CBSE - Chemical Reactions and Equations

Magnesium is a very reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper for removing this layer so that the underlying metal can be exposed to air.

Write the balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Answer

(i) H₂ (g) + Cl₂ (g) → 2HCl (g)

(ii) 3BaCl₂ (s) + Al₂(SO₄)₃ (s) → 3BaSO₄ (s) + 2AlCl₃ (s)

(iii) 2Na(s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g)

Write a balanced chemical equation with state symbols for the following reactions.

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer

(i) BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl (aq) 

(ii) NaOH (aq) + HCl (aq) → NaCL (aq) + H₂O (l)

A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer

(i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

CaO (s) + H₂O (l) → Ca(OH)₂ (aq)
Calcium Oxide (Quick Lime) + Water    → Calcium Hydroxide (Slaked Lime)

Water (H₂O) contains two parts hydrogen and one part oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double of the amount collected in the other.

When an iron nail dipped in the copper sulphate solution than iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore the colour of the copper sulphate solution changes.

The reaction involved here is:

Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)

Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.

Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + NaCl (aq)

In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction.

Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na (s) + O₂ (g) → 2Na₂O₂s)
(ii) CuO (s) + H₂ (g) → Cu (s) + H₂O (l)

Answer

(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H₂) gets oxidised to water (H₂O).

Which of the statements about the reaction below are incorrect?
2PbO (s) + C (s) → 2Pb (s) + CO₂ (g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.

(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all

Ans(i) (a) and (b)

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of a

(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Ans (d) displacement reaction.

What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.

Ans (a) Hydrogen gas and iron chloride are produced.

A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation.

The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer

(a) 3H₂ (g) + N₂ (g) → 2NH₃ (g)

(b) 2H₂S (g) + 3O₂ (g) → 2H₂O (l)3 + 2SO₂ (g)

(c) 3BaCl₂ (aq) + Al2(SO₄)₃ (aq) → 2AlCl₃ (aq) + 3BaSO₄ (s)

(d) 2K (s) + 2H₂O (l) → 2KOH (aq) + H₂ (g)

Balance the following chemical equations.

(i) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(ii) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(iii) NaCl + AgNO₃ → AgCl + NaNO₃
(iv) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Answer

(i) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(ii) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(iii) NaCl + AgNO₃ → AgCl + NaNO₃
(iv) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer

(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(b) Zn + 2AgNO₃ → Zn(NO3)₂ + 2Ag

(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

 Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a)Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium bromide(s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Answer

(a) 2KBr (aq) + BaI₂ (aq) → 2KI (aq) + BaBr₂ (s): Double displacement reaction

(b) ZnCO₃ (s) →  ZnO (s) + CO₂ (g): Decomposition reaction

(c) H₂ (g) + Cl₂ (g) → 2HCl (g): Combination reaction

(d) Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g) : Displacement Reaction

Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example: C (g) + O₂ (g) → CO₂ + Heat Energy

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

Example:
              heat
N₂ + O₂  -------> 2NO

Respiration is considered as an exothermic reaction because in respiration oxidation of glucose takes place which produces large amount of heat energy.

C₆H₁₂O₆ (aq) + 6O₂ (g) → 6CO₂ (g) + 6H₂O (l) + Energy

Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.For Example:
Decomposition Reaction:
                heat
CaCO₃(s) --------> CaO(s) + CO₂(g)

Combination Reaction:
CaO (s) + H₂O (l) → Ca(OH)₂ (aq)

In a displacement reaction, a more reactive element replaces a less reactive element from a compound.
For Example: CuSo₄ (aq) + Zn (s) → ZnSO₄ (aq) + Cu (s)
In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

For Example: Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)

2AgNO₃ (aq) + Cu (s) → Cu(NO₃)₂ (aq) + 2Ag (s)
Silver Nitrate + Copper → Copper Nitrate + Silver

A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.For Example:
Na₂CO₃ (aq) + CaCl₂ (aq) → CaCO₃ (s) + 2NaCl (aq)
Sodium Carbonate + Calcium Chloride → Calcium Carbonate + Sodium Chloride

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction

Oxidation Reaction: It is a chemical reaction in which gain of oxygen or loss of hydrogen takes place.
For example:

In equation (i), H2is oxidized to H2O and in equation (ii), Cu is oxidised to CuO.

Reduction Reaction: It is a chemical reaction in which loss of oxygen or gain of hydrogen takes place.

For example:

In equation (i), CO2is reduced to CO and in equation (ii), CuO is reduced to Cu.

‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented.

Oil and fat containing food items flushed with nitrogen because nitrogen acts as an antioxidant and it prevent them from being oxidised.

Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity

Answer

Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.
For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.
4Fe + 3O₂ + nH₂O → 2Fe₂O₃.nH₂O

Rancidity is the process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.
For example, the taste and smell of butter changes when kept for long.

1. Which of the following is not a physical change?
(a) Boiling of water to give water vapour
(b) Melting of ice to give water
(c) Dissolution of salt in water
(d) Combustion of Liquefied Petroleum Gas (LPG)
Ans. (d) Combustion of Liquefied Petroleum Gas (LPG)
Explanation: Combustion of any substance is a chemical change because new substance is formed after combustion.

2. The following reaction is an example of a
4NH₃(g) + 5O₂ → 4NO + 6H₂O (g)
(i) displacement reaction
(ii) combination reaction
(iii) redox reaction
(iv) neutralisation reaction
(a) (i) and (iv)
(b) (ii) and (iii)
(c) (i) and (iii)
(d) (iii) and (iv
Ans. (c) (i) and (iii)
Explanation: In this reaction, oxygen is displacing hydrogen from ammonia; hence it is a displacement reaction. Moreover, nitrogen is getting oxidized and oxygen is getting reduced. Hence, this is redox reaction.

3. Which of the following statements about the given reaction are correct?
3Fe(s) + 4H₂O(g) →Fe₃O₄(s)+ 4H₂(g)
(i) Iron metal is getting oxidized
(ii) Water is getting reduced
(iii) Water is acting as reducing agent
(iv) Water is acting as oxidizing agent
(a) (i), (ii) and (iii)
(b) (iii) and (iv)
(c) (i), (ii) and (iv)
(d) (ii) and (iv)
Ans. (c) (i), (ii) and (iv)
Explanation: Oxygen is being added to iron, hence iron is getting oxidized. Oxygen is removed from water; hence water is getting reduced. Water is providing oxygen; hence water is the oxidizing agent.

4. Which of the following are exothermic processes?
(i) Reaction of water with quick lime
(ii) Dilution of an acid
(iii) Evaporation of water
(iv) Sublimation of camphor (crystals)
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (iii) and (iv)
Ans. (a) (i) and (ii)
Explanation: Evaporation can happen only after absorbing heat. Sublimation of a solid into gas can happen only after absorbing heat. Hence, (iii) and (iv) are endothermic processes. Rest are exothermic processes.

5. Three beakers labelled as A, B and C each containing 25 mL of water were taken. A small amount of NaOH, anhydrous CuSO4 and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of
the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is(are)correct?
(i) In beakers A and B, exothermic process has occurred.
(ii) In beakers A and B, endothermic process has occurred.
(iii) In beaker C exothermic process has occurred.
(iv) In beaker C endothermic process has occurred.
(a) (i) only
(b) (ii) only
(c) (i) and (iv)
(d) (ii) and (iii)
Ans. (c) (i) and (iv)
Explanation: Exothermic process will increase the temperature of beaker, while endothermic process will reduce the temperature of beaker.

6. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the
observation?
(a) KMnO₄ is an oxidising agent, it oxidises FeSO₄
(b) FeSO₄ acts as an oxidising agent and oxidises KMnO₄
(c) The colour disappears due to dilution; no reaction is involved
(d) KMnO₄ is an unstable compound and decomposes in presence of FeSO₄ to a colourless compound.
Ans. (a) KMnO₄ is an oxidising agent, it oxidises FeSO4 Explanation: Potassium permanganate is a potential oxidizing agent.
The purple colour was because of KMnO₄ ; which disappears once all the permanganate in the solution is utilized

7. Which among the following is(are) double displacement reaction(s)?

(i) Pb + CuCl₂ → PbCl₂ + Cu
(ii) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
(iii) C + O₂→CO₂
(iv) CH₄ +2O₂→ CO₂+H₂O
(a) (i) and (iv)
(b) (ii) only
(c) (i) and (ii)
(d) (iii) and (iv)
Ans. (b) (ii) only
Explanation: In this reaction, sodium and barium are displacing each other from their respective salts. Hence, it is a double displacement reaction.

Which among the following statement(s) is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to
(i) the formation of silver by decomposition of silver chloride
(ii) sublimation of silver chloride
(iii) decomposition of chlorine gas from silver chloride
(iv) oxidation of silver chloride
(a) (i) only
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (iv) only
Ans. (a) (i) only
Explanation: When silver chloride is kept in sunlight: it turns to grey because of formation of silver. This is a decomposition reaction.


Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanies by liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among
the following is (are) true about slaking of lime and the solution formed?
(i) It is an endothermic reaction
(ii) It is an exothermic reaction
(iii) The pH of the resulting solution will be more than seven
(iv) The pH of the resulting solution will be less than seven
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (iii) and (iv)
Ans. (b) (ii) and (iii)
Explanation: Slaking of lime is an exothermic reaction which is evident from liberation of heat. Oxides and hydroxides of metals are alkaline. Hence, pH of the resulting solution will be more than 7.

10. Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?
(i) Displacement reaction
(ii) Precipitation reaction
(iii) Combination reaction
(iv) Double displacement reaction
(a) (i) only
(b) (ii) only
(c) (iv) only
(d) (ii) and (iv)
Ans. Explanation: (c) (iv) only
Refer to explanation to question 7.

11. Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is
(a) 1:1
(b) 2:1
(c) 4:1
(d) 1:2
Ans. (b) 2:1
Explanation: In 1 mole water molecule, there are 2 mole hydrogen and 1 mole oxygen.

12. Which of the following is(are) an endothermic process(es)?
(i) Dilution of sulphuric acid
(ii) Sublimation of dry ice
(iii) Condensation of water vapours
(iv) Evaporation of water
(a) (i) and (iii)
(b) (ii) only
(c) (iii) only
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)
Explanation: When a solid change into gas or a liquid change into gas, it happens because of absorption of heat. Hence, these are endothermic processes.

13. In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be
used in place of lead nitrate?
(a) Lead sulphate (insoluble)
(b) Lead acetate
(c) Ammonium nitrate
(d) Potassium sulphate
Ans. (b) Lead acetate
Explanation: We need a source of lead for making lead iodide, hence options (c) and (d) are ruled out. Lead sulphate is insoluble, so it cannot be used.

14. Which of the following gases can be used for storage of fresh sample of an oil for a long time?
(a) Carbon dioxide or oxygen
(b) Nitrogen or oxygen
(c) Carbon dioxide or helium
(d) Helium or nitrogen
Ans. (d) Helium or nitrogen
Explanation: Oxygen is an oxidizing agent and hence it cannot be used. Helium is an inert gas and hence it can be used. Nitrogen is among the least reactive gases and it is much cheaper than helium. Hence, in most of the cases, Nitrogen is used in packets of oily food to prevent rancidity.

15. The following reaction is used for the preparation of oxygen gas in the laboratory

2KCIO₃(s) heat, catalyst-> 2KCI s +3O₂ (g)
Which of the following statement(s) is(are) correct about the reaction?
(a) It is a decomposition reaction and endothermic in nature
(b) It is a combination reaction
(c) It is a decomposition reaction and accompanied by release of heat
(d) It is a photochemical decomposition reaction and exothermic in nature
Ans. (a) It is a decomposition reaction and endothermic in nature.
Explanation: Potassium chlorate decomposes to give Potassium chloride and oxygen;
hence it is a decomposition reaction. Heat is being supplied to this reaction, so it is an endothermic reaction.

16. Which one of the following processes involve chemical reactions?
(a) Storing of oxygen gas under pressure in a gas cylinder
(b) Liquefaction of air
(c) Keeping petrol in a china dish in the open
(d) Heating copper wire in presence of air at high temperature
Ans. (d) Heating copper wire in presence of air at high temperature.
Explanation: When copper is heated at high temperature in presence of air, it undergoes oxidation to form copper oxide.
2Cu + O₂ -> 2CuO

17. In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products
involved at reaction temperature?
(a) 2H₂(l) + O₂(l) -> 2H₂O (g)
(b)2H₂(g) + O₂ (l)-> 2H₂O (l)
(c) 2H₂(g) + O₂ (g)-> 2H₂O (l)
(d) 2H₂(g) + O₂ (g)-> 2H₂O (g)

ans C
Explanation: At room temperature, hydrogen and oxygen are available as gas, while water is available as liquid.

18. Which of the following are combination reactions?


(a) (i) and (iii)
(b) (iii) and (iv)
(c) (ii) and (iv)
(d) (ii) and (iii)
Ans. (d) (ii) and (iii)
Explanation: In these reactions, two reactants react to form a single product.

N₂ g + 3H₂ -> 2NH₃

This is a combination reaction.

NaOH + CH₃COOH  → CH₃COONa + H₂O

This is neutralization reaction.

Ans. C₂H5OH + CH₃COOH → CH₃COOC₂H₅ + H₂O

This is neutralization reaction.

C₂H₄ + 3O₂ → 2CO₂ + H₂O + Heat

 This is combustion reaction.

Fe₂O₃ + 2Al -> 2Fe + Al₂ + Heat + light

This is displacement reaction.

3Mg + N₂  -> Mg₃N₂

This is combination reaction.

Ans. KI + Cl₂ -> 2KCl + I₂

C₂H₅OH +3O₂ → CO₂ + 3H₂O + Heat 

This is redox reaction.

(a) Pb(NO₃)₂(aq) + 2KI(aq) -> PbI₂(x) + 2KNO₃(y)

Ans. x →s, y →aq

 (b) Cu(s) + AgNO(aq) → Cu(NO₃) ₂ (aq) + x

Ans. x → Ag

(c) Zn (s) + H₂SO₄ (aq) + ZnSO₄ ( x) + H₂ (y )

Ans. x →aq, y →g

(d) CaCO₃  x →  CaO (s)   + CO₂ (g)  

Ans. x  → heat

(a) Decomposition of ferrous sulphate
Ans. Endothermic
(b) Dilution of sulphuric acid
Ans. Exothermic
 (c) Dissolution of sodium hydroxide in water
Ans. Exothermic
(d) Dissolution of ammonium chloride in water
 Ans. Endothermic

(a) 4NH₃ + 5O₂ -> 4NO + 6H₂O 
Ans. Ammonia (NH3) 

(b) H₂O + F₂ -> HF + HOF
Ans. Fluorine 

(c) Fe₂O₃ + 3CO ->2Fe + 3CO₂
Ans.  Carbon monoxide (CO) 

(d) 2H₂ + O₂ -> 2H₂O 
Ans. Hydrogen 

(a) Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O
 
 Ans : Pb₃O₄
 
(b) 2Mg + O₂ → 2MgO

Ans O₂

(c) CuSO₄ + Zn → Cu + ZnSO₄

Ans ; CuSO₄

(d) V₂O₅ + 5Ca → 2V + 5CaO

Ans : V₂O₅

(e) 3Fe + 4H₂O → Cu +H₂O

Ans : H₂O

(f) CuO + H₂ → Cu + H₂O

Ans : CuO

(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations ves sodium chloride and sodium hydrogen carbonate.

Ans: Na₂CO₃ + HCl  → NaCl + NaHCO₃

(b) Sodium hydrogen-carbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.
Ans:
NaHCO₃ + HCl → NaCl + H₂O + CO₂

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide Cu₂, I₂  liberates iodine gas and also forms potassium sulphate. 
Ans: 
 2CuSO₄ + 4 KI → Cu₂I₂ + I₂ + k₂SO₄

KCl+ AgNO₃ → AgCl + KNO₃
 This is a precipitation reaction. This is double displacement reaction. 

2FeSO₄ + Heat → Fe₂O₃ + SO₂  + SO₃
 It is decomposition reaction 

Firefly secrete the enzyme luciferase. This enzyme reacts with luciferin which carries out oxidation of magnesium. This happens in the presence of magnesium and oxygen. Light is emitted because of oxidation of magnesium. Due to this, fireflies glow at night.

When grapes are still on plants, their self-defense mechanism is at work. This prevents fermentation of grapes. When grapes are plucked form plants, then yeast (a type of fungi) carry out fermentation. During fermentation, sugar changes into alcohol. So, it is a chemical change

(a) Evaporation of petrol
 (b) Burning of Liquefied Petroleum Gas (LPG)
 (c) Heating of an iron rod to red hot.
 (d) Curdling of milk
 (e) Sublimation of solid ammonium chloride
Ans. (a), (c) and (e) are physical changes.
(b) and (d) are chemical changes. 

(a) Silver metal does not show any change
(b) The temperature of the reaction mixture rises when aluminium (Al) is added.
(c) The reaction of sodium metal is found to be highly explosive
(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.

Explain these observations giving suitable reasons.
Ans. (a) Silver metal does not react with dilute HCl.
(b) The temperature of the reaction mixture rises when aluminium is added because it is an exothermic reaction.
(c) Reaction of sodium metal is found to be highly explosive because it is an exothermic reaction
(d) When lead is treated with hydrochloric acid, bubbles of hydrogen gas are evolved.
 2Pb + HCl → PbCl₂ + H₂ 

 Calcium oxide

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution. 

Ans. Pb(CH₃COO)₂ + HCl → PbCl₂ + CH₃COOH

This is a Double Displacement reaction 

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas. 

Ans. 2Na + 2C₂H₅ONa + H₂

(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas. 

Ans. 2Fe₂O₃ + 3CO → 2Fe + 3CO₂

This is decomposition reaction.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid Sulphur and liquid water.
 Ans. 
 2H₂O + O₂ -> 2S + 2H₂O 
 
 This is a Redox reaction.

Silver chloride on exposure to sunlight may decompose as per the following reaction.

2 AgCl → 2Ag + Cl₂ 

Therefore, it is stored in dark coloured bottles.

(a) Write the chemical formulae of X and Y

Ans. X is MgO Y is  Mg₃N₂

(b) Write a balanced chemical equation, when X is dissolved in water. 

Ans. MgO + H₂O → Mg(OH)₂ 

Zinc is above hydrogen whereas copper is below hydrogen in the activity series of metals. That is why zinc displaces hydrogen from dilute hydrochloric acid, while copper does not. 

Zn + HCl → ZnCl₂ +H₂ 

Cu + HCl → No reaction

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
Ans. Metals such as silver when attacked by substances around it such as moisture, acids, gases etc, are said to corrode and this phenomenon is called corrosion.
(b) Name the black substance formed and give its chemical formula.
Ans. The black substance is formed because silver (Ag) reacts with H₂S  present in air. It forms thin black coating of silver sulphide (Ag₂S)

(a) Write a balanced chemical equation of the reaction.
Ans. Balanced chemical equation
2Cu(NO₃)₂(s)  → 2CuO +O₂ + 4NO₂(g)

(b) Identity the brown gas X evolved.
 Ans. The brown gas X evolved is nitrogen dioxide (NO₂).
 
(c) Identity the type of reaction.
 Ans. This is a decomposition reaction
 
(d) What could be the pH range of aqueous solution of the gas X?
Ans. Nitrogen dioxide dissolves in water to form acidic solution because it is an oxide of non-metal. Therefore, pH of this solution is less than 7.

(a) CO₂
Ans. When carbon dioxide is passed into lime water, the lime water turns milky. This is the characteristic test for carbon dioxide.
(b) SO₂

 Ans. Sulphur dioxide smells like rotten egg.
(c) O₂
Ans. When a burning matchstick is brought near oxygen, the matchstick burns even more brightly. This is the characteristic test for oxygen.
(d) H₂
Ans. When a burning matchstick is brought near hydrogen gas, the matchstick burns with a pop sound

(a) zinc metal is added to copper sulphate solution?
Ans. Zinc displaces copper to form zinc sulphate. 
 Zn(s) + CuSO₄(aq)  → ZnSO₄(aq) +Cu
 
 
 (b) aluminium metal is added to dilute hydrochloric acid?
Ans. Aluminium reacts with dil.hydrochloric acid to form aluminium chloride. Hydrogen gas is evolved in this rection. 

2Al(s) + 6HCl(aq) → AlCl₃(aq) + 3H₂

(c) silver metal is added to copper sulphate solution?
 Ans. Silver is less reactive than copper. Hence, no reaction takes place. 

The reaction of Zn granules with
 (a) Dilute H₂SO₄ 
 
 Zn(s) +H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) 
 
 (b) Dilute HCl
 
 Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) 
 
 (c) Dilute HNO₃
 
 Reaction with dilute HNO₃  is different as compared to other acids because nitric acid is an oxidising agent and it oxidises H₂  gas evolved to H₂O. 
 
 4Zn(s)  + 10HNO₃(aq) → 4Zn(NO₃)₂(aq) + 5H₂O + N₂O(g)
 
 (d) NaCl solution
Zn(s) +  NaCl(aq)  → Noreaction
 
 (e) NaOH solution 
 
 Zn(s) + NaOH(aq) → NaZnO₂ (aq)     + H₂(g)
                                Sodium Zincate

 (a) Write a balanced chemical equation of the reaction involved.
Ans. Balanced chemical equation

NaSO₃(aq) + BaCl₂(aq) → BaSO₃(s) + 2NaCl(aq)

(b) What other name can be given to this precipitation reaction?
Ans. This reaction is also known as double displacement reaction.
(c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?
Ans. BaSO₃  is a salt of a weak acid (H₂SO₃) , therefore dilute acid such as HCl decomposes barium sulphite to produce sulphur dioxide gas which has the smell of burning sulphur
BaSO₃(s) + HCl → BaCl₂ + H₂O + SO₂(g)

Copper is a nobel metal. It does not react with hydrochloric acid or with nitric acid. But aluminium is more reactive than copper. So, these acids should be kept in copper container. 

Copper is less reactive than zinc and hence zinc chloride solution can be kept in copper container. But aluminium is more reactive than zinc and hence zinc chloride solution cannot be kept in aluminium container. For keeping water; both the containers can be used. However, copper is better.