[5 Mark Question Answer]

Question 14 Marks

Select the chemical changes from the following list of changes $–$
$(a)$ Vaporisation of water into water vapour.
$(b)$ Boiling of milk.
$(c)$ Respiration in mammals.
$(d)$ Rotting of eggs.
$(e)$ Drying of a fruit.
$(f)$ The carbon cycle.
$(g)$ Occurrence of lightening.
$(h)$ Breaking of glass.
$(i)$ Butter turning rancid.
$(j)$ Glowing of an electric bulb.
$(k)$ Crystallisation of a salt from its solution.
$(l)$ Change of seasons.
$(m)$ Preparation of carbon dioxide from calcium carbonate & dilute hydrochloric acid.

Answer

$(a)$ Change of water into water vapours is a $\text{PHYSICAL}$ change as no change in composition, only state changes, is$-$ reversible change on cooling vapour water is formed.
$(b)$ Boiling milk is $\text{PHYSICAL}$ change as no change in composition, temporary and reversible, no new substance with new properties is formed.
$(c)$ Respiration in mammals is a $\text{CHEMICAL}$ change, we breathe in oxygen and breathe out carbon dioxide. It is irreversible and composition changes.
$(d)$ Rotting of eggs is $\text{CHEMICAL}$ change, change in composition takes place, is irreversible process.
$(e)$ Drying of a fruit is $\text{PHYSICAL}$ change as composition remains same and only state changes, no new substance with new properties is produced.
$(f)$ Carbon cycle is a $\text{PHYSICAL}$ change as carbon remains as Garbon, mass of carbon remains same in the cycle, is temporary and only change in state takes place.
$(g)$ Occurance of lightening is a $\text{CHEMICAL}$ change, energy is produced, new substance with new properties is produced.
$(h)$ Breaking of glass is a $\text{PHYSICAL}$ change as no change in composition takes place, no new substance is formed.
$(i)$ Butter turning rancid is a $\text{CHEMICAL}$ change as composition changes, new substance with new properties is formed, irreversible and permanent change.
$(j)$ Glowing of an electric bulb is a $\text{PHYSICAL}$ change as no change in composition of filament takes place is reversible, only filament glows when electricity is passed and returns to its original form on switching off the electric current.
$(k)$ Crystallisation of a salt from its solution is a $\text{PHYSICAL}$ change as only state changes $($crystals are formed$)$, composition does not change, is reversible, no change in mass takes place.
$(l)$ Change of season is a $\text{PHYSICAL}$ change, only change in temperature is seen and is reversible change.
$(m)$ Preparation of carbondioxide from calcium carbonate and dilute hydrochloric acid is a $\text{CHEMICAL}$ change as new substance with new properties is formed, the change is permanent and irreversible.

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Question 24 Marks

Potassium chloride is added to water and stirred. A salt solution is obtained which is then boiled leaving behind a residue.
$(a)$ Is the above change physical or chemical
$(b)$ Name the residue which remains behind after the salt solution is boiled.
$(c)$ Is the change reversible or not reversible.
$(d)$ Are the composition and properties of the original substance altered.
$(e)$ Give a reason why the above experimentation would not be possible, if calcium carbonate is taken, in place of potassium chloride.

Answer

$(a)$ Physical change.
$(b)$ White residue of potassium chloride.
$(c)$ Change is reversible.
$(d)$ No, the composition and properties are not altered.
$(e)$ When in place of $\ce{KCl}$ we take calcium carbonate the above experiment cannot be possible as no new substance with new properties is produced, since $\ce{CaCO_3}$ is insoluble in water. Boiling will also not work as $\ce{CaCO_3}$ is in solid form in water.

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CHEMISTRY [5 Mark Question Answer]

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