[3 Mark Question Answer]

Question 13 Marks

State your observation of the following case :
At the cathode when acidified aqueous copper sulphate solution is electrolyzed with copper electrodes

Answer

Acidified aqueous copper sulphate solution is electrolysed with copper electrodes by electrolysis. The electrolysis of an aqueous solution of copper sulphate using copper electrodes (i.e. using active electrodes) results in the transfer of copper metal from the anode to the cathode during electrolysis. Copper sulphate is ionised in aqueous solution.
Chemical equation:
$CuSO_4 \rightarrow Cu^{2+}+SO_4{ }^{2-}$
The positively charged copper ions migrate to the cathode, where each gains two electrons to become copper atoms which are deposited on the cathode.
$Cu^{2+}+2 e^{-} \rightarrow Cu$
Hence, the colour of copper sulphate changes from blue to colourless.

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Question 23 Marks

Sodium hydroxide solution is added to the solutions containing the ions mentioned in List X. List Y gives the details of the precipitate. Match the ions with their coloured precipitates.

List X	List Y
(i) $Pb^{2+}$	(A) Reddish Brown
(ii) $Fe^{2+}$	(B) White insoluble inexcess
(iii) $Zn^{2+}$	(C) Dirty green
(iv) $Fe^{3+}$	(D) White soluble in excess
(v) $Cu^{2+}$	(E) White soluble in excess
(vi)$ Ca^{2+}$	(F) Blue

Answer

List X	List Y
(i) $Pb^{2+}$	(D) White soluble in excess
(ii)$ Fe^{2+}$	(C) Dirty green
(iii) $Zn^{2+}$	(E) White soluble in excess
(iv) $Fe^{3+}$	(A) Reddish Brown
(v)$ Cu^{2+}$	(F) Blue
(vi)$ Ca^{2+}$	(B) White insoluble inexcess

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Question 33 Marks

Salts A, B, C, D and E undergo reactions (i) to (v) respectively. Identify the anion present in these salts on the basis of these reactions. Tabulate your answers in the format given below:
(i) When silver nitrate solution is added to asolution of A, a white precipitate, insoluble in dilute nitric acid, is formed.
(ii) Addition of dilute hydrochloric acid to B produces a gas which turns lead acetate paper black.
(iii) When a freshly prepared solution of ferrous sulphate is added to a solution of C and concentrated sulphuric acid is gently poured from the side of the test, a brown ring is formed.
(iv) When dilute sulphuric acid is added to D, a gas is produced which turns acidified potassium dichromate solution from orange to green.
(v) Addition of dilute hydrochloric acid to E produces an effervescence. The gas produced turns limewater milky but does not affect acidified potassium dichromate solution.

Salt	Anion
A
B
C
D
E

Answer

Salt	Anion
A	$Cl ^{-}$
B	$S2 ^{-}$
C	$NO _3^{-}$
D	$SO _3^{2-}$
E	$CO _3^{2-}$

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Question 43 Marks

Column A	Column B
1. A substance that turns moist starch iodide paper blue.	A. Ammonium sulphate
2. A compound which release a reddish brown gas on reaction with concentrated sulphuric acid and copper turnings.	B. Lead carbonate
3. A solution of this compound gives dirty green precipitate with sodium hydroxide.	C. Chlorine
4. A compound which on heating with sodium hydroxide produces a gas which forms dense white fumes with hydrogen chloride.	D. Copper nitrate
5. A white solid which gives a yellow residue on heating	E. Ferrous sulphate

Answer

Column A	Column B
1. A substance that turns moist starch iodide paper blue.	C. Chlorine
2. A compound which release a reddish brown gas on reaction with concentrated sulphuric acid and copper turnings.	D. Copper nitrate
3. A solution of this compound gives dirty green precipitate with sodium hydroxide.	E. Ferrous sulphate
4. A compound which on heating with sodium hydroxide produces a gas which forms dense white fumes with hydrogen chloride.	A. Ammonium sulphate
5. A white solid which gives a yellow residue on heating	B. Lead carbonate

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Question 53 Marks

A solution of hydrogen chloride in water is prepared. The following substances are added to separate portions of the solution :

S.NO.	Substance added	Gas evolved	Odour
1	Calcium carbonate	_________	_________
2	Magnesium ribbon	_________	_________
3	Manganese (IV) oxide with heating	_________	_________
4	Sodium sulphide	_________	_________

Answer

S.NO.	Substance added	Gas evolved	Odour
1	Calcium carbonate	$CO_2$	Odourless
2	Magnesium ribbon	$H_2$	Odourless
3	Manganese (IV) oxide with heating	$Cl_2$	Strong Pungent odour
4	Sodium sulphide	$H_2S$	rotten eggs

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Question 63 Marks

Sodium hydroxide solution is added first in a small quantity, then in excess to the aqueous salt solutions of copper (II) sulphate, zinc nitrate, lead nitrate, calcium chloride and iron (III) sulphate. Copy the following table and write the colour of the precipitate in (i) to (v) and the nature of the precipitate (soluble or insoluble) in (vi) to (x)

Aqueous salt solution	Colour of precipitate when NaOH is added in a small quantity	Nature of precipitate (soluble or insoluble) when NaOH is added in excess
Copper (II) sulphate	(i)	(vi)
Zinc nitrate	(ii)	(vii)
Lead nitrate	(iii)	(viii)
Calcium chloride	(iv)	(ix)
Iron (III) Sulphate	(v)	(x)

Answer

Aqueous salt solution	Colour of precipitate when NaOH is added in a small quantity	Nature of precipitate (soluble or insoluble) when NaOH is added in excess
Copper (II) sulphate	(i) Pale blue	(vi) insoluble
Zinc nitrate	(ii) White	(vii) soluble
Lead nitrate	(iii) White	(viii) soluble
Calcium chloride	(iv)White curdy ppt.	(ix) insoluble
Iron (III) Sulphate	(v) Reddish brown	(x) insoluble

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Question 73 Marks

Using Sodium hydroxide solution, how would you distinguish: Ammonium sulphate from sodium sulphate.

Answer

Ammonium sulphate on reacting with sodium hydroxide liberates ammonia gas:
$
\left. NH _4\right)_2 SO _4( aq )+2 NaOH ( I ) \longrightarrow Na _2 SO _4( aq )+2 NH _3( I )+2 H _2 O ( I )
$
Sodium sulphate on reacting with sodium hydroxide undergoes the following change:
$
NaOH + Na _2 SO _4 \longrightarrow NaOH + Na _2 SO _4
$
However the products would be the same as the reactants, therefore providing no observation for a visible chemical reaction.

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Question 83 Marks

Lead nitrate solution treated with calcium chloride solution and products are heated and cooled.

Answer

When lead nitrate solution is treated with calcium chloride solution it forms white ppt. of lead chloride and calcium nitrate
$CaCl _{2( s )} \rightarrow Ca ^{2+}{ }_{( aq )}+ Cl _{( aq )} $
$Pb \left( NO _3\right)_2( aq ) \longrightarrow Pb ^{2+}( aq )+ NO _3^{2-}( aq ) $
$Pb ^{2+}( aq )+ NO _3^{2-}( aq )+ Ca ^{2+}( aq )+ Cl ^{-}( aq ) \longrightarrow PbCl _2+ Ca \left( NO _3\right)_2$
On further heating and cooling the products, calcium nitrate decomposes upon heating to release nitrogen dioxide:
$2 Ca \left( NO _3\right)_2 \rightarrow 2 CaO +4 NO _2+ O _2$

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Question 93 Marks

Common salt solution is added to silver nitrate solution and $NH_3$ solution add to it.

Answer

Common salt solution is added to silver nitrate solution.
$AgNO_{3(aq)}+NaCl_{(aq)} \rightarrow AgCl_{(s)}+NaNO_{3(aq)}$
Common salt is NaCl , in aqueous medium it ionizes to form $Na ^{+}$and $Cl ^{-}$ions.
$NaCl_{(aq)} \rightarrow Na_{(aq)}^{+}+Cl_{(aq)}$
$Cl_{(aq)}^{-}+Ag_{(aq)}^{+} \rightarrow AgCl_{(s)}$
On adding ammonia solution to it, the silver chloride gets dissolved and the following reaction occurs:
$AgCl_{(s)}+2 NH_{3(aq)} \rightarrow\left[Ag\left(NH_3\right)_2\right]_{(aq)}^{+}+Cl_{(aq)}$

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Question 103 Marks

Caustic soda solution is added to $Cu \left( NO _3\right)_2$ solution and product boiled.

Answer

When Caustic soda solution is added to $Cu \left( NO _3\right)_2$ solution, the following reaction takes place:
$
Cu \left( NO _3\right)_{2(a q)}+2 NaOH _{( aq )} \rightarrow Cu ( OH )_{2(5)}+2 NaNO _{3( aq )}
$
When we heat it further, the greenish blue copper hydroxide decomposes to form slightly black ppt. of copper oxide.
$
Cu ( OH )_2( S ) \xrightarrow{\Delta} CuO ( s )+ H _2 O ( g )
$

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Question 113 Marks

$NH _3$ solution is added to $CuSO _4$ solution drop by drop and then in excess.

Answer

When NH 3 solution is added to CUSO4 solution drop by drop, ammonia ions completely react with copper sulphate and precipitates of copper hydroxide are formed. $CuSO _{4( aq )}+2 NH _{3( aq )}+2 H _2 O _{(0)} \rightarrow Cu ( OH )_{2(s)}+\left( NH _4\right)_2 SO _{4( aq )}$
But when we add excess of ammonia, the precipitate dissolves and a soluble complex is formed.
$CuSO_{4(a q)}+4 NH_{3(a q)} \rightarrow\left[Cu\left(NH_3\right)_4\right] SO_{4(a q)}$

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Question 123 Marks

Write the steps needed for flame test ?

Answer

Flame test :
1. Make a loop at the tip of the platinum wire and dip it in conc. HCl.
2. Put it on the non luminous part of the flame, to see if it gives colour. Repeat the process till it gives no colour to the flame.
3. Prepare a paste of the given salt in a watch glass using conc. HCl.
4. Load the loop of the wire with this prepared paste and introduce it into the non luminous flame of the bunsen burner and then observe the colour of the flame indicating different elements.

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Chemistry [3 Mark Question Answer]

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