ + Cu)
4. Double Displacement: AB + CD → AD + CB
(a) Precipitation: NaCl + AgNO₃ → AgCl↓ + NaNO₃
(b) Neutralisation: Acid + Base → Salt + Water
5. Oxidation: Addition of O₂ OR Removal of H₂
6. Reduction: Removal of O₂ OR Addition of H₂
EXOTHERMIC: Heat released | +heat on RIGHT side | E.g. burning, rusting
ENDOTHERMIC: Heat absorbed | +heat on LEFT side | E.g. photosynthesis, N₂ + O₂ → NO
SEPARATING SOLID FROM SOLUTION: (1) Evaporation — heat solution till water evaporates |
(2) Crystallization — cool supersaturated solution to get crystals
GALVANIZATION: Depositing ZINC on iron to prevent rusting
CRYSTAL: Solid with atoms in a regular pattern | Crystallization = process of getting crystals
from solution
ALL THE BEST FOR YOUR EXAM! �