Overview
This chapter introduces physical and chemical changes — two broad ways matter can transform. A physical change alters the form or appearance of a substance without producing a new substance (examples: melting, boiling, cutting, dissolving). A chemical change (chemical reaction) produces one or more new substances with different properties (examples: burning, rusting, digestion). The chapter explains key indicators of chemical change such as change in colour, evolution of gas, formation of a precipitate, change in temperature and irreversibility (with noted exceptions), and contrasts them with characteristics of physical changes. Importance: Understanding these concepts helps students explain everyday phenomena (rusting of iron, cooking, formation of curd, melting of ice) and lays the foundation for later chemistry topics like reactions and conservation laws. The chapter develops observation and experimental skills — recording changes, drawing conclusions, and distinguishing reversible and irreversible processes. Key themes include: definitions and examples of physical vs chemical change, common types of physical changes (change of state, cutting, folding, mixing/dissolving),…
Learning Objectives
- Define physical change and chemical change and give two classroom examples of each
- Distinguish between physical and chemical changes by stating at least three distinguishing features
- Classify a given list of processes (melting, rusting, burning, dissolving, baking) as physical or chemical changes and justify your classification
- Explain five observable indicators of a chemical change (color change, gas evolution, precipitate formation, temperature change, irreversibility) with examples
- Identify whether a described change is reversible or irreversible and provide reasons for your answer
- Predict the type of change (physical or chemical) and likely products for simple everyday transformations such as burning paper, dissolving salt, and curdling milk
- Demonstrate by simple experiments the difference between physical and chemical changes and record the observations and conclusions
- Describe the process of rusting of iron and suggest at least three methods to prevent or slow down rusting
Topics in this chapter
7 topics · tap a topic title to jump straight to it.
Physical Changes
Physical Changes
Key Point: Conservation of mass (in a closed system): mass_initial = mass_final
Definition: A physical change is a change in the form, appearance, or state of a substance without forming a new substance. The chemical identity of the substance remains the same.
Key points
- No new substances are produced; only physical properties (shape, state, size, texture, appearance) change.
- Mass is conserved in a physical change (total mass before = total mass after).
- Many physical changes are reversible (melting/freezing, condensation/evaporation), but some are effectively irreversible (cutting, crushing) though composition remains unchanged.
- Energy may be absorbed or released (heat for melting/boiling, work for stretching), but chemical bonds are not broken or formed in a way that makes new substances.
Types of physical changes
- Change of state: solid <> liquid <> gas (melting, freezing, boiling, condensation, sublimation)
- Change of shape or size: cutting, bending, stretching, crushing
- Mixing and separation: dissolving (solution formation), suspension, filtration, crystallization
How to identify a physical change
- No change in chemical properties (no new smell of chemical reaction, no gas evolution with chemical characteristics, no permanent colour change from reaction).
- Substance can often be recovered by physical methods (evaporation, filtration, distillation, crystallization).
Simple classroom examples and demonstrations
- Melting ice to water and refreezing it.
- Boiling water to produce water vapour (steam).
- Dissolving salt or sugar in water and recovering it by evaporation.
- Cutting paper, crushing a chalk piece, folding aluminium foil.
- Sublimation of dry ice (solid CO2) to gas.
Note: Some physical changes (like dissolving) may look like chemical changes; the test is whether the original substance can be recovered and whether a new substance with new chemical properties is formed.
- Melting of ice to form water (solid → liquid) — reversible by freezing.
- Boiling of water to steam (liquid → gas) — reversible by condensation.
- Dissolving sugar or salt in water — recoverable by evaporation (physical separation).
- Cutting paper or breaking a glass — shape/size change, composition unchanged.
- Sublimation of dry ice (solid CO2 → gas) — direct solid to gas change.
- Stretching a rubber band (elastic deformation) — returns to original shape when released (if within elastic limit).
- \[Conservation of mass (in a closed system): mass_initial = mass_final\]
- \[Density: ρ = mass / volume (useful when shape or volume changes)\]
- \[Heat for temperature change: Q = m · c · ΔT (m = mass\]\[c = specific heat, ΔT = temperature change)\]
- \[Latent heat for a change of state: Q = m · L (L = latent heat of fusion or vaporization)\]
Chemical Changes
Chemical Changes
Key Point: 4Fe + 3O2 → 2Fe2O3 (rusting of iron)
What is a chemical change?
A chemical change (chemical reaction) is a process in which one or more substances (reactants) change into new substances (products) with different chemical and physical properties. In a chemical change the original substances are transformed by breaking/forming chemical bonds.
Key features (how to recognise a chemical change)
- Formation of a new substance (different properties)
- Color change (not due to mixing only)
- Evolution of gas (bubbles, odor)
- Formation of a precipitate (insoluble solid)
- Change in temperature (exothermic: gives out heat; endothermic: absorbs heat)
- Usually not easily reversible by physical means
Why does it happen?
Chemical changes occur because atoms rearrange to form new combinations; energy changes (absorption or release) accompany the making or breaking of chemical bonds.
Conservation of mass
In a chemical reaction, mass is conserved: total mass of reactants = total mass of products (in a closed system). This principle helps balance chemical equations.
Common types of chemical reactions (simple terms)
- Combination (synthesis): two or more reactants form one product (A + B → AB)
- Decomposition: one compound breaks into simpler substances (AB → A + B)
- Displacement: a more reactive element replaces a less reactive one in a compound (A + BC → AC + B)
- Double displacement: exchange of ions between two compounds (AB + CD → AD + CB)
- Combustion: reaction with oxygen producing heat and often CO2 and H2O (fuel + O2 → CO2 + H2O)
Classroom-level examples and explanation
When iron rusts, iron reacts with oxygen to form iron(III) oxide; the new substance is brittle and reddish-brown — this is a chemical change. When vinegar reacts with baking soda, carbon dioxide gas evolves and a gas bubble stream is seen — again a chemical change. Cooking an egg: proteins change chemically and cannot be returned to the raw state.
- Rusting of iron: 4Fe + 3O2 → 2Fe2O3 (iron(III) oxide) — slow chemical change producing rust.
- Burning of natural gas (methane): CH4 + 2O2 → CO2 + 2H2O — fast combustion releasing heat and light.
- Reaction of zinc with dilute hydrochloric acid: Zn + 2HCl → ZnCl2 + H2 (hydrogen gas evolves).
- Decomposition of calcium carbonate on heating: CaCO3 → CaO + CO2 (used in making quicklime).
- Formation of curd from milk: lactose and milk proteins are changed by bacterial action — new substance (curd) forms.
- Neutralization: HCl + NaOH → NaCl + H2O (acid and base form salt and water).
- \[4Fe + 3O2 → 2Fe2O3 (rusting of iron)\]
- \[CH4 + 2O2 → CO2 + 2H2O (combustion of methane)\]
- \[2H2 + O2 → 2H2O (formation of water)\]
- \[CaCO3 → CaO + CO2 (thermal decomposition of calcium carbonate)\]
- \[Zn + 2HCl → ZnCl2 + H2 (metal reacting with acid)\]
- \[HCl + NaOH → NaCl + H2O (neutralization)\]
Comparing Physical and Chemical Changes
Comparing Physical and Chemical Changes
Key Point: General chemical equation format: A + B → C + D (reactants → products).
Overview: Changes in matter are classified as physical or chemical. A physical change alters the appearance or state of a substance without forming a new substance. A chemical change (chemical reaction) produces one or more new substances with different properties.
Physical changes — key points:
- No new substance is formed.
- Usually reversible by physical means (melting/freezing, evaporation/condensation).
- Mass remains the same; no chemical bonds are made or broken (only intermolecular changes).
Chemical changes — key points:
- One or more new substances are formed with different chemical properties.
- Often accompanied by indicators: color change, gas evolution (bubbles), formation of a precipitate, temperature change (heat released or absorbed), or light.
- Usually not easily reversible by simple physical methods.
- Involves breaking and forming chemical bonds; mass is conserved overall (Law of Conservation of Mass).
Particle-level view (simple): In physical change particles keep their identity but rearrange (e.g., ice -> water: H2O molecules move more freely). In chemical change, particles (atoms) are rearranged to form new kinds of molecules (e.g., H2 + O2 -> H2O).
How to tell them apart — practical indicators:
- If you only change state or shape (cutting, freezing, dissolving without reaction) it is likely physical.
- If there is a new smell, color change that can't be removed, gas without heating, light or heat given off, or a new solid appears in a solution, it is likely chemical.
Examples in daily life: Melting ice, boiling water, dissolving sugar in tea — physical. Rusting of iron, burning wood, cooking an egg, souring milk (curdling) — chemical.
Conservation of mass note: In both types of changes the total mass of the closed system remains constant; in chemical reactions atoms are simply rearranged into new substances.
- Melting of ice (physical) — water remains H2O; process is reversible by freezing.
- Boiling water (physical) — liquid to gas; no new substance formed.
- Dissolving sugar in water (physical) — sugar molecules disperse but are chemically unchanged.
- Cutting paper (physical) — size/shape change, same substance.
- Rusting of iron (chemical) — iron + oxygen → iron oxide; new substance forms and properties change.
- Burning wood (chemical) — wood turns to ash, smoke and gases; chemical composition changes.
- \[General chemical equation format: A + B → C + D (reactants → products).\]
- \[Law of Conservation of Mass: mass(reactants) = mass(products) (m_reactants = m_products).\]
- \[Example balanced reaction showing mass conservation: 2H2 + O2 → 2H2O.\]
- \[Heat for temperature change (useful in physical changes): Q = m × c × ΔT (where Q = heat\]\[m = mass\]\[c = specific heat, ΔT = temperature change).\]
- \[Heat for phase change (latent heat): Q = m × L (where L = latent heat of fusion/vaporization).\]
Energy Changes in Reactions
Energy Changes in Reactions
Key Point: ΔH = H_products − H_reactants (ΔH < 0 → exothermic; ΔH > 0 → endothermic)
When substances react, energy is either absorbed from or released to the surroundings. These energy changes are connected with breaking old chemical bonds and forming new ones. Breaking bonds needs energy (an input) while forming bonds releases energy.
There are two main types of reactions based on energy change:
- Exothermic reactions: Net energy is released to the surroundings. Temperature of surroundings rises. Examples include burning wood, combustion of fuels, and many neutralisation reactions.
- Endothermic reactions: Net energy is absorbed from the surroundings. Temperature of surroundings falls. Examples include photosynthesis, thermal decomposition and dissolving certain salts (e.g., ammonium nitrate) in water.
Important ideas:
- Activation energy (Ea): A small amount of energy needed to start most reactions (shown as a hump in energy diagrams).
- Energy profile diagrams: Show energy of reactants and products and help identify whether a reaction is exothermic (products lower than reactants) or endothermic (products higher than reactants).
- Law of conservation of energy: Energy is not created or destroyed in a chemical reaction — it is transferred between the reacting system and surroundings.
- Units: Energy is measured in joules (J) or calories (cal). Heat changes in simple experiments can be calculated using Q = m c ΔT (where Q is heat absorbed or released).
Simple classroom/household observations:
- If a reaction makes the test-tube feel warm → exothermic.
- If a reaction makes the test-tube feel cold → endothermic.
- Burning of paper or wood (combustion) — exothermic: releases heat and light.
- Respiration in our cells — exothermic: glucose is broken down to release energy.
- Photosynthesis in plants — endothermic: absorbs sunlight to make glucose from CO2 and water.
- Dissolving ammonium nitrate in water — endothermic: solution gets cold (used in instant cold packs).
- Mixing quicklime (calcium oxide) with water — exothermic: releases heat (used in self-heating cans).
- Neutralisation of acid and base (e.g., HCl + NaOH) — exothermic: heat is released.
- \[ΔH = H_products − H_reactants (ΔH < 0 → exothermic\]\[ΔH > 0 → endothermic)\]
- \[Q = m × c × ΔT where Q = heat energy (J)\]\[m = mass (kg or g)\]\[c = specific heat capacity (J/kg·°C or J/g·°C), ΔT = change in temperature (°C)\]
- \[Activation energy (Ea): energy barrier that must be overcome for reactants to form products (no single numeric formula here\]\[shown on energy diagrams)\]
Applications and Environmental/Practical Aspects
Applications and Environmental/Practical Aspects
Key Point: General chemical reaction: reactants → products
Introduction: Physical changes involve changes in state or shape without forming new substances (for example: melting, freezing, dissolving). Chemical changes produce new substances with different properties (for example: burning, rusting, fermentation). Both kinds of changes have many useful applications in daily life, industry and agriculture, but they also affect the environment and require careful practical management.
Applications of physical changes:
- Separation and purification: Filtration, evaporation, distillation and centrifugation separate mixtures (used in water purification, sugar refining, and laboratory work).
- Change of state for use or transport: Liquefying gases (e.g., LPG), freezing foods to preserve them, melting metals for casting.
- Mechanical processing: Cutting, grinding, and shaping materials (paper cutting, crushing ores) are physical changes used in manufacturing and recycling.
- Recycling: Sorting and remelting plastics, metals and glass rely on reversible physical changes to conserve resources.
Applications of chemical changes:
- Energy production: Combustion of fuels (coal, natural gas, petrol) releases heat and power for homes, transport and industries.
- Food and fermentation: Fermentation (sugar → alcohol + CO2) produces bread, wine and yoghurt; cooking causes chemical changes that make food digestible.
- Industry and materials: Chemical reactions produce fertilizers, medicines, plastics and building materials (e.g., cement formation from lime).
- Biological processes: Photosynthesis (plants make glucose) and respiration (organisms release energy) are essential chemical changes for life.
Environmental impacts:
- Air pollution and greenhouse effect: Burning fossil fuels produces CO2, SO2, NOx and particulates leading to global warming, smog and respiratory problems.
- Acid rain: SO2 and NOx react with water in air to form acids that damage buildings, soil and aquatic life.
- Water pollution and eutrophication: Discharge of chemicals and excess fertilizers causes algal blooms and oxygen loss in water bodies.
- Waste and non-biodegradable pollutants: Many synthetic chemical products persist in the environment and cause long-term harm.
- Corrosion and resource loss: Chemical changes such as rusting weaken structures and increase repair costs and material consumption.
Practical aspects and safety/management:
- Conservation and recycling: Prefer physical recycling (re-melting, reusing) where possible to save resources and reduce harmful chemical production.
- Pollution control: Use catalytic converters, scrubbers, and filters to reduce emissions from combustion and industry.
- Neutralization and treatment: Neutralize acidic or basic wastes before disposal (acid + base → salt + water) and treat wastewater biologically/chemically to remove pollutants.
- Composting and biodegradation: Encourage biological chemical changes that convert organic waste into useful compost rather than methane-producing landfills.
- Lab and household safety: Use proper storage, ventilation, personal protective equipment and follow instructions to avoid harmful chemical reactions (explosions, toxic gases).
- Monitoring changes: Use indicators (litmus), pH meters, and simple mass/temperature measurements to detect and control chemical changes.
Key practical rule: In a closed system the total mass of reactants equals the total mass of products (conservation of mass). Many environmental solutions aim to reduce harmful chemical changes or redirect them into useful processes (e.g., capturing CO2, composting organic waste).
Summary: Understanding how physical and chemical changes are used and how they affect the environment helps us use materials wisely, reduce pollution, ensure safety and protect natural resources.
- Dissolving sugar in water to make lemonade (physical change)
- Melting ice to obtain water for drinking or cleaning (physical change)
- Rusting of iron gutters and rails (chemical change — causes structural damage)
- Burning wood or petrol for heat and transport (chemical change — releases CO2 and pollutants)
- Cooking an egg: proteins denature and new substances form (chemical change)
- Fermentation of grapes to make wine: C6H12O6 → 2C2H5OH + 2CO2 (chemical change)
- \[General chemical reaction: reactants → products\]
- \[Photosynthesis: 6CO2 + 6H2O + light → C6H12O6 + 6O2\]
- \[Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy\]
- \[Combustion (complete): fuel + O2 → CO2 + H2O + energy\]
- \[Rusting (simplified): 4Fe + 3O2 + 6H2O → 4Fe(OH)3 (often written as hydrated Fe2O3)\]
- \[Neutralization: acid + base → salt + water (e.g.\]\[HCl + NaOH → NaCl + H2O)\]
Practical Work and Experiments
Practical Work and Experiments
Key Point: Mass conservation (closed system): mass of reactants = mass of products (use to check experimental errors).
Practical work and experiments are essential to learn how physical and chemical changes occur. A well-planned experiment helps you observe changes, collect data, and draw conclusions. The typical steps for a school experiment are:
- State the aim or question (What do you want to find out?).
- Write the hypothesis (a simple prediction).
- List materials and safety precautions.
- Identify variables: independent (what you change), dependent (what you measure), and controlled (what you keep the same).
- Describe the procedure clearly so it can be repeated.
- Observe and record results (tables, notes, measurements).
- Draw conclusions and relate them to physical or chemical changes.
Key points when studying physical vs chemical changes during experiments:
- Physical changes: no new substance is formed. Typical signs: change of state (melting, freezing, evaporation), shape, or size; usually reversible (e.g., melting ice → water → refreeze).
- Chemical changes: new substances are produced. Typical signs (indicators): color change, gas evolution (bubbles), formation of a precipitate, change in temperature (exothermic or endothermic), emission of light, and often irreversibility under the same conditions.
Recording and safety: use a table to record observations (time, temperature, mass, appearance). Always wear safety goggles, use heat-resistant mats, work under supervision for reactions that produce gas or heat, and never taste chemicals.
Repetition and controls: repeat trials to confirm results and use a control experiment (where the factor being tested is absent) to compare outcomes.
- Melting of ice (physical change): Solid ice → liquid water. Reversible by cooling.
- Dissolving salt in water (physical change): Salt disperses but no new substance is formed; can be recovered by evaporation.
- Evaporation of water (physical change): Liquid water → vapour; reversible by condensation.
- Rusting of iron (chemical change): Iron + oxygen (+ moisture) → hydrated iron(III) oxide (rust); indicated by reddish-brown deposit and is not easily reversible.
- Reaction of baking soda and vinegar (chemical change): Produces carbon dioxide gas (bubbling) and a new substance (sodium acetate and water).
- Heating copper(II) sulfate pentahydrate (physical/chemical observation): Blue crystals lose water on heating and become white anhydrous copper sulfate (CuSO4·5H2O → CuSO4 + 5H2O).
- \[Mass conservation (closed system): mass of reactants = mass of products (use to check experimental errors).\]
- \[NaHCO3 + CH3COOH → CO2↑ + H2O + CH3COONa (baking soda + acetic acid → carbon dioxide gas + water + sodium acetate)\]
- \[2Mg + O2 → 2MgO (magnesium burning to form magnesium oxide\]\[exothermic chemical change)\]
- \[CuSO4·5H2O → CuSO4 + 5H2O (on heating\]\[hydrated copper sulfate loses water of crystallization)\]
- \[ΔT = T_final − T_initial (useful to record temperature change in exothermic/endothermic reactions)\]
Key Concepts and Definitions
Key Concepts and Definitions
Key Point: Law of Conservation of Mass (qualitative): mass(reactants) = mass(products) (for a closed system).
Overview
Physical and chemical changes describe how matter is transformed. A physical change alters the form or appearance of a substance without forming a new substance. A chemical change (chemical reaction) produces one or more new substances with different chemical properties.
Physical change — definition and features
- Definition: A change in which the physical properties (shape, size, state, appearance) of a substance change but its chemical identity remains the same.
- Features: No new substance formed; usually reversible (not always); mass remains constant in a closed system; examples include changes of state (melting, freezing, evaporation), cutting, bending, dissolving (when no chemical reaction occurs).
Chemical change — definition and features
- Definition: A change in which one or more substances (reactants) transform into new substances (products) with different chemical properties and composition.
- Features: New substances formed; often accompanied by energy change (heat, light); signs include color change, gas evolution, formation of a precipitate, temperature change; often irreversible (many exceptions exist); mass is conserved in a closed system.
Important related terms
- Reactants and products: Substances before and after a chemical reaction (notation: reactants → products).
- Precipitate: An insoluble solid formed when two solutions react.
- Exothermic reaction: Releases heat (temperature of surroundings rises).
- Endothermic reaction: Absorbs heat (temperature of surroundings falls).
- Irreversible/reversible: Many physical changes are reversible (e.g., freezing ⇄ melting); many chemical changes appear irreversible (e.g., burning), though some chemical reactions are reversible under conditions.
Law of Conservation of Mass (simple statement)
Mass of reactants = Mass of products in a chemical change, provided the system is closed.
How to identify a type of change
- Observe: Does a new substance form? (bubbles not from trapped air, color change consistent, precipitate formation) → chemical change likely.
- If only form or state changes (shape, size, state) with no new substance → physical change.
Examples used in class 7 experiments
- Physical: Melting ice, evaporation of water, dissolving salt/sugar in water, cutting paper.
- Chemical: Burning paper/wood, rusting of iron, souring of milk, baking an egg (cooking), reaction of vinegar and baking soda (CO2 formation), formation of silver chloride (white precipitate) when silver nitrate reacts with sodium chloride.
This summary gives the core definitions and criteria taught in Class 7 so students can classify changes and understand observable signs and basic conservation laws.
- Melting of ice to water — physical change (state change; reversible).
- Dissolving sugar in water — physical change (no new substance formed; can be recovered by evaporation).
- Burning of paper or wood — chemical change (new substances like ash and gases form; often irreversible).
- Rusting of iron (Fe + O2 + H2O → hydrated iron(III) oxide) — chemical change (new compound forms; slow, involves oxygen).
- Reaction of vinegar (acetic acid) and baking soda (sodium bicarbonate) — chemical change producing CO2 gas (bubbling).
- Formation of a white precipitate when silver nitrate solution reacts with sodium chloride solution (AgNO3 + NaCl → AgCl(s) + NaNO3) — chemical change (insoluble product forms).
- \[Law of Conservation of Mass (qualitative): mass(reactants) = mass(products) (for a closed system).\]
- \[Chemical reaction notation: reactants → products (example: H2 + O2 → H2O — simplified representation).\]
- \[Exothermic/endothermic energy sign: ΔH < 0 for exothermic (heat released), ΔH > 0 for endothermic (heat absorbed).\]
- \[Precipitation example (word & formula): silver nitrate + sodium chloride → silver chloride (precipitate) + sodium nitrate\]\[AgNO3 + NaCl → AgCl(s) + NaNO3.\]
Key Concepts
- Physical change
- A change that alters the physical properties of a substance without forming a new substance.
- Chemical change
- A change in which new substances with different properties are formed.
- Reactant
- A substance present at the start of a chemical reaction that undergoes change.
- Product
- A substance formed as the result of a chemical reaction.
- Reversible change
- A change that can be undone and the original substance recovered.
- Irreversible change
- A change that cannot be easily reversed to regain the original substance.
- Melting
- Transition of a solid into a liquid on heating at its melting point.
- Freezing (Solidification)
- Transition of a liquid into a solid on cooling at its freezing point.
- Boiling
- Rapid vaporization of a liquid that occurs throughout the liquid at its boiling point.
- Evaporation
- Slow vaporization of a liquid from its surface at temperatures below the boiling point.
- Condensation
- Change of a gas into a liquid when cooled.
- Sublimation
- Transition of a substance directly from solid to gas without passing through the liquid state.
- Dissolution
- Process in which a solute disperses uniformly in a solvent to form a solution.
- Precipitate
- An insoluble solid that forms and separates from a solution during a chemical reaction.
- Oxidation
- A chemical process where a substance combines with oxygen or loses electrons.
- Combustion
- A chemical reaction of a substance with oxygen that releases heat and often light.
- Rusting
- Corrosion of iron by oxygen and moisture, producing hydrated iron(III) oxide (rust).
- Tarnishing
- Slow surface oxidation of metals (like silver) that forms a dull or discolored layer.
- Exothermic change
- A process or reaction that releases heat to the surroundings.
- Endothermic change
- A process or reaction that absorbs heat from the surroundings.
Practice Questions
-
Which of the following is a chemical change? (a) Melting of ice, (b) Cutting paper, (c) Rusting of iron, (d) Dissolving sugar in water. / निम्नलिखित में से कौन-सा रासायनिक परिवर्तन है? (a) बर्फ का पिघलना, (b) कागज काटना, (c) लोहे पर जंग लगना, (d) पानी में चीनी घोलना।
Show answer
(c) Rusting of iron. / लोहे पर जंग लगना। — Rusting produces a new substance (iron oxide) with different properties; the change is irreversible, making it a chemical change. The others are physical changes where no new substance forms. / जंग लगने से एक नया पदार्थ (आयरन ऑक्साइड) बनता है जिसके गुण अलग होते हैं; परिवर्तन अपरिवर्तनीय है, जो इसे रासायनिक परिवर्तन बनाता है। बाकी भौतिक परिवर्तन हैं जिनमें कोई नया पदार्थ नहीं बनता।
-
During a chemical change, which of the following is always conserved? (a) Colour, (b) State of matter, (c) Mass, (d) Shape. / रासायनिक परिवर्तन के दौरान निम्नलिखित में से कौन-सा हमेशा संरक्षित रहता है? (a) रंग, (b) पदार्थ की अवस्था, (c) द्रव्यमान, (d) आकार।
Show answer
(c) Mass. / द्रव्यमान। — According to the Law of Conservation of Mass, the total mass of reactants equals the total mass of products in a closed system. / द्रव्यमान संरक्षण के नियम के अनुसार, एक बंद प्रणाली में अभिकारकों का कुल द्रव्यमान उत्पादों के कुल द्रव्यमान के बराबर होता है।
-
Which of the following is NOT an indicator of a chemical change? (a) Change in colour, (b) Evolution of gas, (c) Change in shape, (d) Formation of a precipitate. / निम्नलिखित में से कौन-सा रासायनिक परिवर्तन का सूचक नहीं है? (a) रंग में परिवर्तन, (b) गैस का निकलना, (c) आकार में परिवर्तन, (d) अवक्षेप का बनना।
Show answer
(c) Change in shape. / आकार में परिवर्तन। — A change in shape (like cutting or bending) is a physical change where no new substance is produced. The other options are indicators of chemical change. / आकार में परिवर्तन (जैसे काटना या मोड़ना) एक भौतिक परिवर्तन है जिसमें कोई नया पदार्थ नहीं बनता। अन्य विकल्प रासायनिक परिवर्तन के सूचक हैं।
-
A reaction that releases heat to the surroundings is called an ________ reaction. / जो अभिक्रिया परिवेश में ऊष्मा छोड़ती है उसे ________ अभिक्रिया कहते हैं।
Show answer
Exothermic / ऊष्माक्षेपी (एक्जोथर्मिक) — In an exothermic reaction, the products have less energy than the reactants, so the extra energy is released as heat to the surroundings (e.g., burning wood, neutralisation). / ऊष्माक्षेपी अभिक्रिया में, उत्पादों की ऊर्जा अभिकारकों से कम होती है, इसलिए अतिरिक्त ऊर्जा परिवेश में ऊष्मा के रूप में निकलती है (जैसे लकड़ी जलाना, उदासीनीकरण)।
-
Dissolving salt in water is a physical change because the salt can be recovered by ________. / पानी में नमक घोलना एक भौतिक परिवर्तन है क्योंकि नमक को ________ द्वारा वापस प्राप्त किया जा सकता है।
Show answer
Evaporation / वाष्पीकरण — When the water is evaporated, the salt is left behind unchanged in its chemical composition. This confirms that dissolving is a physical change. / जब पानी वाष्पित हो जाता है, तो नमक अपनी रासायनिक संरचना में अपरिवर्तित रूप में बच जाता है। यह पुष्टि करता है कि घुलना एक भौतिक परिवर्तन है।
-
True or False: Burning of wood is a reversible change. / सत्य या असत्य: लकड़ी का जलना एक उत्क्रमणीय परिवर्तन है।
Show answer
False / असत्य — Burning wood produces new substances like ash, carbon dioxide, and water vapour. These cannot easily be turned back into wood, so it is an irreversible chemical change. / लकड़ी जलाने पर राख, कार्बन डाइऑक्साइड और जलवाष्प जैसे नए पदार्थ बनते हैं। इन्हें आसानी से वापस लकड़ी में नहीं बदला जा सकता, इसलिए यह एक अपरिवर्तनीय रासायनिक परिवर्तन है।
-
State two differences between a physical change and a chemical change. / भौतिक परिवर्तन और रासायनिक परिवर्तन के बीच दो अंतर बताएं।
Show answer
1. In a physical change no new substance is formed, whereas in a chemical change one or more new substances with different properties are produced. 2. Physical changes are usually reversible (e.g., melting ice can be re-frozen), whereas chemical changes are generally irreversible (e.g., cooked egg cannot be made raw again). / 1. भौतिक परिवर्तन में कोई नया पदार्थ नहीं बनता, जबकि रासायनिक परिवर्तन में अलग-अलग गुणों वाले एक या अधिक नए पदार्थ बनते हैं। 2. भौतिक परिवर्तन आमतौर पर उत्क्रमणीय होते हैं (जैसे पिघली बर्फ को फिर से जमाया जा सकता है), जबकि रासायनिक परिवर्तन सामान्यतः अपरिवर्तनीय होते हैं (जैसे पका अंडा कच्चा नहीं बनाया जा सकता)।
-
Write the balanced chemical equation for the rusting of iron and state the conditions needed. / लोहे के जंग लगने का संतुलित रासायनिक समीकरण लिखें और इसके लिए आवश्यक परिस्थितियाँ बताएं।
Show answer
4Fe + 3O₂ → 2Fe₂O₃ (or more accurately with water: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃, often written as hydrated iron oxide). Conditions needed: presence of both oxygen (from air) and water (moisture). Rusting does not occur in dry air or in oxygen-free water alone. / 4Fe + 3O₂ → 2Fe₂O₃ (या पानी के साथ: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃, जिसे प्रायः जलयोजित आयरन ऑक्साइड के रूप में लिखा जाता है)। आवश्यक परिस्थितियाँ: ऑक्सीजन (हवा से) और पानी (नमी) दोनों की उपस्थिति। सूखी हवा में या केवल ऑक्सीजन-मुक्त पानी में जंग नहीं लगता।
Related Laws & Principles
Explore allFoundational laws & principles connected to this chapter — tap to open in the Laws Explorer.