[3 Mark Question Answer]

Question 13 Marks

(a) Acids dissolve in water to produce positively charged ions. Draw the structure of the positive ion.
(b) Elements Q and S react together to form an ionic compound. Under normal conditions, which physical state will the compound QS exist in?
(c) Can Q and S, both be metals? Justify your answer.

Answer

(b) The compound QS is likely to be in the solid state and is crystalline in nature.
(c) No, Q and S cannot be metals. It is because, an ionic compound is formed only, if one element donates electron as metal and the other element accepts it as non-metal. If both Q and S are donors of electrons, then, no ionic compound is formed.

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Question 23 Marks

You are provided with the list of chemicals below in the box:

Using suitable chemicals from the list given below, write balanced chemical equations for the preparation of the salts mentioned below:
(i) Lead carbonate
(ii) Ammonium sulphate

Answer

(i) $Pb \left( NO _3\right)_2( aq )+ Na _2 CO _3( aq ) \longrightarrow 2 NaNO _3( aq )+ PbCO _3 \downarrow$ (ppt.)
(ii) $2 NH _4 OH ( aq )+ H _2 SO _4($ dil. $) \longrightarrow\left( NH _4\right)_2 SO _4( aq )+2 H _2 O ( l )$
(iii) $2 Al ( s )+6 HCl$ (dil.) $\longrightarrow 2 AlCl _3( aq )+3 H _2(g)$

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Question 33 Marks

Give one chemical test to distinguish between the following pairs of compounds.
(i) Zinc sulphate solution and zinc chloride solution.
(ii) Iron(II) chloride solution and iron(III) chloride solution.
(iii) Calcium nitrate solution and calcium chloride solution.

Answer

(i) Treat each of the solution with barium chloride solution.
In case of zinc chloride no visible reaction takes place.
In case of zinc sulphate a white precipitate of barium sulphate is formed.
(ii) Treat each of the solution with dilute sodium hydroxide solution.
In case of iron(II) chloride a dirty green precipitate of iron(II) hydroxide is formed.
In case of iron(III) chloride a reddish brown precipitate of iron(III) hydroxide is formed.
(iii) Treat each of the solution with silver nitrate solution.
In case of calcium nitrate, no visible reaction takes place.
In case of calcium chloride, a curdy white ppt. of silver chloride is formed.

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Question 43 Marks

Starting with copper chips and concentrated nitric acid, describe briefly how you can prepare black copper oxide.

Answer

Dissolve copper turnings in conc. nitric acid. The following reaction takes place with the formation of copper nitrate.
Cu + 4HNO₃(conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O
Filter the reaction mixture and transfer the clear solution of copper nitrate to a china dish. Heat the solution to dryness and then heat strongly. The copper nitrate decomposes to form copper oxide. Go on heating till brown fumes of nitrogen dioxide stop coming. The residue is copper oxide.
$2 Cu \left( NO _3\right)_2 \xrightarrow{\Delta} 2 CuO +4 NO _2+ O _2$

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Question 53 Marks

Write the observations and balanced equations for the following reactions :
(i) Sodium hydroxide is added drop by drop, till in excess, in a solution of zinc sulphate.
(ii) Ammonium hydroxide is added first in small quantity and then in excess to a solution of copper sulphate.
(iii) Excess of ammonium hydroxide is added to a substance obtained by the action of hydrochloric acid in silver nitrate.

Answer

(i) Observations : First a white gelatin like precipitate of zinc hydroxide is formed, which dissolves in excess of sodium hydroxide to form a clear solution of sodium zincate.

(ii) Observations :
(1) With small amount of ammonium hydroxide, it forms a bluish white precipitate.
(2) With excess of ammonium hydroxide, the precipitate dissolves to form deep blue coloration.

(iii) Observations :
(1) When hydrochloric acid is added to silver nitrate solution it forms a curdy white precipitate of silver chloride.
(2) The silver chloride (ppt.) dissolves in excess of ammonium hydroxide solution to form colourless solution.

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Question 63 Marks

What is observed when :
(i) Hydrogen sulphide gas is passed through lead acetate solution?
(ii) Neutral litmus solution is added to sodium hydrogen carbonate solution?
(iii) A small piece of iron is placed in copper sulphate solution?

Answer

(i) A black precipitate of lead (II) sulphide is formed.
(ii) The colour of litmus solution changes to blue colour.
(iii) The piece of iron is coated with reddish deposit of copper.

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Question 73 Marks

Solution A is a strong acid
Solution B is a weak acid
Solution C is a strong alkali
(i) Which solution contains solute molecules in addition to water molecules?
(ii) Which solution will give a gelatinous white precipitate with zinc sulphate solution? The precipitate disappears when an excess of the solution is added.
(iii) Which solution could be a solution of glacial acetic acid?
(iv) Give an example of a solution which is a weak alkali.

Answer

(i) Solution B contains solute molecules in addition to water molecules.
(ii) Solution C gives a white gelatinous precipitate; when zinc sulphate dissolves in excess solution of C.
(iii) Solution B can be glacial acetic acid.
(iv) Calcium hydroxide solution (limewater) is a weak alkali.

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Question 83 Marks

Zinc metal and aluminium metal are amphoteric in nature. What do you understand by the statement? Support your answer by writing fully balanced chemical equations.

Answer

Zinc as well as aluminium metals react both with acids as well as alkalis to form a salt and displace hydrogen gas. Such metals which react, both with acids as well as alkalis are known as amphoteric in nature.
Zn + H₂SO₄(dil) → ZnSO₄ + H₂(g) [basic nature]
Zn + 2NaOH → Na₂ZnO₂ + H₂ (g) [acidic nature]
2AI + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂(g) [acidic nature]
2Al + 3H₂SO₄ (dil) → Al₂(SO₄)₃ + 3H₂ (g) [basic nature]

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Question 93 Marks

Define the following terms giving one example in each case (i) An acid (ii) A base (iii) An acid salt.

Answer

(i) A compound which on dissolving in water furnishes hydronium ions as the only positively charged ions is called an acid. For example, $HCl \rightleftharpoons H ^{+}+ Cl ^{-}$.
(ii) A compound which reacts with the H⁺ ions of an acid to form salt and water as the only product is called a base. For example :
NaOH + HNO₃ → NaNO₃ + H₂O.
(iii) A salt formed by the partial replacement of H⁺ ions of an acid by a basic radical is called an acid salt. For example, NaHSO₄.

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Question 103 Marks

Show ionically why (i) phosphoric acid, (ii) sulphuric acid and (iii) acetic acid are called acids.

Answer

Any substance which furnishes H⁺(aq) ions as the only positively charged ions in an aqueous solution is called an acid.
(i) $H _3 PO _4+3 H _2 O \rightleftharpoons 3 H _3 O ^{+}+ PO _4^{3-}$
(ii) $H _2 SO _4+2 H _2 O \rightleftharpoons 2 H _3 O ^{+}+ SO _4^{2-}$
(iii) $CH _3 COOH + H _2 O \rightleftharpoons H _3 O ^{+}+ CH _3 COO ^{-}$
As the above substance furnish only H₃O⁺ positively charged ions in the aqueous solution, therefore, they are acids.

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Question 113 Marks

State two chemical properties each with equations for (i) a solution containing H⁺(aq) ions (ii) a solution containing OH^- ions.

Answer

(i) The solution containing H⁺(aq) ions is an acidic solution.
(1) It reacts with active metals to liberate hydrogen gas.
Zn + 2H⁺ → Zn²⁺+ H₂(2) It reacts with metallic carbonates to liberate carbon dioxide gas and water.
$CO _3{ }^{2-}+2 H ^{+} \longrightarrow H _2 O + CO _2$
(ii) The solution containing OH^- ions is basic (alkaline) in nature.
(1) It react with acids to form salt and water as the only products.
ХОН + HCI → XCI + H₂O
(2) It reacts with ammonium salts on warming and liberates ammonia gas.
NH₄Cl + XOH → XCI + NH₃ + H₂O

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Question 123 Marks

Why is sulphuric acid a dibasic acid? Give three reasons. Support your answer with equations, wherever possible.

Answer

(i) A molecule of sulphuric acid furnishes 2H⁺(aq) ions in an aqueous solution.
$H _2 SO _4+2 H _2 O \rightleftharpoons 2 H _3 O ^{+}+ SO _4^{2-}$
(ii) A molecule of sulphuric acid dissociates in two steps in water
$\begin{array}{l} H _2 SO _4+ H _2 O \rightleftharpoons H _3 O ^{+}+ HSO _4^{-} \\ HSO _4^{-}+ H _2 O \rightleftharpoons H _3 O ^{+}+ SO _4^{2-}\end{array}$
(iii) A molecule of sulphuric acid forms two series of salts, i.e., hydrogen sulphates (acid salt) and sulphates (normal salt)
$\begin{array}{l} NaOH + H _2 SO _4 \longrightarrow NaHSO _4+ H _2 O \\ 2 NaOH + H _2 SO _4 \longrightarrow Na _2 SO _4+ H _2 O \end{array}$

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Chemistry [3 Mark Question Answer]

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