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NCERT Solutions for CBSE Class 7 Science Chapter 5: Physical and Chemical Changes

CBSE Class 7 Science Chapter 5 Physical and Chemical Changes introduces one of the most fundamental concepts in chemistry — how matter transforms around us every single day. From the morning dew evaporating to the rusting gate in your neighbourhood, students learn to classify changes scientifically. This NCERT Solutions guide provides detailed, step-by-step answers to every question in Chapter 5, following the exact terminology and examples from the official textbook. Whether your child is preparing for a unit test, revision before term exams, or simply needs clarity on why burning is irreversible while boiling is not, these solutions offer the depth and accuracy that CBSE marking schemes reward.

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Key takeaways

  • ✓Physical changes alter only form or state without creating new substances, while chemical changes produce entirely new materials with different properties
  • ✓Rusting of iron is a chemical change requiring moisture and oxygen; prevention methods include painting, oiling, and galvanisation with zinc coating
  • ✓Crystallisation is a physical change used to purify substances like salt and copper sulphate by controlled evaporation and cooling
  • ✓Galvanisation protects iron from corrosion by depositing a thin zinc layer, commonly seen on water pipes and bridge structures
  • ✓CBSE Class 7 Science Chapter 5 Physical and Chemical Changes carries approximately 8-10 marks across term exams through MCQs, short answers and diagram-based questions
  • ✓Every NCERT in-text question and end-of-chapter exercise from Class 7 Science Chapter 5 requires clear identification of change type with scientific reasoning
  • ✓Understanding reversibility is key: physical changes are mostly reversible (water to ice to water), chemical changes are permanent (burnt paper cannot return to paper)

Understanding Physical Changes: Definition and Characteristics

Physical changes modify the physical properties of a substance — such as size, shape, state, or appearance — without altering its chemical composition. When ice melts into water, the H₂O molecules remain identical; only the arrangement and energy change. CBSE Class 7 Science Chapter 5 Physical and Chemical Changes emphasises that most physical changes are reversible. If you tear paper into pieces, the chemical nature (cellulose fibres) stays the same; you have simply changed the size. Similarly, dissolving sugar in water is a physical change because evaporating the water will give you sugar crystals back. The NCERT textbook lists several daily-life examples: stretching a rubber band, inflating a balloon, magnetising iron, and the formation of clouds. During the monsoon, water evaporates from lakes (physical change), forms clouds (physical change), and rains back down (physical change) — completing a reversible cycle. In CBSE exams, questions often ask students to identify whether a given scenario involves only physical transformation. The key test: Can you get the original substance back by simple physical means like heating, cooling, or filtering? If yes, it is a physical change. No new substance with new properties is formed. Understanding this core principle helps students tackle not just Class 7 Science Chapter 5, but also builds a foundation for stoichiometry and reaction classification in Classes 9 and 10.
  • No new substance formed — chemical identity remains unchanged throughout the process
  • Generally reversible — original substance can be recovered by reversing the conditions (heating, cooling, dissolving, filtering)
  • Change in state, size, shape or physical form — like solid to liquid, large pieces to small pieces, straight wire to coiled wire
  • Energy absorbed or released is usually small — compared to the massive energy changes in chemical reactions like combustion
  • Examples from NCERT: melting ice, boiling water, dissolving salt, cutting wood, breaking glass, condensation of steam

Understanding Chemical Changes: Formation of New Substances

A chemical change results in the formation of one or more new substances with properties entirely different from the original material. CBSE Class 7 Science Chapter 5 Physical and Chemical Changes defines this transformation as a chemical reaction. When magnesium ribbon burns in air, it forms magnesium oxide — a white powder that is not the shiny grey metal anymore. The NCERT textbook emphasises five clear indicators of a chemical change: formation of a gas (bubbles when vinegar meets baking soda), formation of a precipitate (curd from milk), change in colour (silver ornament turning black), change in temperature (heat released when quicklime meets water), and change in smell (the foul odour of a rotten egg). Most chemical changes are irreversible under normal conditions. You cannot unbake a cake or un-burn charcoal. During photosynthesis, green plants convert carbon dioxide and water into glucose and oxygen — a chemical change that sustains all life but cannot be easily reversed in the kitchen. The CBSE marking scheme for Class 7 Science solutions expects students to justify their answer: if a question asks 'Is digestion of food a physical or chemical change?', the complete answer must state 'Chemical change, because food is broken down into simpler molecules like glucose and amino acids with entirely new properties, and this process is irreversible.' Around 3-4 marks in term exams come from such justification-based questions tied directly to NCERT examples in Chapter 5.
  • New substance(s) formed — with different chemical composition and properties than reactants
  • Usually irreversible — cannot get original substances back by simple physical methods
  • Energy change is significant — heat, light or sound often released or absorbed during the reaction
  • Observable signs include gas evolution, precipitate formation, colour change, temperature change, smell change
  • Examples from NCERT Class 7 Science: burning of paper, rusting of iron, cooking of food, digestion, ripening of fruits, souring of milk

Comparing Physical and Chemical Changes: NCERT Table Format

The NCERT textbook for CBSE Class 7 Science Chapter 5 Physical and Chemical Changes includes a clear comparison table that frequently appears in exam questions. Students must memorise these distinctions because 2-mark and 3-mark questions in CBSE term exams often ask 'Differentiate between physical and chemical changes with two examples each.' The table format is the most concise way to present this answer and score full marks. When iron rusts, the reddish-brown layer of iron oxide has magnetic properties different from iron, cannot conduct electricity as well, and is brittle — all signs of a new substance. By contrast, when the same iron is merely heated and glows red-hot, it cools back to the same iron with identical magnetic and conductive properties. Understanding these distinctions helps students analyse everyday phenomena: why does a candle burning create a chemical change (wax to carbon dioxide and water vapour) while candle wax melting is physical (solid wax to liquid wax, same chemical formula)? The Class 7 Science solutions in this guide use comparison tables extensively because they are memory-efficient and exam-friendly. During revision, students should practice filling in blank comparison tables without referring to notes — a technique that significantly improves recall during the actual exam.

Detailed NCERT In-Text Question Solutions: Activity-Based Learning

CBSE Class 7 Science Chapter 5 Physical and Chemical Changes contains several in-text questions embedded within the chapter narrative. These are not merely exercises but are integrated with hands-on activities that students are expected to perform or understand. For instance, Activity 5.1 asks students to observe an iron nail left in copper sulphate solution. After a few hours, the blue solution turns light green and a reddish-brown deposit forms on the nail. The NCERT question asks: 'What type of change is this?' The complete answer is: 'Chemical change. Iron displaces copper from copper sulphate solution, forming iron sulphate (light green) and depositing copper metal (reddish-brown) on the nail. New substances with different properties are formed, and the change is irreversible.' Similarly, Activity 5.3 involves heating a magnesium ribbon until it burns with a brilliant white flame, leaving behind white ash (magnesium oxide). Students must write: 'The greyish-white powder (magnesium oxide) is a new substance formed by the chemical combination of magnesium and oxygen from air. This is a chemical change because the product has properties entirely different from magnesium metal.' Each in-text answer should reference observable evidence (colour change, gas evolution, energy release) and conclude with the classification. During practicals and unit tests, these activity-based questions fetch 2-3 marks each, and incomplete answers lose at least one mark for missing the reasoning or the classification.
  • Activity 5.1 (Iron nail in copper sulphate) — Chemical change; displacement reaction forms new substances
  • Activity 5.2 (Heating sugar solution) — Physical change during evaporation; crystallisation gives back sugar crystals
  • Activity 5.3 (Burning magnesium ribbon) — Chemical change; new compound magnesium oxide with different properties
  • Activity 5.4 (Mixing vinegar and baking soda) — Chemical change; gas (carbon dioxide) evolved, indicating new substances formed
  • Each activity answer must state: observation, type of change, reason with evidence, reversibility status

End-of-Chapter Exercise Solutions: Question-by-Question Breakdown

The NCERT Class 7 Science textbook concludes Chapter 5 with a set of exercises that form the backbone of exam preparation. Question 1 typically asks students to classify a list of ten changes as physical or chemical: tearing paper (physical), digestion of food (chemical), melting butter (physical), rusting of almirah (chemical), boiling water to form steam (physical), passing electric current through water to form hydrogen and oxygen (chemical), dissolving common salt in water (physical), making a fruit salad (physical), burning kerosene in a lantern (chemical), and formation of clouds (physical). Each classification must include a one-line reason to score the full mark. Question 2 often requires students to state differences between physical and chemical changes — the table format discussed earlier is ideal. Question 3 may present a scenario: 'A drop of lemon juice was put on a piece of washing soda. Bubbles were formed. What type of change is this?' Full answer: 'Chemical change. Lemon juice (citric acid) reacts with washing soda (sodium carbonate) to produce carbon dioxide gas (bubbles), water, and a salt. New substances are formed and the change is irreversible.' Questions 4 and 5 typically cover rusting and crystallisation in detail, which are treated as separate sections in this guide. The CBSE marking scheme allocates 10 marks to these exercises in term exams, with 1 mark per short answer and 2-3 marks for explanatory answers. Practising these exercises repeatedly, writing answers in complete sentences, and timing oneself are proven strategies to maximise scores in Class 7 Science Chapter 5.

Rusting of Iron: Conditions, Process and Real-World Impact

Rusting is the most extensively discussed chemical change in CBSE Class 7 Science Chapter 5 Physical and Chemical Changes. When iron objects are exposed to moist air (containing both oxygen and water vapour), they develop a reddish-brown coating called rust, which is chemically hydrated iron(III) oxide (Fe₂O₃·xH₂O). The NCERT textbook specifies that rusting requires two conditions simultaneously: presence of oxygen and presence of moisture. This is why iron articles rust faster during the monsoon season in Indian cities like Mumbai, Kolkata and Chennai, where humidity remains high. A simple experiment in the textbook asks students to keep three iron nails: one in dry air, one in boiled distilled water (which has no dissolved oxygen) covered with oil, and one in tap water exposed to air. Only the third nail rusts significantly. Rusting is a slow chemical change that weakens iron structures — bridges, railings, water pipes, and even the Eiffel Tower require constant maintenance against rust. The economic cost is enormous: India spends thousands of crores annually on corrosion prevention and replacement of rusted infrastructure. From a CBSE exam perspective, questions on rusting can be asked in multiple formats: short answer (2 marks) asking for two conditions, diagram-based questions showing experimental setup (3 marks), or long answer (5 marks) covering conditions, prevention methods, and economic impact. Students should be able to write the word equation: Iron + Oxygen (from air) + Water (moisture) → Hydrated Iron(III) Oxide (rust).
  • Two essential conditions: moisture (water vapour) and oxygen from air must both be present
  • Accelerated by: salt (coastal areas see faster rusting), acids (polluted industrial air), and higher temperatures
  • Rust formation is slow but continuous — once started, the flaky rust layer exposes fresh iron beneath, continuing the cycle
  • Rust is chemically different from iron: brittle, non-magnetic, reddish-brown, does not conduct electricity
  • NCERT experiment: three nails in dry air, boiled water with oil layer, and tap water prove the dual necessity of oxygen and moisture

Prevention of Rusting: Painting, Greasing and Galvanisation

The NCERT textbook for CBSE Class 7 Science Chapter 5 dedicates significant space to rust prevention methods because this knowledge has direct practical application. The core principle behind all methods is simple: prevent contact between iron, oxygen, and moisture. Painting is the most common household method — the paint layer acts as a barrier. This is why window grilles, gates, and metal furniture are painted regularly. Applying grease or oil creates a thin film that repels water; bicycle chains, machine parts, and tools are greased for this reason. Galvanisation is the most durable industrial method. A thin layer of zinc metal is deposited on iron or steel articles through electroplating or hot-dip galvanising. Zinc is more reactive than iron and preferentially reacts with oxygen and moisture, forming a protective zinc oxide layer. Even if the zinc coating is scratched, the zinc continues to protect the underlying iron — a phenomenon called sacrificial protection. CBSE exams often ask: 'Why is galvanised iron used for water pipes and roofing sheets?' Full answer: 'Galvanised iron is coated with zinc, which prevents rusting by acting as a protective barrier and sacrificing itself to protect the iron beneath. This makes galvanised iron long-lasting and suitable for outdoor and moist environments like water supply systems and rooftops exposed to rain.' Other methods mentioned in Class 7 Science notes include electroplating with chromium (for shiny car parts), alloying iron with other metals to make stainless steel (which contains chromium and nickel and resists rusting), and keeping iron articles dry and well-ventilated.
  • Painting — barrier coating; must be renewed every few years; used for gates, grilles, metal furniture
  • Applying grease or oil — temporary moisture barrier; needs frequent reapplication; used for tools, machinery, bicycle chains
  • Galvanisation — durable zinc coating; sacrificial protection; used for water pipes, roofing sheets, buckets, nails
  • Electroplating with chromium or nickel — shiny protective layer; used for car parts, bathroom fittings, kitchen utensils
  • Alloying to make stainless steel — permanent solution; expensive; used for cutlery, surgical instruments, modern architecture

Crystallisation: Process, Procedure and Purification Applications

Crystallisation is introduced in CBSE Class 7 Science Chapter 5 Physical and Chemical Changes as a physical process used to purify solid substances. When a hot saturated solution is allowed to cool slowly, pure crystals of the solute separate out. NCERT describes the step-by-step procedure for obtaining pure copper sulphate crystals from an impure sample. First, prepare a saturated solution by dissolving impure copper sulphate powder in the minimum amount of water while heating. Any insoluble impurities are removed by filtration. The clear blue solution is then heated to concentrate it further by evaporating some water. Finally, the concentrated hot solution is allowed to cool slowly without disturbance. Beautiful blue crystals of pure copper sulphate (CuSO₄·5H₂O) appear as the solution cools. The impurities remain dissolved in the mother liquor (the remaining solution). Students often confuse crystallisation with evaporation. The key difference: in evaporation, you heat until all water is gone and a solid residue remains (which may include impurities). In crystallisation, you cool a saturated solution slowly, so only the pure solute crystallises out while impurities stay in solution. CBSE exam questions ask: 'Why is crystallisation better than evaporation for purification?' Answer (2 marks): 'Crystallisation gives purer crystals because impurities remain dissolved in the mother liquor and do not crystallise. Evaporation gives a mixture of solute and impurities.' Common salt is purified on an industrial scale from seawater using solar evaporation followed by crystallisation. Sugar, alum (used in water purification), and many medicines are also purified by crystallisation. This section of Class 7 Science Chapter 5 often appears as a 3-mark question asking for the complete procedure in steps.

Galvanisation: Industrial Process and Everyday Applications

Galvanisation is the process of coating iron or steel articles with a thin layer of zinc to protect them from rusting. Although introduced in CBSE Class 7 Science Chapter 5 Physical and Chemical Changes as a rust prevention method, the NCERT textbook provides enough detail for students to answer 2-3 mark questions specifically on galvanisation. The industrial process typically involves dipping cleaned iron articles into molten zinc at around 450°C (hot-dip galvanising). Alternatively, zinc can be electroplated onto iron using an electric current. The zinc layer serves a dual protective role. First, it acts as a physical barrier preventing moisture and oxygen from reaching the iron surface. Second, zinc is more reactive than iron (higher in the reactivity series, which students learn in Class 10), so even if the coating is scratched, zinc preferentially reacts with oxygen and moisture, sacrificing itself while the iron beneath remains protected. This is called galvanic or sacrificial protection. Galvanised iron (GI) sheets and pipes are ubiquitous in India — roofing sheets on lakhs of houses, water supply pipes in cities and villages, electricity poles in rural areas, buckets, dustbins, and fencing wire are all galvanised. The zinc coating gives these articles a characteristic spangled appearance and a dull grey colour. CBSE Class 7 Science solutions should clearly distinguish between painting (simple barrier, no chemical protection) and galvanising (barrier plus sacrificial protection). A typical 3-mark exam question: 'What is galvanisation? Why are iron pipes used for carrying water galvanised?' Model answer: 'Galvanisation is the process of depositing a protective zinc coating on iron or steel articles. Iron pipes used for water supply are galvanised because water provides moisture which causes rusting. The zinc coating prevents contact between iron and moisture. Even if scratched, zinc reacts preferentially with oxygen and water, protecting the iron. This ensures long life for water pipes.'
  • Hot-dip method — iron article dipped in molten zinc at ~450°C; forms a metallic bond for durability
  • Electro-galvanising — zinc electroplated using electric current; gives thinner, more uniform coating for precision parts
  • Dual protection — physical barrier (prevents contact) + sacrificial protection (zinc reacts instead of iron even when scratched)
  • Common galvanised articles in India: roofing sheets, water tanks, pipes, buckets, barbed wire, electricity poles, highway crash barriers
  • Lifespan: properly galvanised articles can last 30-50 years even in harsh coastal or monsoon environments

Common Mistakes in CBSE Class 7 Science Chapter 5 Exams

Students frequently lose marks in CBSE Class 7 Science Chapter 5 Physical and Chemical Changes due to avoidable mistakes. The most common error is incomplete classification. When asked to classify a change, students write 'physical change' or 'chemical change' without any reasoning. CBSE marking schemes mandate a justification — no reason means loss of at least one mark. For example, for 'formation of curd from milk,' the answer must be: 'Chemical change, because milk proteins are converted into lactic acid and other compounds with different taste and texture. This change is irreversible.' Another frequent mistake is confusing reversibility with physical change. Students assume all physical changes are reversible and all chemical changes are irreversible, but the NCERT textbook clarifies this: most, not all. Tearing paper is physical but effectively irreversible (you cannot restore the original sheet). Some chemical changes can be reversed under special conditions, though not by simple physical means. In diagram-based questions (like the three-nail rusting experiment), students often draw the setup but forget to label the conditions ('Nail A: dry air,' 'Nail B: boiled water + oil layer,' 'Nail C: tap water + air') or omit the conclusion — which nail rusts and why. This costs 1-2 marks. In crystallisation procedure questions, missing even one step (like filtering to remove insoluble impurities or adding dilute acid to prevent hydrolysis) results in mark deduction. Writing 'heat the solution and crystals will form' is wrong — you must cool a saturated solution slowly. For rusting prevention, simply writing 'galvanisation' without explaining how zinc protects iron is incomplete. Finally, many students write long paragraphs when a bulleted list or table would be clearer and save time. Practising answers in the NCERT format, underlining key terms, and writing step-by-step procedures improve scores significantly in Class 7 Science solutions.
  • Not providing reasoning — always justify your classification with observable evidence (colour change, gas, irreversibility, new substance)
  • Confusing reversible with physical — some physical changes are irreversible (tearing), some chemical changes can be reversed (not by simple means)
  • Incomplete diagrams — label all parts, state conditions, and write conclusion in activity-based questions
  • Missing steps in crystallisation — filter hot solution, cool slowly, do not evaporate to dryness
  • Vague answers on galvanisation — must explain zinc's dual role: barrier + sacrificial protection
  • Paragraph answers instead of points — use bullets, tables, or numbered steps for clarity and speed

How CBSETUTOR.ai Helps Master Physical and Chemical Changes

For students who find it challenging to distinguish between physical and chemical changes or struggle with writing complete NCERT-style answers, CBSETUTOR.ai offers a 24×7 AI tutor that has ingested every page of the Class 7 Science textbook and beyond. A student can photograph any question from their worksheet or school exam — whether it is 'Explain with an example why burning of candle is both a physical and chemical change' or 'Design an experiment to prove both oxygen and moisture are necessary for rusting' — and receive a step-by-step solution in seconds, formatted exactly as CBSE expects. The AI tutor explains concepts using real-world examples familiar to Indian students (why stainless steel utensils do not rust, how salt accelerates rusting in coastal cities like Visakhapatnam, why sugar factories use crystallisation). It supports detailed doubt resolution: if a child is confused about whether dissolving salt is reversible, the AI explains the evaporation-crystallisation cycle with a worked example. Parents across India use CBSETUTOR.ai at a flat ₹999/month for all subjects and all classes from 6 to 12 — one simple price with a 3-day free trial and no credit card required upfront. For CBSE Class 7 Science Chapter 5 Physical and Chemical Changes, the platform provides not just answers but also comparison tables, labelled diagrams for experiments, memory tips (like 'NEW GELS' for signs of chemical change: New substance, Energy change, Water/gas evolved, Gas evolved, Energy, Light, Sound, Smell change), and practice questions with instant feedback. It is like having a patient science teacher available at 10 pm when doubts arise during homework.
  • Photo upload — snap any question from NCERT, school worksheet, or test paper and get CBSE-format answers instantly
  • Concept clarity — explains WHY rusting needs both oxygen and moisture, not just WHAT the conditions are
  • Memory aids — mnemonics, comparison tables, and visual cues to remember differences and classification rules
  • Practice mode — generates similar questions on physical/chemical changes, crystallisation, rusting with auto-grading
  • ₹999/month flat for Classes 6-12, all subjects — 3-day free trial, no credit card needed to start

Exam Strategy and Marks Weightage for Chapter 5

In the CBSE Class 7 Science term exams (both periodic tests and the annual examination), Chapter 5 Physical and Chemical Changes typically accounts for 8-10 marks out of the 80-mark theory paper. The question pattern includes 2-3 multiple-choice questions (1 mark each), 2-3 very short answer questions (1 mark each asking for one example or one difference), 3-4 short answer questions (2-3 marks each requiring explanations or procedures), and occasionally one long answer or case-based question (5 marks covering rusting prevention or crystallisation in detail). To maximise marks, students should memorise the comparison table (physical vs chemical changes) verbatim — this fetches easy 3 marks in 'tabulate the differences' questions. Practice drawing and labelling the three-nail rusting experiment diagram; this appears almost every year. For crystallisation, write the procedure in five clear numbered steps as shown in NCERT; missing any step loses marks. When classifying changes, always use the two-part answer format: classification + reason. For example, 'Burning of coal: Chemical change. Coal reacts with oxygen to form carbon dioxide, heat, and ash. New substances are formed and the change is irreversible.' Underline key terms like 'new substances,' 'irreversible,' 'chemical composition changed' — examiners look for these keywords. Time management is crucial: do not spend more than 2 minutes on a 2-mark question. Use the last 10 minutes to review answers and ensure every question has a classification or conclusion statement. Students who consistently score 9-10 out of 10 in this chapter follow a simple strategy: read NCERT thoroughly, solve all in-text and exercise questions in writing (not just mentally), and revise using self-made flashcards with one change on the front ('rusting of iron') and classification + reason on the back. This active recall method is far more effective than passive rereading.
  • MCQs (2-3 marks) — usually test definitions, examples, or odd-one-out (which is NOT a chemical change?)
  • Very short answers (2-3 marks) — one-word to one-sentence answers: name two methods to prevent rusting, give one example of a slow chemical change
  • Short answers (6-9 marks total) — explanations, differences, reasons: Why is rusting a chemical change? How is crystallisation different from evaporation?
  • Diagram-based (3 marks) — draw and label rusting experiment setup, state observation and conclusion
  • Long answer / case-based (0-5 marks) — detailed procedure for crystallisation, comprehensive rust prevention methods with examples, or a real-life scenario requiring application of concepts

Connecting Chapter 5 to Daily Life and Higher Classes

One reason CBSE includes Physical and Chemical Changes in the Class 7 Science syllabus is its direct connection to observable everyday phenomena and its foundational role for chemistry in higher classes. Every time a student sees dew forming on grass (physical change: condensation), milk turning sour overnight in summer (chemical change: bacterial fermentation producing lactic acid), or an iron gate that was fine last monsoon now showing rust spots (slow chemical change), they are observing the principles of Chapter 5. Cooking is a treasure trove of both change types: boiling potatoes (physical: state change of water), frying them (chemical: proteins and carbohydrates undergo Maillard reaction creating new flavours and brown colour), and adding salt (physical: dissolving). The NCERT textbook encourages students to maintain a 'change diary' for one week, noting ten changes they observe at home or school and classifying each — this exercise deepens understanding far beyond rote learning. In Class 8, students build on this foundation with chapters on metals and non-metals (where rusting is revisited in detail), combustion (a specific type of chemical change), and pollution (where rusting and corrosion are environmental concerns). In Class 9, the chapter on 'Is Matter Around Us Pure?' revisits crystallisation as a separation technique. Class 10 chemistry chapters on chemical reactions and equations formalise the word equations for rusting (4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃, which dehydrates to rust) and introduce the concept of redox reactions, where rusting is explained as oxidation of iron. Understanding physical and chemical changes at the Class 7 level — with clear examples, correct terminology, and the ability to justify classifications — makes these advanced topics in ICSE and CBSE Classes 9-10 significantly easier. Students who master Chapter 5 thoroughly find stoichiometry, balancing equations, and reaction prediction much more intuitive in senior secondary.

Frequently asked questions

How many marks does CBSE Class 7 Science Chapter 5 Physical and Chemical Changes carry in term exams?+
Chapter 5 typically carries 8-10 marks in CBSE Class 7 Science term exams, distributed across MCQs (2-3 marks), very short answers (2-3 marks), short answers requiring explanation (4-5 marks), and occasionally a diagram-based or long answer question (3-5 marks). The exact distribution varies by school and examination pattern, but students should prepare all NCERT in-text and exercise questions thoroughly to cover the likely question types.
Is rusting of iron a physical change or chemical change, and how should I write the answer in exams?+
Rusting of iron is a chemical change. In CBSE exams, write the complete answer as: 'Rusting is a chemical change because iron reacts with oxygen from air and moisture to form a new substance called rust (hydrated iron(III) oxide), which has entirely different properties — it is reddish-brown, brittle, and non-magnetic. This change is irreversible; we cannot get pure iron back from rust by simple physical methods.' This format ensures full marks by including classification, reason, evidence, and irreversibility.
What is the difference between evaporation and crystallisation in CBSE Class 7 Science Chapter 5?+
Evaporation involves heating a solution until all the solvent evaporates, leaving behind a solid residue that may contain impurities. Crystallisation involves preparing a hot saturated solution, filtering it, and then cooling it slowly so that pure crystals of the solute form while impurities remain dissolved in the mother liquor. Crystallisation is a better purification method than evaporation because it yields purer crystals by separating the desired substance from impurities.
Why is galvanised iron preferred for water pipes and how should I explain this in a 3-mark answer?+
A complete 3-mark answer: 'Galvanised iron is preferred for water pipes because (1) it is coated with a protective layer of zinc which prevents rusting, (2) water provides continuous moisture which would otherwise cause iron to rust rapidly, and (3) even if the zinc coating is scratched, zinc being more reactive than iron reacts preferentially with oxygen and moisture, protecting the underlying iron through sacrificial protection. This ensures the pipes last for decades without corroding and contaminating the water supply.'
My child confuses when a change is reversible or irreversible. How can NCERT Class 7 Science Chapter 5 help clarify this?+
The NCERT textbook explains that most physical changes are reversible (you can get the original substance back by reversing the process: ice to water to ice), while most chemical changes are irreversible (you cannot unburn paper or un-rust iron). However, there are exceptions: tearing paper is a physical change but practically irreversible; certain chemical reactions can be reversed under special laboratory conditions but not by simple heating or cooling. The key test for students: If new substances with different properties are formed, it is chemical and usually irreversible.
What are the five main signs that a chemical change has occurred, according to CBSE Class 7 Science Chapter 5?+
The five observable signs of a chemical change mentioned in NCERT Class 7 Science are: (1) evolution of a gas (bubbles when vinegar is added to baking soda), (2) formation of a precipitate (solid formed when two solutions mix), (3) change in colour (silver turning black, iron turning reddish-brown), (4) change in temperature (heat released or absorbed, vessel becomes hot or cold), and (5) change in smell (fresh milk has a sweet smell, curd has a sour smell). Presence of any of these signs usually indicates a chemical change has taken place.
How do I write the step-by-step procedure for crystallisation of copper sulphate to score full marks?+
Write in five numbered steps exactly as per NCERT: (1) Take a cupful of water in a beaker, add a few drops of dilute sulphuric acid, and heat. (2) Add impure copper sulphate powder gradually with stirring until no more dissolves (saturated solution). (3) Filter the hot solution to remove insoluble impurities. (4) Collect the clear blue filtrate in a china dish and allow it to cool slowly without disturbance. (5) After some time, beautiful blue crystals of pure copper sulphate appear. Collect the crystals, wash with a little cold water, and dry between filter papers. Missing any step can cost marks.
Will my child fall behind in Class 8 or Class 10 chemistry if concepts from Chapter 5 are not clear now?+
Yes, absolutely. CBSE Class 7 Science Chapter 5 Physical and Chemical Changes lays the conceptual foundation for multiple topics in higher classes. Class 8 revisits rusting in the metals chapter, and combustion is introduced as a specific chemical change. In Class 10, the entire 'Chemical Reactions and Equations' chapter assumes students can identify and classify changes, write word equations (like the rusting equation), and understand energy changes. If your child cannot confidently distinguish physical from chemical changes now, balancing equations and understanding reaction types in Class 10 will be significantly harder. Strengthening this foundation now prevents future struggles.
Can CBSETUTOR.ai help if my child struggles with writing proper NCERT-style answers for Class 7 Science Chapter 5?+
Yes, CBSETUTOR.ai is designed precisely for this. Students can upload a photo of any question from their NCERT textbook, school worksheet, or test paper, and the AI tutor provides step-by-step answers in the exact format CBSE examiners expect — with classification, reasoning, examples, and proper scientific terminology. It also offers practice questions on physical and chemical changes, rusting, crystallisation, and galvanisation with instant feedback. The platform covers all of Class 7 Science (and Classes 6-12 across subjects) for a flat ₹999/month with a 3-day free trial, no credit card needed. It is available 24×7, so doubts can be resolved immediately during homework or late-night revision.
What is the word equation for rusting of iron that I should remember for CBSE Class 7 Science exams?+
For Class 7, write the simple word equation: Iron + Oxygen (from air) + Water (moisture) → Rust (Hydrated Iron(III) Oxide). Some textbooks and teachers may accept: Iron + Oxygen + Water → Iron Oxide (Rust). The chemical formula Fe₂O₃·xH₂O is not required at the Class 7 level — it is introduced in Class 10 when students learn to write balanced chemical equations with symbols and formulae. Focus on understanding and writing the word equation correctly with all three reactants.
How is painting different from galvanisation as a rust prevention method in Class 7 Science Chapter 5?+
Painting provides only a physical barrier that prevents moisture and oxygen from reaching the iron surface. If the paint cracks or peels, the exposed iron starts rusting immediately. Galvanisation provides a dual protection: (1) the zinc coating acts as a physical barrier like paint, but (2) zinc is chemically more reactive than iron, so even if the coating is scratched, zinc reacts with oxygen and moisture instead of iron, sacrificing itself to protect the iron beneath. This sacrificial protection makes galvanisation far more durable and long-lasting than painting, especially in harsh outdoor or moist environments.
Are there any memory tricks to remember the differences between physical and chemical changes for CBSE Class 7 Science Chapter 5?+
Yes, use the mnemonic 'RNC' for chemical changes: Reversibility (usually NOT reversible), New substance (formed with different properties), Composition (chemical composition changes). For physical changes, remember 'RSS': Reversible (generally yes), Same substance (chemical identity unchanged), State/Shape/Size (only physical properties change). Another useful memory aid for signs of chemical change is 'GETCS': Gas evolved, Energy change (heat/light), Temperature change, Colour change, Smell change. Creating and practicing with flashcards using these mnemonics significantly improves recall during exams, as reported by thousands of students using CBSETUTOR.ai study techniques.

Related resources

CBSE Class 7 Science Chapter 5 Physical and Chemical Changes Worksheet with AnswersClass 7 Science Chapter 5 Physical and Chemical Changes — Formulas & Key PointsImportant Questions: CBSE Class 7 Science Chapter 5 Physical and Chemical ChangesCBSE Class 7 Science Chapter 5 Physical and Chemical Changes — NotesAI Tutor for Class 7: The Smart Alternative to TuitionAI Tutor for Class 7 Science: Learn Faster with Instant HelpOnline Class 9 Science Tutor in Mumbai – 24/7 CBSE-Aligned AI CoachingNCERT Solutions for CBSE Class 9 Mathematics Chapter 1: Number Systems

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Class 7Science
CBSE Class 7 Science Chapter 11 Light — 20 MCQs with Answers
Class 7Science
CBSE Class 7 Science Chapter 9 Motion and Time — 20 MCQs with Answers
Class 7Science
CBSE Class 7 Science Chapter 10 Electric Current and its Effects — 20 MCQs with Answers
Class 7Science
CBSE Class 7 Science Chapter 6 Respiration in Organisms — 20 MCQs with Answers

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