Changes Around Us: Physical and Chemical

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physical

chemical reaction

rusting, chemical

calcium carbonate

combustion, chemical

magnesium oxide, light

physical, reversed

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weathering

erosion

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bioluminescence

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A–4, B–1, C–2, D–3, E–5

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A change in which only physical properties such as shape, size or state of a substance change and no new substance is formed. Example: melting of ice.

A change in which one or more new substances are formed. Rusting is a chemical change because a new substance, iron oxide (rust), is formed.

Carbon dioxide present in exhaled air reacts with lime water (calcium hydroxide) to form an insoluble white substance, calcium carbonate, which makes the liquid appear milky.

Magnesium + Oxygen → Magnesium oxide + Heat + Light.

A chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Combustible substances: wood and paper (also cotton, kerosene).

The covered candle does not get a continuous supply of air (oxygen); so once the trapped oxygen is used up, the flame gets extinguished.

The minimum temperature at which a substance catches fire is called its ignition temperature.

(i) A combustible substance (fuel), (ii) oxygen, and (iii) heat that allows the fuel to reach its ignition temperature.

Synthetic cloth can melt and stick to the skin, causing severe burns; so a cotton or woollen blanket should be used instead.

Melting of wax, its solidification and evaporation are physical changes, while the burning of wax vapour to form new substances is a chemical change.

The production of light (without heat) by a chemical change in living organisms. Example: fireflies.

Desirable: ripening of fruits (or cooking of food); Undesirable: rusting of iron (or decay of stored food).

The physical and chemical changes in rocks caused by temperature, roots, water, etc. Weathering finally leads to the formation of soil.

Reversible changes can return to the original form (melting of ice), while irreversible changes cannot (chopping of vegetables or making popcorn).

Focusing sunrays heats the paper; its temperature rises until it reaches the ignition temperature of paper and it catches fire.

The process in which rock pebbles, soil and sediments are broken down and moved from one place to another. Natural forces: wind and flowing water.

A new substance, carbon dioxide, is produced with a fizzing sound; formation of a new substance indicates a chemical change.

It causes long-term environmental impact; e.g., increased fuel consumption raises carbon dioxide levels in the atmosphere.

Carbon dioxide reacts with lime water to form insoluble calcium carbonate, turning the lime water milky; this is a reliable indicator of CO₂.

Constant erosion by flowing water wears them down, making them smoother over time.

Introduction: Changes around us are broadly classified into physical and chemical changes based on whether new substances are formed.
Key Points:
1. No new substance (physical) — In a physical change, only shape, size, or state changes; no new substance is formed. Examples: melting ice, cutting paper.
2. New substance formed (chemical) — In a chemical change, one or more new substances are produced through a chemical reaction. Examples: rusting of iron, burning of magnesium.
3. Reversibility — Physical changes are often reversible (ice to water and back), while chemical changes are generally irreversible (burnt paper cannot return).
Conclusion: The key difference lies in whether a new substance is formed — physical changes do not form one, chemical changes do.

Introduction: The reaction of exhaled air with lime water demonstrates a chemical change by forming a new, insoluble substance.
Key Points:
1. Set-up — Fill one-fourth of a glass tumbler with lime water and blow exhaled air through a straw into it.
2. Observation — The lime water turns milky (cloudy), and after some time a white substance settles at the bottom.
3. New substance — Carbon dioxide in exhaled air reacts with lime water to form insoluble calcium carbonate, confirming a new substance is formed.
Word Equation: Calcium hydroxide (Lime water) + Carbon dioxide → Calcium carbonate (Insoluble substance) + Water
Conclusion: The milky appearance and white deposit confirm that a chemical change has occurred, producing a new substance.

Introduction: Mixing vinegar with baking soda produces a gas that can be identified, confirming a chemical change.
Key Points:
1. Mixing — Add a pinch of baking soda to a teaspoonful of vinegar in a test tube; a fizzing sound and gas bubbles form.
2. Testing the gas — Pass the gas through freshly prepared lime water in another test tube; the lime water turns milky.
3. Confirmation — The milky lime water confirms the gas is carbon dioxide, a new substance not present before, proving a chemical change.
Word Equation: Vinegar + Baking soda → Carbon dioxide + Other substances
Conclusion: Since a new substance (carbon dioxide) is formed, the reaction between vinegar and baking soda is a chemical change.

Introduction: Combustion is a chemical reaction in which a substance reacts with oxygen, producing heat and/or light.
Key Points:
1. Activity (oxygen needed) — Two identical candles are lit; one is covered with a glass tumbler and it extinguishes due to lack of oxygen.
2. Three requirements — Combustion needs a combustible substance (fuel), oxygen (from air), and heat to reach ignition temperature.
3. Fire triangle — These three requirements together form the "fire triangle"; removing any one stops combustion.
Conclusion: Combustion occurs only when all three elements of the fire triangle — fuel, oxygen, and heat — are present together.

Introduction: A magnifying glass can focus sunrays onto paper, raising its temperature until it catches fire.
Key Points:
1. Focusing sunrays — The magnifying glass concentrates sunlight into a small, bright spot on the paper, heating it steadily.
2. Temperature rises — The paper's temperature increases over time until it reaches its ignition temperature and starts burning.
3. No flame needed — This shows a substance can burn without an existing flame, as long as heat raises it to ignition temperature.
Conclusion: The magnifying glass supplies enough heat to reach the paper's ignition temperature, causing it to catch fire without a matchstick.

Introduction: The burning of a candle is a process that simultaneously involves both physical and chemical changes.
Key Points:
1. Physical changes — The wax melts, is drawn up the wick, evaporates due to heat, and can solidify again — all changes of state without new substances.
2. Chemical change — The wax vapour burns to produce a flame, forming new substances like carbon dioxide and water vapour.
3. Simultaneous occurrence — Melting and evaporation (physical) and burning of vapour (chemical) happen together in the same burning candle.
Conclusion: The burning candle demonstrates that physical and chemical changes can occur together within a single process.

Introduction: Whether a change is desirable or undesirable depends on the situation in which it occurs.
Key Points:
1. Undesirable situation — The decomposition of food during storage is undesirable because it causes spoilage and waste.
2. Desirable situation — The same decomposition process is desirable when food waste is converted into compost, enriching soil fertility.
3. Same change, different value — The chemical change of decomposition is harmful during food storage but useful for making compost.
Conclusion: A change can be undesirable in one context and desirable in another, depending on how it is used.

Introduction: Weathering is the gradual breaking down of rocks through physical and chemical changes over time.
Key Points:
1. Physical weathering — Temperature changes, growing roots, and freezing water in cracks break large rocks into smaller pieces.
2. Chemical weathering — Water or chemicals in water react with minerals in rocks, changing their composition (e.g., basalt turning red).
3. Soil formation — These broken-down rock particles, mixed with organic matter over long periods, eventually form soil.
Conclusion: Weathering, through combined physical and chemical changes, gradually converts rocks into the soil that supports life.

Introduction: Erosion is the process by which rock pebbles, soil, and sediments are broken down and moved by wind and flowing water.
Key Points:
1. Process — Natural forces like wind and flowing water transport rock and sediment from one location to another, smoothing rocks over time.
2. Sediment deposition — When wind or water speed decreases, transported materials settle at the bottom of lakes or oceans and harden into new rocks.
3. Irreversibility — These changes take place over thousands of years, making it practically impossible to reverse them.
Conclusion: Erosion is mostly irreversible because it occurs over extremely long periods and permanently relocates and transforms materials.

Introduction: Some changes caused by human activities have lasting environmental consequences.
Key Points:
1. Fuel consumption — Increased use of fuels in cars, trains, and aeroplanes raises carbon dioxide levels in the atmosphere, contributing to pollution.
2. Paint drying — Drying paint on walls and furniture releases substances through evaporation, causing atmospheric pollution.
3. Long-term impact — These ongoing changes accumulate over time and are difficult to reverse, harming the environment.
Conclusion: Human activities like burning fuels and using paints cause chemical changes that lead to long-lasting environmental damage.

Introduction: The classification of a change as physical or chemical depends on whether a new substance is formed, not on reversibility alone.
Key Points:
1. Rusting — new substance — Rusting produces a new brown substance, iron oxide, which is different from the original iron; hence it is a chemical change.
2. Crushing chalk — no new substance — Crushing chalk only changes its size and shape; the powder is still chalk with no new substance formed, so it is physical.
3. Reversibility not the criterion — Both changes are hard to reverse, but the formation of a new substance is what distinguishes rusting as chemical.
Conclusion: Rusting is a chemical change because a new substance (iron oxide) forms, while crushing chalk is a physical change because no new substance is produced.

Introduction: The gas produced by burning a candle can be tested for carbon dioxide using lime water.
Key Points:
1. Collecting the gas — Cover a burning candle with a glass tumbler; the flame extinguishes, and carbon dioxide collects inside.
2. Adding lime water — Add a small amount of lime water to the petri dish under the inverted tumbler.
3. Observation — The lime water turns milky, confirming the presence of carbon dioxide gas from the burnt candle.
Conclusion: The turning of lime water milky confirms that the gas produced by burning a candle is carbon dioxide.

Introduction: Ignition temperature is the minimum temperature at which a substance catches fire.
Key Points:
1. Matchstick activity — When a lighted matchstick is brought near paper, the paper catches fire almost immediately.
2. Reason — The matchstick's temperature is already higher than the ignition temperature of paper, so combustion starts quickly.
3. Definition confirmed — Ignition temperature is the lowest temperature at which a substance begins to burn without an external flame.
Conclusion: The matchstick supplies heat above the paper's ignition temperature, causing it to catch fire rapidly.

Introduction: The colour change of basalt rock from black to red indicates a chemical change in its composition.
Key Points:
1. Iron content — Basalt rock contains iron, which reacts when its surface is exposed to water or air containing water vapour.
2. New substance formed — The iron reacts to form iron oxide, which gives the rock surface a red colour — a new substance different from the original.
3. Chemical change confirmed — Since a new substance (iron oxide) with different properties is produced, it is a chemical change.
Conclusion: The formation of red iron oxide from the iron in basalt confirms this is a chemical change, not merely a physical one.

Introduction: We observe changes occurring around us using our senses of sight, smell, touch, hearing, and taste.
Key Points:
1. Sight — We see changes in colour or appearance, such as banana developing brown spots over time.
2. Smell — We detect changes in odour, such as the strong smell that develops as a banana ripens.
3. Touch and hearing — We feel warmth when ice melts on the hand, and hear the fizzing sound when vinegar reacts with baking soda.
Conclusion: Our senses help us detect and classify the various changes happening around us in daily life.

(a) Tap water only forms bubbles with no change in appearance; lime water turns milky. (b) Carbon dioxide from exhaled air. (c) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water. A new substance (calcium carbonate) is formed, so it is a chemical change.

(a) Carbon dioxide. (b) Formation of a new gas (CO₂) with fizzing, and lime water turning milky. (c) With baking soda and water no new substance is formed; only dissolving occurs, so it is a physical change, not a chemical change.

(a) It did not get a continuous supply of air (oxygen). (b) Carbon dioxide. (c) Oxygen is required for combustion; combustion produces carbon dioxide (from carbon of wax and oxygen of air); without oxygen, the flame gets extinguished.

(a) The minimum temperature at which a substance catches fire. (b) Focusing sunrays heated the paper to its ignition temperature. (c) Both show that a combustible substance (paper) and heat are needed; the matchstick was already hotter than the ignition temperature, while the magnifying glass supplied the required heat.

(a) Melting, evaporation and solidification of wax are physical changes. (b) Burning of wax vapour to form new substances is a chemical change. (c) Faraday delivered lectures called 'Chemical History of a Candle'; he used the candle to explain differences between physical and chemical processes like melting, vapourisation and combustion, showing both can occur in one process.

(a) Rust (iron oxide); it is a chemical change. (b) Desirable, because it converts waste into useful compost. (c) Rusting—chemical change (new substance formed); composting—chemical change (decomposition by bacteria/fungi); paint drying—releases substances by evaporation causing pollution, a change involving evaporation.

(a) Weathering and erosion. (b) Iron in basalt reacted with water/air to form iron oxide (red). (c) Breaking of rocks is a physical change and the formation of iron oxide is a chemical change; together (weathering) they form soil, while erosion moves sediments and forms new rocks over thousands of years.

(a) Rust (iron oxide). (b) Melting of ice can be reversed. (c) Rusting—chemical, irreversible; chopping vegetables—physical, irreversible; melting of ice—physical, reversible.

(a) Tearing of paper, melting of ice, folding of clothes. (b) Rusting, curdling of milk, ripening of fruits, burning of magnesium, mixing baking soda with vinegar. (c) Burning of a candle involves both—melting/evaporation of wax (physical) and burning of wax vapour (chemical).

(a) Chopping/peeling/cutting fruits and vegetables, and collecting peels into the pot (physical). (b) Decomposition of peels into compost by bacteria and fungi (chemical). (c) Chopping/collecting—physical (no new substance); decomposition—chemical (new substance, compost); germination and growth into plants—chemical (new substances/life processes formed).

Introduction: Changes around us are classified as physical or chemical based on whether new substances are formed during the process.
Key Points:
1. Physical change defined — A physical change is one in which a substance undergoes a change in physical properties like shape, size, or state, and no new substance is formed.
2. Physical change examples — Melting of ice cubes, folding paper into shapes, and crushing chalk into powder all change appearance without forming new substances.
3. Chemical change defined — A chemical change is one in which one or more new substances are formed through a chemical reaction.
4. Chemical change examples — Rusting of iron forms iron oxide, burning of magnesium forms magnesium oxide, and mixing vinegar with baking soda produces carbon dioxide.
5. Key difference — In physical changes the substance remains the same only in a different form, while in chemical changes entirely new substances with different properties are produced.
Conclusion: The fundamental distinction is that physical changes do not produce new substances, whereas chemical changes always produce one or more new substances.

Introduction: Blowing exhaled air into lime water is a classic activity that demonstrates both a chemical change and a test for carbon dioxide.
Key Points:
1. Set-up — Take a glass tumbler, fill one-fourth with lime water, and blow exhaled air into it through a straw.
2. Observation — The lime water turns milky (cloudy), and after some time a white substance settles at the bottom of the tumbler.
3. New substance formed — Carbon dioxide from exhaled air reacts with lime water (calcium hydroxide) to form calcium carbonate, an insoluble white substance, confirming a chemical change.
4. Test for carbon dioxide — The turning of lime water milky is used as a standard test to detect the presence of carbon dioxide gas.
5. Word equation — Calcium hydroxide (Lime water) + Carbon dioxide → Calcium carbonate (Insoluble substance) + Water.
Conclusion: This activity confirms a chemical change by forming a new substance (calcium carbonate) and simultaneously serves as a reliable test for carbon dioxide gas.

Introduction: Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light, requiring three key elements.
Key Points:
1. Definition — Combustion is a chemical reaction where a combustible substance reacts with oxygen, producing heat and/or light.
2. Three requirements (fire triangle) — Combustion requires a combustible substance (fuel), oxygen (from air), and heat to reach ignition temperature.
3. Activity — set-up — Two identical candles are placed on two petri dishes and lit; one candle is covered with a glass tumbler.
4. Activity — observation — The uncovered candle continues to burn, while the covered candle extinguishes after some time due to lack of oxygen.
5. Confirmation — Adding lime water to the covered dish turns it milky, confirming carbon dioxide was formed from the carbon of wax and oxygen of air.
Conclusion: The activity demonstrates that oxygen is essential for combustion, and the fire triangle shows that fuel, oxygen, and heat are all necessary.

Introduction: The burning of a candle is a process that simultaneously involves both physical and chemical changes, a subject famously studied by Michael Faraday.
Key Points:
1. Physical change — melting — The heat of the flame melts the solid wax near the wick, changing it from solid to liquid.
2. Physical change — evaporation and solidification — The liquid wax is drawn up the wick and evaporates into wax vapour; spilt wax solidifies back into solid wax.
3. Chemical change — burning of vapour — The wax vapour burns in the flame, reacting with oxygen to form new substances like carbon dioxide and water.
4. Both together — Melting, evaporation, and solidification (physical) and the burning of vapour (chemical) occur simultaneously in the same candle.
5. Faraday's contribution — Michael Faraday delivered a series of lectures called "Chemical History of a Candle," using the candle to explain differences between melting, vapourisation, and combustion.
Conclusion: The burning candle demonstrates that physical and chemical changes can occur together in a single process, a concept explored by Faraday.

Introduction: Changes around us can be grouped into those that can be reversed and those that cannot be reversed.
Key Points:
1. Reversible changes — These are changes where the original substance or object can be brought back; examples: melting ice (refreezes), boiling water (condenses back), and inflating a balloon (air escapes, balloon returns).
2. Irreversible changes — These are changes where the original form cannot be recovered; examples: chopping vegetables, making popcorn, and burning wood.
3. Some physical changes cannot be reversed — Although no new substance is formed, some physical changes like crushing chalk or chopping vegetables cannot be reversed because the original shape and size are permanently altered.
4. Chemical changes are mostly irreversible — Changes like rusting or burning produce new substances that cannot be converted back to the original material.
5. Reversibility not the only criterion — A change being irreversible does not necessarily mean it is chemical; crushing chalk is physical yet irreversible.
Conclusion: Reversible changes allow recovery of the original substance, while irreversible changes do not, and some physical changes are irreversible despite not forming new substances.

Introduction: Changes around us can be desirable (useful) or undesirable (harmful), and the same change can fall into either category depending on context.
Key Points:
1. Desirable changes — Useful changes in daily life include converting milk to curd, ripening of fruits, and cooking of food.
2. Undesirable changes — Harmful changes include rusting of iron and the decay or spoilage of stored food.
3. Same change — decomposition of food — Decomposition is undesirable when it spoils stored food but desirable when food waste is converted into compost.
4. Situation matters — Whether a change is desirable or undesirable depends on where and why it occurs, not on the change itself.
5. Human activities — Increased fuel consumption and paint drying cause atmospheric pollution, which is undesirable for the environment.
Conclusion: The desirability of a change is determined by the situation; the same change can be beneficial or harmful depending on the context.

Introduction: Weathering and erosion are slow natural processes that break down rocks and transport materials, shaping the Earth's surface over thousands of years.
Key Points:
1. Weathering — physical — Temperature changes, growing roots, and freezing water in cracks physically break large rocks into smaller pieces.
2. Weathering — chemical — Water or chemicals in water react with rock minerals, changing their composition (e.g., basalt turning red due to iron oxide).
3. Soil formation — The combined physical and chemical weathering of rocks, mixed with organic matter over time, eventually forms soil.
4. Erosion — Wind and flowing water break down and transport rock pebbles, soil, and sediments from one location to another, smoothing rocks.
5. New rock formation — When wind or water slows down, transported sediments settle at the bottom of oceans or lakes, harden over time, and form new rocks.
Conclusion: Weathering breaks rocks into soil-forming material, while erosion transports sediments that eventually harden into new rocks; both are mostly irreversible.

Introduction: The reaction between vinegar and baking soda produces a gas that can be tested to confirm a chemical change.
Key Points:
1. Mixing — Take a teaspoonful of vinegar in a test tube and add a pinch of baking soda (sodium hydrogen carbonate) to it.
2. Observation — A fizzing, bubbling sound is heard and gas bubbles form immediately, indicating a reaction is occurring.
3. Testing the gas — Pass the gas through freshly prepared lime water in another test tube using a straw or delivery arrangement.
4. Result — The lime water turns milky, confirming the gas produced is carbon dioxide, a new substance not present before.
5. Word equation — Vinegar + Baking soda → Carbon dioxide + Other substances.
Conclusion: The formation of carbon dioxide (a new substance) confirms that the reaction between vinegar and baking soda is a chemical change.

Introduction: Bioluminescence and combustion both involve chemical changes that produce light, but they differ significantly in their characteristics.
Key Points:
1. Bioluminescence defined — Bioluminescence is the production of light (without heat) by living organisms through a chemical change.
2. Firefly example — Fireflies are insects that emit light in gardens or fields during late evenings; this light is produced by a chemical change in their bodies without producing heat.
3. Combustion defined — Combustion is a chemical reaction where a substance reacts with oxygen, producing both heat and light.
4. Burning magnesium example — When magnesium ribbon burns, it forms magnesium oxide (a new substance) and produces both heat and light.
5. Key difference — Bioluminescence produces light without heat, while combustion always produces heat and/or light along with new substances.
Conclusion: Bioluminescence (fireflies) produces light without heat, while combustion (burning magnesium) produces both heat and light, though both are chemical changes.

Introduction: Human activities cause changes that have long-term environmental impacts, but steps can be taken to reduce them.
Key Points:
1. Fuel consumption — Increased use of fuels in cars, trains, and aeroplanes increases carbon dioxide in the atmosphere, contributing to global warming and pollution.
2. Paint and chemicals — Drying of paint on walls, doors, and furniture releases substances through evaporation, causing atmospheric pollution.
3. Long-term impact — These changes accumulate over time, are difficult to reverse, and harm ecosystems and air quality.
4. Reduce fuel use — Using public transport, carpooling, and switching to cleaner energy sources can reduce carbon dioxide emissions.
5. Eco-friendly practices — Using biodegradable products and reducing chemical-based paints and waste can lower atmospheric pollution.
Conclusion: Human activities like burning fuels and using paints cause lasting environmental damage, which can be reduced through cleaner energy and eco-friendly practices.