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Condensation is the process in which a gas changes directly into its liquid state at a fixed temperature upon cooling.Rusting of iron is classified as a slow change because it requires a long duration of many days, months, or years to complete.An endothermic change is a change in which heat energy is absorbed from the surroundings, such as when dissolving ammonium chloride in water.Non-periodic changes are changes that do not occur at fixed intervals of time, such as coughing or volcanic eruptions.Salt can be recovered from a salt solution by completely evaporating the water over heat, which leaves a white residue of salt behind in the dish.Two examples of natural changes are the changing of seasons and the melting of glaciers.The chemical equation for respiration is Glucose + Oxygen → Carbon dioxide + Water + Energy.The physical properties of a substance include its shape, size, color, and state of matter.Digestion is a chemical change because complex food items are broken down by enzymes into simpler substances with completely different chemical compositions.A periodic change is a change that recurs regularly at fixed intervals of time, such as the swinging of a clock’s pendulum.The ripening of a tomato is a chemical change because its internal chemical composition changes, a new substance is formed, and it results in an irreversible color shift from green to red.Water droplets appear because the water vapour present in the surrounding air condenses upon touching the cold outer surface of the glass.A pop sound is heard when a lighted matchstick is brought near the test tube, confirming the presence of hydrogen gas.Two examples of fast changes are the bursting of crackers and the burning of a matchstick.Magnetising iron is a physical change because no new substance is formed, and the iron retains its original chemical properties.When heated strongly with sulphur, iron reacts to form iron sulphide and entirely loses its magnetic property, so it is no longer attracted to a magnet.A physical change is a change in which only the physical properties of a substance change without forming any new substance or altering its chemical composition.Human-made changes are those changes that occur specifically due to human efforts or activities, such as weaving cloth or cooking food.Three substances that undergo sublimation are camphor, ammonium chloride, and solid carbon dioxide.An exothermic change is a change in which heat energy is evolved or released into the surroundings, such as during the burning of fuels.Curdling of milk is irreversible because milk cannot be recovered from curd by reversing any of the conditions.Evaporation takes place only from the surface of the liquid, whereas boiling takes place throughout the entire bulk of the liquid.Mixing them is a physical change because the iron filings can be easily separated from the sulphur powder using a magnet.Rainfall is desirable for farmers during the sowing of seeds but becomes undesirable when it occurs during the harvesting of crops.A chemical change is a change in which the chemical composition and properties of a substance are altered, resulting in the formation of one or more new substances.
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Chapter: 02. Physical And Chemical Changes
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