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Chapter: 02. Physical And Chemical Changes
Natural vs. Human-Made Changes
Points of Comparison
Natural Changes
Human-Made Changes
Definition
Changes that occur naturally and are beyond the control of human beings.
Changes that occur due to human efforts or activities.
Control
Beyond human control.
Controlled/initiated by human activities.
Examples
* Change of day and night* Change of seasons* Melting of glaciers* Formation of coal* Eruption of a volcano* Earthquake* Growth of a seedling
* Cooking of food* Burning of oil (fuel)* Weaving of cloth* Making chapatis* Production of food grains* Making a paper boat
Slow vs. Fast Changes
Points of Comparison
Slow Changes
Fast Changes
Timeframe to Complete
Takes a longer time (hours, days, months, or years).
Takes a shorter time (seconds or minutes).
Examples
* Seedling growing into a tree* Rusting of iron* Tooth decay* Change of seasons* Formation of coal and petroleum* Germination of seeds* Growth in living organisms
* Burning of a matchstick* Bursting of crackers* Beating of the heart* Lighting of an electric bulb* Burning of a piece of paper* Bursting of an inflated balloon
Desirable vs. Undesirable Changes
Points of Comparison
Desirable Changes
Undesirable Changes
Definition
Changes that are beneficial to humankind and we want them to take place.
Changes that are harmful to humankind and we do not want them to take place.
Humankind’s Stance
Wanted / Beneficial.
Unwanted / Harmful.
Examples
* Formation of curd from milk* Cooking of food* Drying of clothes* Ripening of fruits
* Flooding of rivers* Spoilage of food* Occurrence of earthquakes* Occurrence of diseases
Dual Nature Context
* Cutting trees: Desirable for making paper/furniture.* Rainfall: Desirable for farmers during sowing of seeds.
* Cutting trees: Undesirable because it causes soil erosion/flooding.* Rainfall: Undesirable at the time of harvesting.
Periodic vs. Non-Periodic Changes
Points of Comparison
Periodic Changes
Non-Periodic Changes
Intervals of Occurrence
Occur at fixed intervals of time.
Do not occur at fixed intervals of time.
Examples
* Occurrence of day and night* Sunset and sunrise* Full moon and new moon night* High and low tides* Swinging of the pendulum of a clock
* Landslides* Eruption of volcanoes* Coughing* Thunder and lightning* Occurrence of diseases
Reversible vs. Irreversible Changes
Points of Comparison
Reversible Changes
Irreversible Changes
Definition
Changes that can be reversed easily by reversing the conditions.
Changes that cannot be reversed even by reversing the conditions.
Recovery of Original Substance
Possible to obtain the substance in its original form.
Not possible to obtain the substance in its original form.
Examples
* Melting of ice* Boiling of water* Dissolving of common salt in water* Glowing of an electric bulb* Stretching of a rubber band* Melting of wax
* Burning of paper* Curdling of milk* Ageing of human beings* Weathering of rocks* Ripening of fruits* Grinding of wheat into flour* Cooking of food* Rusting of iron* Burning of a candle
Physical vs. Chemical Changes
Points of Comparison
Physical Changes
Chemical Changes
Formation of New Substances
No new substance is formed.
One or more new substances are formed.
Nature of Change
Generally temporary.
Permanent.
Reversibility
Generally reversible by simple physical methods.
Generally irreversible by simple physical methods.
Chemical Composition
Remains the exact same.
Changes.
Properties Affected
Only physical properties (size, shape, colour, and state) change.
Both physical and chemical properties change.
Change in Energy
Generally, no overall change in energy takes place.
Overall change in energy takes place (absorbed or released).
Examples
* Melting/freezing of water* Glowing of an electric bulb* Crushing/rolling of chalk paste* Boiling/condensation of water* Dissolving salt/sugar in water* Sublimation of camphor/ammonium chloride* Stretching of a rubber band* Magnetisation of iron
* Curdling of milk* Cooking of food* Photosynthesis* Digestion and respiration* Burning of fuels/paper/candle/leaves* Explosion of fire crackers* Browning of cut fruits* Addition of zinc to hydrochloric acid* Rusting of iron
Evaporation vs. Boiling
Points of Comparison
Evaporation
Boiling
Process Rate
Slow process.
Fast process.
Site of Occurrence
Takes place only from the surface of the liquid.
Takes place throughout the entire liquid.
Temperature Condition
Takes place at all temperatures (below the boiling point).
Takes place at a particular (fixed) temperature.
Cooling Effect
Causes cooling.
Does not cause cooling.
Endothermic vs. Exothermic Changes
Points of Comparison
Endothermic Changes
Exothermic Changes
Heat Energy
Heat energy is absorbed.
Heat energy is evolved (released).
Temperature Effect
Surrounding area/container becomes cold.
Surrounding area/container becomes hot.
Examples
* Vaporisation of spirit or perfume on skin* Melting of ice on the palm* Dissolving glucose in water* Dissolving ammonium chloride in water
* Burning of fuels (coal, wood, LPG, petrol, diesel)* Burning of a matchstick* Dissolving quicklime in water
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