Answer the questions.[Che-5M]

Question 15 Marks

What happens when (Write the balanced equation involved) -
i. Copper is heated in air?
ii. Aluminum oxide is reacted with hydrochloric acid?
iii. Potassium reacts with water?
iv. Cinnabar is heated in air?
v. Aluminium oxide reacts with sodium hydroxide?

Answer

i. Copper is heated in air
Copper is heated in the presence of air to produce black copper oxide.
$2 Cu(s)+O_2(g) \longrightarrow 2 CuO(s)$
ii. Aluminum oxide is reacted with hydrochloric acid
Aluminum oxide is treated with hot dilute hydrochloric acid to produce aluminum chloride and water.
$Al_2 O_3(s)+6 HCl(aq) \longrightarrow 2 AlCl_3(aq)+3 H_2 O(l)$
iii. Potassium reacts with water
When potassium combines with water, potassium hydroxide and hydrogen gas are formed.
$2 K(s)+2 H_2 O(l) \longrightarrow 2 KOH(aq)+H(g)$
iv. Cinnabar is heated in air
When cinnabar is burned in air, it is converted to mercuric oxide, which produces sulfur dioxide gas.
$2 HgS(s)+3 O_2(g) \longrightarrow 2 HgO(s)+2 SO_2(g)$
v. Aluminum oxide reacts with sodium hydroxide
Sodium aluminate and water are formed when aluminum oxide combines with sodium hydroxide.
$Al_2 O_3(s)+2 NaOH(aq) \longrightarrow 2 NaAlO_2(aq)+H_2 O(l)$

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Question 25 Marks

Compound ' A ', when dissolved in water, gives compound ' B ' and liberates heat. Compound ' A ' is used in whitewashing. Compound ' B ' reacts with $CO _2$ to form a white precipitate of compound ' C '. Identify compounds ' A ', ' B ' and ' C ' Also write the equations involved.

Answer

$\underset{\substack{\text { Usedfor } \\ \text { Whitewashing }}}{ 'A'} \xrightarrow[\substack{\text { Exothermic } \\ \text { reaction }}]{+ H _2 O } {'B'} \xrightarrow{ CO _2} \underset{\text { Whiteppt. }}{'C'}$
Quicklime ( CaO ) is used for whitewashing. So ' A ' is CaO .
$Cao + H _2 O \rightarrow \underset{\substack{\text { Calcium } \\ \text { Carbonate }}}{ Ca ( OH )_2}+$ Heat

' B ' is $\mathrm{Ca}(\mathrm{OH})_{2}$
$Ca ( OH )_2( aq )+ CO _2( aq )+ CO _2(g) \longrightarrow \underset{\substack{\text { CalciumCarbonate } \\ \text { (White ppt.) }}}{ CaCO _3(s)}$

'C' is $\mathrm{CaCO}_{3}$.

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Science Answer the questions.[Che-5M]

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