[2 Mark Question Answer]

Question 12 Marks

A piece of magnesium strip is dropped into a beaker containing dilute hydrochloric acid.

Answer

When a piece of magnesium strip is dropped in dilute hydrochloric acid, hydrogen gas is evolved and hence it is a chemical change.
$Mg +$ dil. $\ce{2 HCl }\rightarrow \ce{MgCl _2}+ H _2 \uparrow$
as new substances $\ce{MgCl_2}$ and $H _2$ with new properties are produced and the reaction is irreversible.

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Question 22 Marks

An iron nail kept open in the atmosphere rusts.

Answer

Rust of iron is a chemical change as iron rust $\left[ \ce{Fe_2 O_2} \times H _2 O \right]$ is brown flaky powder having different composition from original iron metal, is irreversible change.
$4 Fe +3 O _2 \xrightarrow{+2 \times H _2 O } \underset{\substack{\text { (Iron rust) }}}{2 Fe _2 O _3 \times H _2 O }$

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Question 32 Marks

Sulphur when burnt in air evolves a gaseous acidic product

Answer

Is a chemical change as sulphur $($yellowish green$)$ on burning produces $\ce{SO_2}$ gas which is acidic in nature and a new substance with new properties is formed.

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Question 42 Marks

A bright light is seen evolved when a strip of magnesium is heated.

Answer

Magnesium is heated producing a bright light to form $\ce{MgO}$ in air is a chemical change as the composition changes $\ce{(MgO}$ white powder$)$ is formed, is irreversible. We cannot get magnesium back from $\ce{MgO}$.

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Question 52 Marks

Copper carbonate on heat gives copper oxide and carbon dioxide.

Answer

Copper carbonate $[ \ce{CuCO3}]$ green colour on heating gives copper oxide $[$ Black colour $\ce{CuO} ]$ and $\ce{CO_2}$.
Hence composition is changed new substances $\ce{CuO} [ Black ]$ and $CO _2$ with new properties are produced and it is irreversible reaction hence it is a chemical change.
$\underset{\text { Green }}{ CuCO _3} \xrightarrow{\Delta} \underset{\text { Black }}{ CuO }+ CO _2$

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Question 62 Marks

Compare the energy changes involved during a physical and a chemical change.

Answer

Physical change : No gain or loss of energy during the physical change. Energy required for completion of a physical change is released when the change is reversed
Chemical change : Energy is either absorbed or evolved during the chemical change. Energy in the form of light, percussion $($pressure$)$ or generally heat energy is required for a chemical change.

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Question 72 Marks

Give a reason why $–$
$(a)$ Freezing of water is a reversible change while burning of a candle $–$ an irreversible change,
$(b)$ Change of seasons is a periodic change while change of weather is a non$-$periodic change.

Answer

$(a)$ Freezing of water is reversible change as on heating frozen ice changes to water by heating.
Burning of a candle is irreversible change as on reversing the conditions we can not get candle back out of $CO$, water vapours and wax vapours.
$(b)$ Change of season is periodic change as it occurs at regular intervals. Change of weather is non$-$periodic change because, it does not occur periodically at regular interval. Weather can change suddenly and it may rain in the evening.

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Question 82 Marks

Change $–$ is the law of nature which occurs in everyday life. State when a substance $[$ue. matter$]$ undergoes a kind of change.

Answer

Change is the law of nature which occurs in everyday life. A substance $[$i.e. matter$]$ undergoes a kind of change when subjected to energy changes. Matter is said to undergo certain changes when energy is added to the matter or removed from the matter.
Examples of Changes are :
$(i)$ Ice melts on supplying heat to it.
$(ii)$ Milk changes into curd when a little curd is added to hot milk.

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CHEMISTRY [2 Mark Question Answer]

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