Get authentic, comprehensive, and scoring-oriented solutions for DAV Class 6 Science Chapter 5: Changes Around Us from the official DAV textbook The Living World. Here you will find step-by-step verified answers to all questions in Something To Know (Sections A, B, C, D, E), structured 3-column physical vs chemical change comparison tables, Value-Based Questions, and authentic DAV Board Examination Questions (PYQs).
🔄 Chapter 5 Quick Revision & Key Concepts Summary:
- Change: An alteration in the shape, size, state, position, temperature, or chemical composition of an object.
- Reversible vs Irreversible: Reversible change can be reversed to obtain original material back (e.g., melting of ice, stretching of rubber band). Irreversible change cannot be reversed; original material cannot be recovered (e.g., burning of paper, curdling of milk).
- Physical vs Chemical: Physical change alters only physical properties without forming any new chemical substance (e.g., cutting vegetables, boiling water). Chemical change forms one or more entirely new substances with completely different chemical properties (e.g., rusting of iron, cooking of food, burning of fuel).
- Slow vs Fast Changes: Slow change takes hours, days, or years to occur (e.g., rusting of iron, germination of seed, change of seasons). Fast change takes place within seconds or minutes (e.g., burst of a firecracker, burning of matchstick, lightning).
- Periodic vs Non-periodic: Periodic change repeats at regular intervals of time (e.g., phases of moon, day and night, heartbeat). Non-periodic change does not repeat at regular intervals (e.g., earthquakes, forest fires, road accidents).
- Energy in Changes: Endothermic change absorbs heat energy (e.g., melting of chocolate/ice, evaporation). Exothermic change releases heat and light energy (e.g., burning of fuels, bursting firecrackers).
Comprehensive 3-Column Comparison: Physical vs Chemical Change
| Feature / Parameter | Physical Change | Chemical Change |
|---|---|---|
| 1. Formation of New Substance | No new substance is formed | One or more entirely new substances are formed |
| 2. Reversibility | Usually reversible (temporary change) | Usually irreversible (permanent change) |
| 3. Chemical Composition | Chemical composition of the original substance remains unchanged | Chemical composition changes completely with new bonds |
| 4. Energy Change | Small amount of energy absorbed or evolved | Large amount of energy usually absorbed or released |
| 5. Typical Examples | Melting of ice, tearing paper, dissolving salt in water | Burning paper, rusting iron, milk turning into curd |
A. Fill in the blanks.
1. All irreversible changes bring a permanent change in a substance.
Explanation: Once an irreversible change occurs, the original substance cannot be reformed by reversing the conditions.
2. A change, in which the substance produced can get back to the original form, is called a reversible change.
Explanation: Reversible changes are temporary and do not alter the fundamental chemical identity of the substance.
3. Bending of an iron rod is a physical change.
Explanation: Bending changes only the shape and geometry of the rod; the iron material remains chemically identical.
4. Burning of fuel is a fast chemical change.
Explanation: Combustion of fuel (such as LPG or petrol) takes place rapidly, producing heat, light, carbon dioxide, and water vapour within seconds.
5. In a chemical change, new substances with different properties are formed.
Explanation: Chemical reactions reorganize atoms, forming new products whose physical and chemical properties differ completely from the reactants.
B. Write True or False for the following statements.
1. Glowing of an electric bulb is a fast change.
Answer: True
Explanation: The tungsten filament heats up and emits bright light immediately upon closing the electric switch.
2. Deforestation is a reversible change.
Answer: False
Correction: Deforestation destroys centuries-old forest ecosystems, topsoil, and biodiversity; it is an irreversible ecological change.
3. Burning of paper is a temporary change.
Answer: False
Correction: Burning paper produces ash, smoke, and gases; it is a permanent chemical change.
4. Cutting of an apple is a chemical change.
Answer: False
Correction: The physical act of slicing an apple into pieces is a physical change (though the subsequent browning when exposed to air is chemical).
5. Making of fruit salad is a chemical change.
Answer: False
Correction: Mixing chopped fruits creates a heterogeneous physical mixture without chemical reaction, so it is a physical change.
6. Changing milk into curd is a physical change.
Answer: False
Correction: Lactic acid bacteria ferment lactose into lactic acid, forming a new sour substance; therefore, curdling is a chemical and irreversible change.
C. Tick the correct option.
1. Which of the following can be considered as a fast change?
(a) rusting of iron
(b) burst of a cracker
(c) growth of a child
(d) formation of curd
Answer: (b) burst of a cracker
Reason: An explosive chemical reaction occurs instantaneously within a fraction of a second.
2. Evaporation of water is a—
(a) physical and reversible change
(b) physical and irreversible change
(c) chemical and reversible change
(d) chemical and irreversible change
Answer: (a) physical and reversible change
Reason: Water changes from liquid to vapour without new substance formation, and water vapour can be condensed back into liquid water.
3. Which one of these changes is a reversible change?
(a) baking of a cake
(b) tearing of a paper
(c) melting of ice
(d) cooking of food
Answer: (c) melting of ice
Reason: Water obtained from melted ice can be refrozen into solid ice by cooling below 0°C.
4. Which one of these is an irreversible change?
(a) stretching of a rubber band
(b) knitting of a sweater from yarn
(c) blooming of a flower
(d) inflating a balloon
Answer: (c) blooming of a flower
Reason: Once a flower opens from a bud, it can never revert back into a closed bud.
5. A piece of paper undergoes a chemical change when it is—
(a) folded
(b) torn
(c) burnt
(d) rolled
Answer: (c) burnt
Reason: Burning paper combines cellulose with oxygen, producing carbon dioxide, water vapour, and ash.
D. Answer the following questions in brief.
1. Define the following terms:
(a) Reversible change: A change in which the original substance can be recovered by reversing the conditions (e.g., melting of ice, stretching of a spring).
(b) Chemical change: A change in which one or more new substances with entirely different chemical properties and composition are formed (e.g., rusting of iron, burning of wood).
(c) Physical change: A change in which only the physical properties (shape, size, state) of a substance alter without any new substance being formed (e.g., melting of wax, dissolving sugar in water).
2. Distinguish between fast and slow changes. Give two examples of each type.
Answer:
• Fast Change: Changes that take place over very short intervals of time (seconds or minutes).
Examples: (1) Burning of a matchstick; (2) Bursting of a balloon.
• Slow Change: Changes that take a long time (hours, days, months, or years) to complete.
Examples: (1) Rusting of an iron nail; (2) Growth of a tree from a sapling.
3. Give two examples each of reversible and irreversible change.
Answer:
• Reversible changes: (1) Melting of butter on a warm pan; (2) Freezing of water into ice.
• Irreversible changes: (1) Cooking of vegetables; (2) Curdling of milk.
4. State some of the irreversible biological changes that take place in nature.
Answer:
1. Growth of living organisms: Growth of a seedling into a tall banyan tree, and growth of a human baby into an adult.
2. Ripening of fruits: A green, sour raw mango ripening into a sweet, yellow mature fruit.
3. Yellowing and falling of leaves: Autumn leaves shedding from deciduous trees cannot rejoin branches.
4. Ageing and death of animals: Natural biological ageing processes in animals and humans.
E. Answer the following questions.
1. Give two differences between reversible and irreversible changes.
Answer:
| Parameter | Reversible Change | Irreversible Change |
|---|---|---|
| 1. Recovery of Original State | Original substance can be easily obtained by reversing conditions | Original substance cannot be recovered by any physical method |
| 2. Permanency | Temporary change | Permanent change |
| 3. Nature of Products | No new chemical products are formed | New products with different properties are produced |
| 4. Examples | Melting of ice, evaporation of water, glowing bulb | Burning paper, cooking chapati, digestion of food |
2. ‘Formation of curd is an irreversible as well as a chemical change.’ Justify this statement.
Answer: The conversion of milk into curd is justified as both a chemical and irreversible change due to the following scientific facts:
1. Why it is a Chemical Change: When a small amount of starter curd is added to lukewarm milk, Lactobacillus bacteria ferment the milk sugar (lactose) into lactic acid. The lactic acid coagulates milk proteins, producing curd. Curd has entirely different chemical composition, taste, and texture from milk; an entirely new substance is formed.
2. Why it is an Irreversible Change: Once milk is transformed into curd, no physical or chemical process can revert the sour curd back into liquid sweet milk. It is a permanent change.
3. Water cycle is a natural change. Classify it as a (i) physical or chemical change (ii) a reversible or irreversible change. Justify your answer.
Answer:
• (i) Classification as Physical Change: The water cycle is a physical change. Throughout the cycle, water transitions between solid (snow/hail), liquid (rain, river, sea), and gas (water vapour/clouds) without altering its fundamental chemical molecule (H2O). No new chemical substance is formed at any step.
• (ii) Classification as Reversible Change: The water cycle is a reversible change. Liquid water evaporates into water vapour due to solar heat, and this vapour condenses back into liquid water drops when cooled in the upper atmosphere. Because the process operates in a closed continuous loop, it is reversible.
4. How can we say that burning of candle is a chemical change but the glowing of bulb is a physical change?
Answer:
• Burning of a Candle (Chemical Change): When the wick of a candle burns, the molten wax acts as fuel and undergoes combustion with atmospheric oxygen. It produces entirely new chemical substances—carbon dioxide gas, water vapour, soot (carbon particles), and releases heat and light energy. The burnt wax is consumed permanently and cannot be recovered.
• Glowing of an Electric Bulb (Physical Change): When electric current passes through the bulb, the tungsten filament becomes red-hot and glows due to high electrical resistance. When the switch is turned off, the filament cools down and returns to its original appearance and properties without any loss of mass or formation of any new chemical substance.
Value-Based Questions (VBQ) & HOTS (Higher Order Thinking Skills)
💡 Value-Based Question (VBQ):
Scenario: Aman’s father noticed that the wooden wheels of their ancestral bullock cart had become loose. Instead of throwing them away, a blacksmith heated an iron rim until it expanded, slipped it smoothly over the wooden wheel, and poured cold water. The cooled iron rim contracted tightly around the wood.
Values & Scientific Principle:
1. Valuing Traditional Technology & Sustainability: Repairing tools using scientific methods saves resources and prevents wasteful disposal.
2. Scientific Application: Metals undergo expansion on heating (physical change) and contraction on cooling (reversible physical change), creating an unbreakable mechanical grip.
🧠 Higher Order Thinking Skill (HOTS):
Question: When a candle burns, both physical and chemical changes take place simultaneously. Identify and explain both changes clearly.
Dual Changes in Burning Candle:
1. Physical Change: The heat of the flame causes solid wax near the wick to melt into liquid wax. Molten wax trickles down and solidifies again into wax. Because it is only a change of physical state from solid to liquid and back, with no new substance formed, melting of wax is a physical change.
2. Chemical Change: Liquid wax is drawn up the wick by capillary action and vaporizes. Wax vapour burns in air (combustion), reacting with oxygen to produce carbon dioxide, water vapour, heat, and light. Because wax is permanently converted into new gaseous products that cannot be reconstituted into wax, burning of wax vapour is a chemical change.
🎯 DAV Examination Corner: Authentic Board Exam Questions (PYQs 2022–23 & 2023–24)
Below are authentic questions directly retrieved from the DAV Public Schools Common Examination (Half-Yearly & Annual Board Papers) covering Chapter 5:
Q1. [DAV Half-Yearly Exam 2023–24 • 1 Mark VSA]
A cauliflower is cut into pieces but there is no new product formed. Identify the change.
Model Answer: Physical change. Cutting alters only the size and physical shape of the vegetable; no new chemical substance is formed.
Q2. [DAV Half-Yearly Exam 2023–24 • 1 Mark MCQ]
Melting of Cadbury chocolate shows the type of change. Choose the correct answer: (a) Energy absorbed (b) Energy released
Model Answer: (a) Energy absorbed. Melting is an endothermic physical change requiring heat absorption from surroundings to transition solid fat into liquid.
Q3. [DAV Half-Yearly Exam 2023–24 • 2 Marks SA-I]
Some sugar was heated in a porcelain dish using a burner.
(a) What type of change is this?
(b) Illustrate another example of the same type of change.
Model Answer:
(a) Heating sugar strongly causes charring, turning it into black carbon and water vapour. This is an irreversible chemical change.
(b) Another example is the burning of paper or piece of wood into black ash.
Q4. [DAV Half-Yearly Exam 2023–24 • 3 Marks SA-II]
Differentiate between slow and fast changes and give appropriate examples in each case.
Model Answer:
• Slow Change: Changes that take a long time (hours, days, months, or years) to complete. Examples: Rusting of iron, curdling of milk, growth of human hair.
• Fast Change: Changes that occur very quickly, within seconds or minutes. Examples: Burning of a matchstick, bursting of a firecracker, switching on an electric light bulb.
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