What is weathering of rocks?

Physical and chemical changes in rocks caused by climate growing roots freezing water and chemicals

The process of sediments being broken down and moved by natural forces like wind and flowing water

The hardening of soil into rock

What is bioluminescence?

Light produced by a chemical change in living organisms without heat (like fireflies)

Is rusting a physical or a chemical change?

Chemical change because new substance iron oxide is formed

Both physical and chemical equally

Is the melting of wax in a burning candle a physical or chemical change?

Physical change because wax changes state but no new substance forms

What substance forms when vinegar (acetic acid) reacts with baking soda?

Carbon dioxide and other substances

Which of the following is an example of a reversible change?

Ice melting into water which can be refrozen

Why does a candle covered by a glass tumbler go out after some time?

The covered candle uses up the available oxygen and without a continuous supply of oxygen combustion cannot continue

The candle melts faster under the glass

Michael Faraday used a candle to teach science. What makes a burning candle suitable for demonstrating BOTH physical and chemical changes?

A burning candle simultaneously demonstrates physical changes (melting solidification evaporation of wax) and chemical change (combustion of wax vapour) — making it a complete study of change types in one object

How can you test if the gas formed when vinegar and baking soda react is carbon dioxide?

Pass the gas through lime water — if lime water turns milky the gas is carbon dioxide

What is the difference between a reversible and an irreversible change?

A reversible change can return to the original state (ice melting) while an irreversible change cannot (burning paper rusting cooking) — because new substances form in irreversible changes that cannot easily be converted back

Irreversible changes are always slower

Reversible changes always involve heat

Why is decomposition of food waste described as both undesirable AND desirable depending on context?

Food spoilage in storage is undesirable (wasted food) but the same decomposition process is desirable when converting food waste into compost — demonstrating that whether a change is desirable depends entirely on context

All decomposition is undesirable

Decomposition only happens slowly

What is the fire triangle and why must ALL three conditions be present simultaneously?

The fire triangle is just a symbol

The fire triangle (fuel + oxygen + ignition temperature) represents the three necessary conditions for combustion — removing ANY one extinguishes fire — this is why smothering fire with a blanket (removes oxygen) or cooling with water (lowers temperature) both work

How does the burning candle demonstrate that physical and chemical changes can occur simultaneously in a single process?

The candle shows both simultaneously — the wax melts (physical — state change) and is carried up the wick where it evaporates (physical) and then the wax vapour undergoes combustion (chemical — new substances CO2 and water form) — proving both types can coexist

Only chemical changes occur when burning

Only physical changes occur when burning

How does erosion connect to the formation of soil — and why is this a slow process?

Weathering breaks rocks into smaller pieces through physical and chemical changes — erosion moves those pieces by water and wind — they settle as sediments which harden into new rock — over thousands of years this cycle produces soil — the extreme slowness reflects the timescale of geological processes

Soil forms quickly from erosion

Soil forms only from biological processes

Why does increased burning of fuels in cars and aircraft increase carbon dioxide in the atmosphere — and why is this a chemical change?

Combustion is a chemical change where fuel reacts with oxygen to form CO2 and water — with millions of vehicles and aircraft burning fuels daily the cumulative chemical change is increasing atmospheric CO2 concentrations — demonstrating that chemistry connects to climate

Fuel evaporates without reacting

CO2 is produced only by factories

Compare physical and chemical changes in the context of weathering of rocks. What does this reveal about how the two types of changes interact?

Weathering involves both — physical changes (temperature cracks root growth) break rocks into smaller pieces without forming new substances — chemical changes (iron in basalt reacting with water/air to form red iron oxide) alter the chemical composition — both processes work together over geological time to transform rocks into soil

Only physical changes cause weathering

Only chemical changes cause weathering

Why is the reaction between lime water and carbon dioxide important as a laboratory test — and what does it demonstrate about chemical changes?

The reaction Ca(OH)2 + CO2 → CaCO3 + H2O produces an insoluble white precipitate (calcium carbonate) that visibly turns lime water milky — the formation of an entirely new substance that is insoluble confirms a chemical change — this visual evidence makes it an ideal diagnostic test for carbon dioxide

The test only works at high temperatures

Why do physical changes in rocks (breaking due to temperature changes and freezing water) ultimately lead to chemical changes (iron oxide formation)? What does this reveal about the relationship between physical and chemical processes?

Physical and chemical changes are unrelated

Physical breakdown of rocks increases the surface area exposed to water and air — increasing the contact points for chemical reactions to occur — physical changes thus CREATE CONDITIONS for chemical changes — demonstrating that the two categories of change are interconnected rather than independent

Chemical changes cause physical changes

Physical changes are more important

How does the concept of ignition temperature explain why an entire forest does not spontaneously combust on a hot summer day — even though it contains abundant combustible material and oxygen?

Ignition temperature must be reached at a specific point — ambient summer temperatures typically do not reach the ignition temperature of wood — fire requires a specific initiation event (lightning dry heat focused on a small area) to raise temperature above ignition threshold — once started the heat from combustion provides the ignition temperature to propagate

Summer heat is not warm enough for any combustion

NCERT Solutions Class 7 Science Chapter 5 – Changes Around Us: Physical and Chemical

Name: Changes Around Us: Physical and Chemical – Class 7 Science
Uploaded: 2026-07-23T05:13:01+00:00
Description: Changes around us can be physical or chemical. Physical changes alter the appearance or state of a substance without forming a new substance, while chemical changes produce new substances, often with heat, light, or gas. Some changes are reversible, some are not, and they can be desirable or undesirable.

Class 7 Science

Last Updated: 23 July 2026

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Chapter Introduction

Changes around us can be physical or chemical. Physical changes alter the appearance or state of a substance without forming a new substance, while chemical changes produce new substances, often with heat, light, or gas. Some changes are reversible, some are not, and they can be desirable or undesirable.

हिंदी में (Hindi)

हमारे चारों ओर होने वाले परिवर्तन भौतिक या रासायनिक हो सकते हैं। भौतिक परिवर्तन में पदार्थ का रूप या अवस्था बदलती है, पर नई पदार्थ नहीं बनती, जबकि रासायनिक परिवर्तन में नई पदार्थ बनती है, अक्सर गर्मी, रोशनी या गैस के साथ। कुछ परिवर्तन उलटे जा सकते हैं, कुछ नहीं, और ये लाभकारी या हानिकारक हो सकते हैं।

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Important Points

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Practice MCQ Questions (25 questions)

Q1. What is a physical change?

Q2. What is a chemical change?

Q3. What happens when lime water is blown into with exhaled breath?

Q4. What are the three requirements for combustion?

Q5. What is ignition temperature?

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Frequently Asked Questions (FAQ)

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Tough Words (Glossary)

combustion

Meaning: A chemical reaction where a substance burns in oxygen releasing heat and/or light

अर्थ: दहन; ऑक्सीजन में जलने की रासायनिक क्रिया जिसमें ऊष्मा/प्रकाश निकलता है

"Combustion of magnesium produces a white ash."

ignition temperature

Meaning: The lowest temperature at which a substance catches fire

अर्थ: ज्वलनांक; वह न्यूनतम तापमान जिस पर पदार्थ आग पकड़ता है

"Paper burns when it reaches its ignition temperature."

chemical change

Meaning: A change that forms one or more new substances

अर्थ: रासायनिक परिवर्तन; जिसमें नए पदार्थ बनते हैं

"Rusting is a chemical change."

physical change

Meaning: A change in shape, size, or state with no new substance formed

अर्थ: भौतिक परिवर्तन; आकार/आवस्था बदलती पर नया पदार्थ नहीं बनता

"Melting ice is a physical change."

bioluminescence

Meaning: Light produced by a living organism due to a chemical reaction

अर्थ: जैव-दीप्ति; जीवों द्वारा रासायनिक क्रिया से उत्पन्न प्रकाश

"Fireflies glow due to bioluminescence."

extinguish

Meaning: To put out a fire or flame

अर्थ: बुझाना; आग को समाप्त करना

"Covering the candle with a glass extinguished the flame."

lime water

Meaning: A solution of calcium hydroxide in water

अर्थ: चूने का पानी; कैल्शियम हाइड्रॉक्साइड का विलयन

"Blowing into lime water makes it milky."

calcium carbonate

Meaning: A white insoluble solid formed in some reactions

अर्थ: कैल्शियम कार्बोनेट; एक सफेद अविलेय ठोस

"Calcium carbonate forms when CO₂ reacts with lime water."

insoluble

Meaning: Not able to dissolve in a liquid

अर्थ: अविलेय; जो द्रव में नहीं घुलता

"Chalk is insoluble in water."

precipitate

Meaning: A solid that forms and settles from a liquid during a reaction

अर्थ: अवक्षेप; विलयन से बनने वाला ठोस

"A white precipitate appeared in the test tube."

erosion

Meaning: Wearing away and movement of soil or rock by wind or water

अर्थ: कटाव; हवा/पानी द्वारा मिट्टी या चट्टान का बह जाना

"River water causes erosion of the banks."

weathering

Meaning: Breaking down of rocks by physical and chemical processes

अर्थ: अपक्षय; भौतिक व रासायनिक प्रक्रियाओं से चट्टानों का टूटना

"Weathering turns big rocks into soil over time."

sediment

Meaning: Material that settles at the bottom of water

अर्थ: अवसाद; पानी के तल में जमा पदार्थ

"Fine sediment collected at the lake bed."

basalt

Meaning: A dark volcanic rock

अर्थ: बेसाल्ट; एक गहरा ज्वालामुखीय पत्थर

"Basalt turned reddish due to oxidation."

oxidation

Meaning: A chemical reaction where a substance combines with oxygen

अर्थ: ऑक्सीकरण; ऑक्सीजन के साथ संयोजन की क्रिया

"Iron undergoes oxidation to form rust."

rusting

Meaning: Formation of brown iron oxide on iron in moist air

अर्थ: जंग लगना; आर्द्र वायु में लोहे पर भूरी परत बनना

"Keeping iron wet speeds up rusting."

vapourisation

Meaning: Change of a liquid into vapour (gas)

अर्थ: वाष्पीकरण; द्रव का वाष्प में बदलना

"Vapourisation of water forms steam."

condensation

Meaning: Change of gas into liquid on cooling

अर्थ: संघनन; गैस का ठंडा होकर द्रव बनना

"Condensation formed drops on the bottle."

solidification

Meaning: Change of liquid into solid on cooling

अर्थ: ठोसकरण; द्रव का ठंडा होकर ठोस बनना

"Solidification turns melted wax solid again."

evaporation

Meaning: Slow change of liquid to vapour at the surface

अर्थ: वाष्पन; सतह से द्रव का धीरे-धीरे वाष्प में बदलना

"Evaporation dries wet clothes."

state (of matter)

Meaning: Form in which matter exists: solid, liquid, or gas

अर्थ: अवस्था; ठोस/द्रव/वायु रूप

"Water exists in all three states."

reaction

Meaning: A process where substances change into new substances

अर्थ: अभिक्रिया; पदार्थों का नए पदार्थों में बदलना

"The reaction produced bubbles."

fume hood

Meaning: A ventilated enclosure for safely handling gases/vapours

अर्थ: धूम-हुड; वाष्प/गैस से सुरक्षित कार्य हेतु वायु-संचालित कक्ष

"Burn sulfur only under a fume hood."

deflagrating spoon

Meaning: A long-handled metal spoon for burning substances safely

अर्थ: दहन-चम्मच; सुरक्षित जलाने हेतु लंबा धातु चम्मच

"Sulfur was burnt in a deflagrating spoon."

tumbler

Meaning: A glass or cup without a handle

अर्थ: गिलास; बिना हैंडल वाला पात्र

"Add lime water to the tumbler."

petri dish

Meaning: A shallow dish used in experiments

अर्थ: पेट्री डिश; प्रयोगों में प्रयुक्त उथला पात्र

"Place the candle on a petri dish."

wick

Meaning: The string in a candle that draws up melted wax

अर्थ: बाती; मोमबत्ती में मोम ऊपर खींचने वाली डोरी

"The wick soaked up the melted wax."

fizzing

Meaning: A hissing sound due to gas bubbles forming

अर्थ: फुफकार/झाग की आवाज; गैस बनने से आने वाली ध्वनि

"Mixing soda and vinegar caused fizzing."

bubbling

Meaning: Formation of bubbles in a liquid

अर्थ: बुलबुलाना; द्रव में बुलबुले बनना

"Bubbling showed a gas was produced."

exhale

Meaning: To breathe out

अर्थ: श्वास छोड़ना; साँस बाहर निकालना

"We exhale carbon dioxide."

milky

Meaning: Cloudy white in appearance

अर्थ: दूधिया; धुंधला सफेद

"Lime water turned milky."

combustible

Meaning: Able to catch fire and burn

अर्थ: ज्वलनशील; जो आसानी से जल सके

"Paper is a combustible substance."

fuel

Meaning: A substance burned to produce heat or energy

अर्थ: ईंधन; ऊष्मा/ऊर्जा देने हेतु जलाया जाने वाला पदार्थ

"Wood is used as a fuel in villages."

oxygen

Meaning: A gas in air essential for burning and breathing

अर्थ: ऑक्सीजन; जलने व श्वास के लिए आवश्यक गैस

"Oxygen supports combustion."

carbon dioxide

Meaning: A gas exhaled by us and formed in many reactions

अर्थ: कार्बन डाइऑक्साइड; हमारे द्वारा छोड़ी जाने वाली गैस

"Carbon dioxide turned lime water milky."

vinegar

Meaning: A sour liquid containing acetic acid

अर्थ: सिरका; एसीटिक अम्ल वाला खट्टा तरल

"Vinegar reacts with baking soda."

baking soda

Meaning: Sodium hydrogen carbonate, a basic salt

अर्थ: बेकिंग सोडा; सोडियम हाइड्रोजन कार्बोनेट, क्षारीय लवण

"Baking soda produced bubbles with vinegar."

reversible

Meaning: Able to be changed back to the original form

अर्थ: उलटनीय; जिसे वापस पूर्व रूप में लाया जा सके

"Melting of ice is reversible."

irreversible

Meaning: Not able to return to the original form

अर्थ: अनुलटनीय/अपरिवर्तनीय; जिसे वापस न किया जा सके

"Making popcorn is irreversible."

decomposition

Meaning: Breaking down of a substance into simpler parts

अर्थ: अपघटन; पदार्थ का सरल भागों में टूटना

"Food decomposition makes compost."

desirable

Meaning: Good or useful to have

अर्थ: वांछनीय; लाभकारी

"Curd formation is a desirable change."

undesirable

Meaning: Not wanted or harmful

अर्थ: अवांछनीय; हानिकारक

"Food spoilage is undesirable."

settle

Meaning: To sink and rest at the bottom

अर्थ: बैठना/जमना; तल में जमा होना

"The white solid settled at the bottom."

dissolve

Meaning: To mix uniformly in a liquid

अर्थ: घुलना; द्रव में समान रूप से मिलना

"Sugar dissolves in water."

formation

Meaning: The act of creating or coming into being

अर्थ: निर्माण; बनना

"The formation of rust takes time."

evaporate

Meaning: To change from liquid to gas

अर्थ: उड़ना/वाष्पित होना; द्रव से गैस बनना

"Water will evaporate in the sun."

condense

Meaning: To change from gas to liquid

अर्थ: संघनित होना; गैस से द्रव बनना

"Steam condenses on a cool surface."

ignite

Meaning: To start burning

अर्थ: प्रज्वलित करना; आग लगना

"The match ignited the paper."

magnifying glass

Meaning: A lens that makes objects appear bigger

अर्थ: आवर्धक काँच; वस्तुओं को बड़ा दिखाने वाला लेंस

"Use a magnifying glass to focus sunlight."

heat

Meaning: Energy that makes things warm or hot

अर्थ: ऊष्मा; गर्मी देने वाली ऊर्जा

"Heat speeds up many changes."

light

Meaning: Visible form of energy that helps us see

अर्थ: प्रकाश; देखने में सहायक ऊर्जा

"The reaction released heat and light."

moist

Meaning: Slightly wet

अर्थ: नम; थोड़ा गीला

"Rust forms faster in moist air."

dazzling

Meaning: Extremely bright

अर्थ: चमचमाता/चकाचौंध करने वाला

"Magnesium burns with a dazzling flame."

cloudy

Meaning: Not clear; full of tiny particles

अर्थ: धुंधला; अस्पष्ट

"The solution turned cloudy."

expand

Meaning: To increase in size or volume

अर्थ: फैलना/विस्तृत होना

"Metals expand on heating."

contract

Meaning: To decrease in size

अर्थ: सिकुड़ना

"Wax contracts when it cools."

suspend

Meaning: To keep particles floating in a liquid

अर्थ: स्थगित/झूलते रहना; द्रव में कणों का तैरना

"Soot particles were suspended in air."

odor

Meaning: A smell

अर्थ: गंध/सुगंध

"The fruit had a strong odor."

settling time

Meaning: Time taken for particles to deposit

अर्थ: अवसादन समय; कणों के तल पर बैठने का समय

"The settling time of the solid was short."

flammable

Meaning: Easily set on fire

अर्थ: ज्वलनशील; आसानी से जलने वाला

"Kerosene is highly flammable."

reaction equation

Meaning: A short way to show a chemical change using symbols

अर्थ: अभिक्रिया समीकरण; प्रतीकों से रासायनिक परिवर्तन का संक्षिप्त रूप

"Write the reaction equation for rusting."

vapour

Meaning: Gas form of a substance normally liquid or solid

अर्थ: वाष्प; सामान्यतः द्रव/ठोस का गैसीय रूप

"The wax turned into vapour near the flame."

residue

Meaning: A small amount of substance left behind

अर्थ: अवशेष; बचा हुआ पदार्थ

"A black residue remained after burning."

climatic

Meaning: Related to climate

अर्थ: जलवायवीय; जलवायु से संबंधित

"Climatic changes affect weathering."

sedimentary rock

Meaning: Rock formed from compacted sediments

अर्थ: अवसादी शैल; अवसादों से बनी चट्टान

"Sand layers later form sedimentary rock."