CBSE Class 7 Science Chapter 05 Changes Around Us Physical And Chemical MCQs Set 03

Science Objective Questions and Answers: Chapter 05 Changes Around Us Physical And Chemical

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Question: When a sheet of paper is torn into small pieces, the original paper cannot be reassembled into its former shape. What type of change is this, and why?
A. A physical change, because no new substance is formed, even though the shape has altered
B. A chemical change, because the paper's appearance has changed significantly
C. A reversible change, because paper can theoretically be pressed back together
D. An undesirable change, because it damages a useful material
Show Answer & Explanation

Answer: (A) A physical change, because no new substance is formed, even though the shape has altered

Explanation:
Tearing paper changes only its physical property (shape and size), but the material itself—paper—remains paper. No new substance is formed through a chemical reaction, making it a physical change. The chapter explicitly explains that physical changes involve alterations in shape, size, or state while the substance stays the same.

Question: During the lime water test, why does the liquid turn milky or cloudy when air is blown through it?
A. Oxygen from the exhaled air mixes with lime water and creates visible bubbles
B. Carbon dioxide from exhaled air reacts with lime water to form an insoluble white substance
C. The lime water temperature drops when exposed to cool exhaled air
D. Pressure from the air causes lime water molecules to reorganize into a cloudlike state
Show Answer & Explanation

Answer: (B) Carbon dioxide from exhaled air reacts with lime water to form an insoluble white substance

Explanation:
The chapter states that carbon dioxide from exhaled air reacts with lime water (calcium hydroxide) to produce calcium carbonate, an insoluble white substance that makes the liquid appear milky. This is presented as proof of a chemical reaction forming a new substance.

Question: A candle is lit and burns steadily. Which of the following best describes the sequence of changes the wax undergoes?
A. The wax evaporates directly without melting first, then burns in the flame
B. The wax melts, flows up the wick through capillary action, evaporates due to heat, and the vapour burns
C. The wax solidifies again immediately after melting, creating a continuous cycle
D. The wax burns directly without any intermediate melting or evaporation stage
Show Answer & Explanation

Answer: (B) The wax melts, flows up the wick through capillary action, evaporates due to heat, and the vapour burns

Explanation:
• The chapter explains that candle wax melts from the heat of the flame
• The liquid wax travels up the wick
• Heat from the flame causes the wax to evaporate into vapour
• This vapour is what actually burns, producing the flame
This sequence demonstrates why both physical changes (melting, evaporation) and chemical change (combustion) occur in the single process of burning a candle.

Question: In Activity 5.6, when sunlight is focused through a magnifying glass onto paper, the paper eventually catches fire without any initial flame touching it. What concept does this experiment illustrate?
A. That heat alone, without oxygen, is sufficient to start combustion
B. That sunlight contains a chemical substance capable of burning paper directly
C. That a substance must reach a minimum temperature, called its ignition temperature, to catch fire
D. That paper is more flammable than other materials because it contains combustible compounds
Show Answer & Explanation

Answer: (C) That a substance must reach a minimum temperature, called its ignition temperature, to catch fire

Explanation:
The chapter defines ignition temperature as the lowest temperature at which a substance catches fire. The focused sunlight gradually heats the paper until it reaches paper's ignition temperature, at which point it ignites. The chapter notes that the matchstick example works faster because the matchstick is already hotter than paper's ignition temperature.

Question: According to the fire triangle, which of the following statements correctly identifies all three requirements for combustion to occur?
A. A fuel source, the presence of air, and bright light from any source
B. A combustible substance, oxygen gas, and heat sufficient to reach ignition temperature
C. Paper or wood as fuel, atmospheric oxygen, and a flame source
D. Any flammable material, nitrogen from the air, and environmental heat
Show Answer & Explanation

Answer: (B) A combustible substance, oxygen gas, and heat sufficient to reach ignition temperature

Explanation:
The fire triangle explicitly lists three requirements: a combustible substance (fuel), oxygen, and heat that allows the fuel to reach its ignition temperature. These are the only universal conditions needed—the chapter uses various examples (paper with sunlight, candle with flame, magnesium ribbon with air) to show that oxygen and heat are essential, though their sources may vary.

Question: Iron nails rusting is a commonly cited example. Why is this classified as a chemical change rather than simply a physical weathering of the surface?
A. Because the nails change colour and appearance over time
B. Because the surface texture becomes rough and corroded
C. Because a new substance, iron oxide (rust), with different properties is formed through a chemical reaction
D. Because the rusting process is slow and occurs naturally outdoors
Show Answer & Explanation

Answer: (C) Because a new substance, iron oxide (rust), with different properties is formed through a chemical reaction

Explanation:
Rusting forms a chemically distinct new substance—iron oxide—which is not simply iron that has changed appearance. The chapter explains that in chemical changes, one or more new substances are formed through a chemical reaction. Since rust has entirely different properties from the original iron, this confirms it is a chemical change.

Question: In the activity where a balloon is inflated and then pricked with a pin versus one that is deflated slowly by hand, what does comparing these two outcomes demonstrate?
A. That rubber is not a durable material for balloons
B. That the same material (rubber) can appear to undergo a different fate depending on how the change happens, but the original substance remains unchanged
C. That pricking a balloon causes a chemical change while deflating it causes a physical change
D. That the air inside the balloon becomes trapped if the balloon is punctured
Show Answer & Explanation

Answer: (B) That the same material (rubber) can appear to undergo a different fate depending on how the change happens, but the original substance remains unchanged

Explanation:
Both outcomes—pricking or deflating—return the balloon to its uninflated state, but the balloon material (rubber) is still rubber in both cases. This illustrates that physical changes can happen in different ways, yet no new substance is formed. The material's essential identity persists despite the change in form.

Question: When heating lemon juice and adding baking soda creates carbon dioxide gas, and this gas is then passed through lime water which turns milky, what can be concluded about the reaction that occurred?
A. A physical change took place because gas was produced
B. A chemical change occurred because a new substance (carbon dioxide) was formed that can be detected by its reaction with lime water
C. Both reactants simply changed state without forming anything new
D. The reaction produced only heat and light without any new matter
Show Answer & Explanation

Answer: (B) A chemical change occurred because a new substance (carbon dioxide) was formed that can be detected by its reaction with lime water

Explanation:
The chapter shows that mixing vinegar (or lemon juice) with baking soda produces carbon dioxide gas—a new substance not present before the reaction. The detection of this gas through the lime water test (turning milky) provides evidence of a chemical change. The chapter represents this as: Vinegar + Baking soda → Carbon dioxide + Other substances.

Question: Bud-bearing rose stems, brown spots on bananas, and the temperature of bottled water all change over time as noted at the chapter's opening. What is the primary purpose of presenting these varied examples at the start?
A. To show that only living things undergo changes in nature
B. To demonstrate that changes are observable in many different forms around us and lead into classifying what types of changes they represent
C. To prove that physical changes happen faster than chemical changes
D. To explain why some changes are desirable while others cause decay
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Answer: (B) To demonstrate that changes are observable in many different forms around us and lead into classifying what types of changes they represent

Explanation:
The opening examples represent a range of observable changes—physical (temperature change), chemical (browning/ripening), and growth-related (bud to flower). The chapter then asks students to consider 'What kinds of changes are they talking about?' establishing the foundation for the lesson's central task of categorizing changes into physical and chemical types.

Question: Synthetic blankets should never be used to extinguish a fire on someone's clothing. What is the scientific reason for this caution?
A. Synthetic materials burn hotter than natural fabrics and spread the fire faster
B. Synthetic materials can melt from the heat and stick to the skin, causing additional injury
C. Synthetic materials do not absorb moisture needed to cool the flames
D. Synthetic materials contain chemicals that react with fire and release toxic gases
Show Answer & Explanation

Answer: (B) Synthetic materials can melt from the heat and stick to the skin, causing additional injury

Explanation:
The chapter explicitly states in the 'Science and Society' box that synthetic blankets or cloth should never be used because 'these can melt and stick to the skin.' This caution emphasizes a material science principle about melting points and the hazard of melted polymer adhesion.

Question: Black basalt rock gradually develops a red-coloured surface layer when exposed to water or moist air over extended periods. What causes this colour change?
A. The black colour fades naturally over time due to UV exposure from sunlight
B. Iron in the basalt chemically reacts with oxygen or water to form iron oxide, which is red
C. Water physically washes away the black layer to reveal red rock underneath
D. Weathering physically breaks the rock into smaller pieces that appear reddish
Show Answer & Explanation

Answer: (B) Iron in the basalt chemically reacts with oxygen or water to form iron oxide, which is red

Explanation:
The chapter explains that basalt containing iron undergoes a chemical change when exposed to water or air with water vapour over time. The iron oxidizes to form iron oxide, producing the red-coloured layer. This is explicitly presented as a chemical change during weathering, not merely a physical alteration.

Question: Baking soda dissolves in water when stirred, but no fizzing or bubble formation occurs. Going by the distinction between physical and chemical changes, what type of change is this?
A. A chemical change, because the baking soda has disappeared from view
B. A physical change, because the baking soda particles mix and dissolve but no new substance with different properties is formed
C. A reversible change that cannot occur in the opposite direction
D. An example of combustion involving baking soda and water
Show Answer & Explanation

Answer: (B) A physical change, because the baking soda particles mix and dissolve but no new substance with different properties is formed

Explanation:
The chapter repeatedly contrasts the reaction of baking soda with vinegar (which produces carbon dioxide gas—a chemical change) with the dissolving of baking soda in water alone (no gas, no reaction). Dissolution is a physical change because the substance still dissolves but remains essentially baking soda; no new compound forms. The chapter hints at this distinction in the exploratory section.

Question: When cornhusks are heated and transformed into popcorn, this is a change that cannot be reversed. Which explanation best justifies why popcorn cannot return to its original corn kernel form?
A. The high temperature permanently damages the molecular structure, making reassembly impossible
B. Popcorn has expanded due to steam and pressure, rupturing the kernel coat permanently, and no reassembly process can restore the original tight structure
C. The starch in corn chemically converts to a different substance that cannot be converted back
D. Popcorn is less dense than kernels, so the missing material has been lost
Show Answer & Explanation

Answer: (B) Popcorn has expanded due to steam and pressure, rupturing the kernel coat permanently, and no reassembly process can restore the original tight structure

Explanation:
The chapter's discussion of irreversible changes emphasizes that some changes produce permanent structural alterations. In popcorn, heat causes moisture to turn to steam, building internal pressure that ruptures the kernel coat and causes permanent expansion. Unlike freezing and melting (which are reversible), or even burning (which is chemical but studied as irreversible), the physical rupture and expansion in popcorn creation cannot be undone—the original compact structure is destroyed.

Question: Wrapping a blanket around a person removes one critical element from the fire triangle. Which element does cutting off the air supply eliminate?
A. Heat, because the blanket insulates and cools the fire
B. Fuel, because the blanket prevents the clothing from reaching ignition temperature
C. Oxygen, because the blanket stops air from reaching the burning material
D. Light, because the blanket blocks the flame from being visible
Show Answer & Explanation

Answer: (C) Oxygen, because the blanket stops air from reaching the burning material

Explanation:
The fire triangle shown in the chapter lists three sides: fuel, oxygen, and heat. Wrapping with a blanket removes oxygen by blocking the air supply to the fire. The chapter explicitly states: 'This cuts off the supply of air, and the fire gets extinguished.' This demonstrates that without one of the three essential elements, combustion cannot continue.

Question: Fireflies produce light through a chemical change that occurs without generating heat. What is this special biological light-producing process called?
A. Combustion, which releases light energy instead of heat
B. Bioluminescence, a type of light production without heat in living organisms
C. Photosynthesis, which converts chemical energy to visible light
D. Ignition, which occurs at very low temperatures in insects
Show Answer & Explanation

Answer: (B) Bioluminescence, a type of light production without heat in living organisms

Explanation:
The chapter's 'Fascinating Facts' section identifies fireflies as insects that emit light produced by a chemical change. It specifically names this process 'bioluminescence,' defining it as 'light production (without heat) in living organisms.' This contrasts with typical combustion, which releases both heat and light.

Question: Wet clothes drying over time and ice melting into liquid water are both reversible physical changes. However, they involve different processes. Which pair correctly matches each change to its reverse?
A. Drying can be reversed by evaporating water again; melting can be reversed by freezing
B. Drying can be reversed by condensation; melting can be reversed by refreezing into ice
C. Drying can be reversed by heating; melting can be reversed by cooling
D. Drying reverses through freezing; melting reverses through heating
Show Answer & Explanation

Answer: (B) Drying can be reversed by condensation; melting can be reversed by refreezing into ice

Explanation:
The chapter explains that drying (evaporation) can be reversed by condensation, and melting (state change from solid to liquid) can be reversed by refreezing. These are presented as paired reversible processes—both are physical changes because no new substance forms, only state or physical form changes.

Question: A student reads that deposits of sand and soil collect at the base of mountains due to natural processes involving both rock breaking and material movement. What are these processes collectively called?
A. Erosion, the movement and deposition of broken rock fragments by wind and water
B. Weathering, the breaking down of rocks through physical and chemical processes that eventually leads to soil formation
C. Combustion, the burning away of rock surfaces to create sediment
D. Oxidation, the chemical reaction of rock minerals with oxygen to produce new sediments
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Answer: (B) Weathering, the breaking down of rocks through physical and chemical processes that eventually leads to soil formation

Explanation:
The chapter distinguishes between weathering (the breaking down of rocks through physical and chemical processes) and erosion (the movement of broken rock by natural forces). The accumulation of 'heaps of sand, soil, and stones' at mountain bases results from weathering breaking rocks apart into smaller pieces. The chapter explicitly states that weathering 'eventually leads to the formation of soil.'

Question: Among reversible versus irreversible changes, chopped vegetables are listed as an example that cannot be reversed. Why is this particular change classified as irreversible?
A. Because vegetables are living tissues that die when cut, preventing regrowth
B. Because once the cellular structure is cut apart, there is no practical way to restore the vegetable to its original size and shape
C. Because the vitamin content degrades when the vegetable is exposed to air after chopping
D. Because chemical reactions between cut cell surfaces produce new compounds
Show Answer & Explanation

Answer: (B) Because once the cellular structure is cut apart, there is no practical way to restore the vegetable to its original size and shape

Explanation:
The chapter notes in Table 5.2 that 'chopped vegetables cannot return to their original size and shape.' This illustrates that while the substance remains vegetable matter (not a chemical change), the physical damage—the cutting apart—cannot be undone. The severed cellular structure and pieces cannot reassemble into the whole vegetable, making this a permanent, irreversible physical change.

Question: Decomposition of food waste can be both desirable in some situations and undesirable in others. Which example best illustrates this dual nature?
A. Food spoils during storage (bad), but can be burned for fuel (good)
B. Food rots in the trash (bad), but transforms into compost in a controlled composting process (good)
C. Food cooks when heated (good), but burns if overheated (bad)
D. Food grows mold (bad), but yeast ferments it (good)
Show Answer & Explanation

Answer: (B) Food rots in the trash (bad), but transforms into compost in a controlled composting process (good)

Explanation:
The chapter explicitly explains: 'A change that is undesirable in some situations may be desirable in other situations. For example, decomposition of food can be very useful in converting food waste into compost.' Uncontrolled decay during storage is undesirable because it ruins food; the same decomposition process in a composting system is desirable because it creates useful soil amendment.

Question: During erosion, fine sand and pebbles collect on lake and river bottoms after being carried by flowing water. What happens to these deposited sediments over an extended period?
A. They remain loose sediment indefinitely and never change form
B. They harden over time and eventually become new rocks through a geological process
C. They dissolve completely when exposed to standing water for years
D. They are immediately recycled back into the water cycle as evaporation begins
Show Answer & Explanation

Answer: (B) They harden over time and eventually become new rocks through a geological process

Explanation:
The chapter states: 'When the speed of the water or wind decreases, such as in an ocean or a lake, the material transported during erosion settles down at the bottom. These sediments harden over time and become new rocks.' This explains the complete erosion cycle and demonstrates that physical changes in geology can transform loose material into solid rock through consolidation and compaction.

Question: Combustible substances like wood, paper, and kerosene are commonly grouped together. What defining characteristic do all these materials share?
A. They are made of carbon and hydrogen elements only
B. They are capable of undergoing combustion reactions when they reach their ignition temperature and oxygen is present
C. They produce smoke and ash when burned
D. They are all derived from ancient organisms and contain stored solar energy
Show Answer & Explanation

Answer: (B) They are capable of undergoing combustion reactions when they reach their ignition temperature and oxygen is present

Explanation:
The chapter defines combustible substances as materials that 'undergo combustion reactions.' Combustion requires three elements (fuel, oxygen, and heat to reach ignition temperature). All the materials listed—wood, paper, cotton, kerosene—meet this criterion: they can react with oxygen and produce heat and/or light when ignited. The chapter groups them by this functional ability rather than their origin or composition.

Question: At the base of mountains, heaps of material collect over very long periods through weathering and erosion. What are these heaps of sand, soil, and stones specifically called?
A. Erosion deposits
B. Weathered fragments
C. Sediments
D. Bedrock fragments
Show Answer & Explanation

Answer: (C) Sediments

Explanation:
The chapter introduces the term 'sediments' in section 5.7.1, defining them as 'heaps of sand, soil, and stones lying at the base of mountains.' Sediments are specifically the broken-down fragments of rock that result from weathering and are moved by erosion, eventually settling in new locations.

Question: Rolling dough balls into flat chapatis is listed in Activity 5.1 as a change to observe. If a freshly made chapati is then rolled back into a ball shape, does this action represent reversal of the original change?
A. No, because rolling dough into a flat shape creates a chemical change that cannot be undone
B. Yes, because the original shape can be restored even though the dough has been flattened
C. No, because the dough has cooled and can no longer be shaped back
D. Yes, only if the chapati is returned to the kitchen while still hot
Show Answer & Explanation

Answer: (B) Yes, because the original shape can be restored even though the dough has been flattened

Explanation:
Rolling dough into a chapati and then back into a ball are both physical changes—the dough itself remains dough, only its shape changes. The chapter's definition of reversible changes includes the ability to return to original form. Unlike popcorn (which ruptures permanently) or chopped vegetables (physically separated), dough can be re-shaped, making this a reversible physical change.

Question: Rising fuel consumption in cars, trains, and aeroplanes is linked to long-term environmental harm. What specific atmospheric consequence does increased fuel burning cause?
A. Depletion of oxygen in the atmosphere, making the air difficult to breathe
B. Increased amount of carbon dioxide in the atmosphere, contributing to environmental problems
C. Formation of toxic nitrogen compounds that cause acid rain everywhere
D. Reduction of the ozone layer due to combustion byproducts
Show Answer & Explanation

Answer: (B) Increased amount of carbon dioxide in the atmosphere, contributing to environmental problems

Explanation:
The chapter explicitly states: 'the increased consumption of fuels in cars, trains, aeroplanes, etc., is increasing the amount of carbon dioxide in the atmosphere.' This is presented as a long-term environmental impact resulting from human activity and chemical changes (combustion) on a large scale.

Question: When paint dries on walls and furniture, this is considered an undesirable change because of its atmospheric effect. What specifically is released during this drying process?
A. Carbon monoxide gas that depletes oxygen
B. Water vapour that increases humidity and causes mold
C. Chemical substances through evaporation that cause atmospheric pollution
D. Paint particles that accumulate as solid waste
Show Answer & Explanation

Answer: (C) Chemical substances through evaporation that cause atmospheric pollution

Explanation:
The chapter states: 'Drying of paint on walls, doors, furniture, etc., releases many substances through evaporation, causing atmospheric pollution.' This describes a physical change (evaporation) that has an undesirable environmental consequence, demonstrating that not all changes are beneficial despite being natural.

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Chapter 05 Changes Around Us Physical And Chemical Objective Questions & Solutions for Class 7 Science

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