Chemical Changes and Reactions Class 9 ICSE Important Questions with Answers

Answer: (c) Rusting of iron.

Rusting forms a new substance, hydrated iron(III) oxide, and is irreversible, so it is a chemical change. The other three are physical changes forming no new substance.

Answer: (b) Exothermic combination.

Two reactants combine to form a single product and heat is released, so it is an exothermic combination reaction.

Answer: (b) Light (sunlight).

The decomposition of silver chloride into silver and chlorine is a photochemical reaction brought about by sunlight; this is why silver salts are used in photography.

Answer: (d) Double decomposition (precipitation).

$BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)$BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq). The ions exchange partners and an insoluble white precipitate of $BaSO_4$BaSO₄ forms, so it is a double decomposition (double displacement) reaction that forms a precipitate.

Answer: (a) $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$CaCO₃ → (Δ) CaO + CO₂ absorbs heat continuously, so it is endothermic and R correctly explains why A is true.

$NaOH+HCl\rightarrow NaCl+H_2O+\text{heat}$NaOH+HCl→ NaCl+H₂O+heat
The two compounds exchange their ions, so it is a double decomposition reaction (a neutralisation). The test tube becomes warm, so it is exothermic.

The blue colour of the copper sulphate solution slowly fades and a reddish-brown deposit of copper forms on the zinc strip.

$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$Zn + CuSO₄ → ZnSO₄ + Cu

Zinc, being more reactive, displaces copper, so this is a displacement reaction.

(i) $H_2 + Cl_2 \rightarrow 2HCl$H₂ + Cl₂ → 2HCl — combination reaction.

(ii) $Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃ — double decomposition (precipitation); a yellow precipitate of $PbI_2$PbI₂ forms.

(iii) $NH_4NO_2 \xrightarrow{\Delta} N_2\uparrow + 2H_2O$NH₄NO₂ → (Δ) N₂↑ + 2H₂O — thermal decomposition reaction.

(i) By electricity (electrolysis of water):

$2H_2O \xrightarrow{\text{electricity}} 2H_2\uparrow + O_2\uparrow$2H₂O → (electricity) 2H₂↑ + O₂↑

An electric current decomposes acidified water into hydrogen at the cathode and oxygen at the anode.

(ii) By a catalyst (decomposition of hydrogen peroxide):

$2H_2O_2 \xrightarrow{MnO_2} 2H_2O + O_2\uparrow$2H₂O₂ → (MnO₂) 2H₂O + O₂↑

Manganese dioxide acts as a catalyst, speeding up the decomposition without being used up.

1. Change of colour: e.g. bright silver iron turns to reddish-brown rust on reacting with air and moisture.

2. Evolution of a gas: e.g. zinc with dilute sulphuric acid gives off hydrogen, $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$Zn + H₂SO₄ → ZnSO₄ + H₂↑.

3. Formation of a precipitate / change in temperature: e.g. mixing $BaCl_2$BaCl₂ and $Na_2SO_4$Na₂SO₄ gives a white precipitate of $BaSO_4$BaSO₄; and quicklime with water releases heat (temperature rise).

(i) $Na_2SO_4(aq)+BaCl_2(aq)\rightarrow BaSO_4(s)\downarrow+2NaCl(aq)$Na₂SO₄(aq)+BaCl₂(aq)→ BaSO₄(s)↓+2NaCl(aq): white precipitate of barium sulphate.

(ii) $AgNO_3(aq)+NaCl(aq)\rightarrow AgCl(s)\downarrow+NaNO_3(aq)$AgNO₃(aq)+NaCl(aq)→ AgCl(s)↓+NaNO₃(aq): white, curdy precipitate of silver chloride.

(iii) $Pb(NO_3)_2(aq)+2KI(aq)\rightarrow PbI_2(s)\downarrow+2KNO_3(aq)$Pb(NO₃)₂(aq)+2KI(aq)→ PbI₂(s)↓+2KNO₃(aq): yellow precipitate of lead iodide.

In each, the two compounds exchange their ions (both partners are displaced), so the reaction is called double displacement, or double decomposition. Because an insoluble solid forms, these are also precipitation reactions.

(a)

• Exothermic reaction: releases heat to the surroundings. Example: $C + O_2 \rightarrow CO_2 + \text{heat}$C + O₂ → CO₂ + heat.

• Endothermic reaction: absorbs heat from the surroundings. Example: $N_2 + O_2 \xrightarrow{\text{heat}} 2NO$N₂ + O₂ → (heat) 2NO.

(b)

1. Combination: $A + B \rightarrow AB$A + B → AB

2. Decomposition: $AB \rightarrow A + B$AB → A + B

3. Displacement: $A + BC \rightarrow AC + B$A + BC → AC + B

4. Double decomposition: $AB + CD \rightarrow AD + CB$AB + CD → AD + CB

(c) Respiration in living cells (slow oxidation of glucose) releases heat and is exothermic; another example is the slaking of lime, $CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}$CaO + H₂O → Ca(OH)₂ + heat.

(a)

(i) Decomposition: on passing electricity, water breaks up: $2H_2O \xrightarrow{\text{electricity}} 2H_2\uparrow + O_2\uparrow$2H₂O → (electricity) 2H₂↑ + O₂↑.

(ii) Combination: water is formed when hydrogen burns in oxygen: $2H_2 + O_2 \rightarrow 2H_2O$2H₂ + O₂ → 2H₂O.

(b) The thermite reaction:
$Fe_2O_3+2Al\rightarrow2Fe+Al_2O_3+\text{heat}$Fe₂O₃+2Al→2Fe+Al₂O₃+heat
Aluminium, being more reactive, removes oxygen from iron(III) oxide and takes the place of iron, so this is a displacement reaction. It is strongly exothermic: the iron formed is molten and fills the gap in the rails.

(c)

(i) Light given out: magnesium ribbon burns with a dazzling white light, $2Mg+O_2\rightarrow2MgO+\text{heat}+\text{light}$2Mg+O₂→2MgO+heat+light.

(ii) Light absorbed: silver chloride decomposes in sunlight, $2AgCl\xrightarrow{\text{sunlight}}2Ag+Cl_2$2AgCl→ (sunlight)2Ag+Cl₂.

(iii) Electricity produced: the chemical reactions inside a dry cell or a simple voltaic cell produce an electric current.

(iv) Electricity absorbed: electrolysis of acidified water, $2H_2O\xrightarrow{\text{electricity}}2H_2+O_2$2H₂O→ (electricity)2H₂+O₂.

(i) On heating, the green crystals lose their seven molecules of water of crystallisation and turn into white anhydrous ferrous sulphate: $FeSO_4\cdot 7H_2O \xrightarrow{\Delta} FeSO_4 + 7H_2O$FeSO₄· 7H₂O → (Δ) FeSO₄ + 7H₂O.

(ii) The reddish-brown residue is ferric oxide, $Fe_2O_3$Fe₂O₃.

(iii) $2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2\uparrow + SO_3\uparrow$2FeSO₄ → (Δ) Fe₂O₃ + SO₂↑ + SO₃↑

(iv) The final step is a thermal decomposition reaction (the single compound breaks down on heating into simpler substances).