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NCERT Solutions for class 10 Science Chapter 1 – Chemical Reactions and Equations

Back Exercise

Question 1.
Which of the following statements about the reaction given below are incorrect ?

(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 are incorrect
Answer:
(i) a and b are both incorrect
Pb is getting oxidised to PbO in backward reaction.
CO₂ is getting reduced to C in backward reaction.

Question 2.

The above reaction is an example of :
(a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction
Answer:
(d). It is an example of displacement reaction. The aluminium (Al) metal has displaced iron (Fe) from Fe₂O₃ when the reaction is carried in aqueous solution. Please note that Al lies above Fe in the reactivity series.

Question 3.
What happens when dilute hydrochloric acid is added to iron fillings ? 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.
Answer:
(a). hydrogen gas and Iron chloride are produced according to the reaction

Question 4.
What is a balanced chemical equation ? Why should chemical equations be balanced ? (CBSE 2013)
Answer:
Balanced chemical equation :- A chemical equation which has an equal number of atoms of all the elements on both sides of the chemical equation. The chemical equations have to be balanced to fulfill the requirement of law of conservation of mass. According to the law, In a chemical reaction, the mass of reactants must be the same as the mass of products and this is possible only if the atoms of different elements of the reactants and products are equal.

Question 5.
Translate the following statements into chemical equations and 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 precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer:
(a) The symbol equation for the reaction is :
H₂ + N₂ ———> NH₃
The balancing of equation is done in the following steps :
Step I: Let us count the number of atoms of all the elements of the reactants and the products on both sides of the equation.

A simple look at the equation reveals that neither the number of H nor of N atoms are equal on both sides of the equation.
Step I: In order to equate the number of H atoms on both sides, put coefficient 3 before H₂ on the reactant side and coefficient 2 before NH₃ on the product side.
3H₂ + N₂ ———–> 2NH₃
Step III: On counting, the number of N atoms on both sides of the equation are also the same (2). This means that the equation is balanced.
(b) The symbol equation for the reaction is :
H₂S + O₂ ———–> H₂O + SO₂
The balancing of equation is done in the following steps :
Step I: Let us count the number of atoms of all the elements on both sides on the equation.

A simple look at the equation reveals that the number of H and S atoms are equal on both sides. At the same time, the number of O atoms are not equal.
Step II: In order to equate the number of O atoms, put coefficient 3 before O₂ on the reactant side and coefficient 2 before SO₂ on the product side.
H₂S + 3O₂ ———–> H₂O + 2SO₂
Step III: O atoms are still not balanced. To achieve this, put coefficient 2 before H₂O on the product side.
H₂S + 3O₂ ————> 2H₂O + 2SO₂
Step IV: To balance S atoms, put coefficient 2 before H₂S on the reactant side.
2H₂S + 3O₂ ————> 2H₂O + 2SO₂
On inspection, the number of atoms of all the elements in both sides of the equation are equal. Therefore, the equation is balanced.
(c) The symbol equation for the reaction is :
BaCl₂ + Al₂(SO₄)₃ ———-> A1Cl₃ + BaSO₄
The balancing of equation is done in the following steps :
Step I: Let us count the number of atoms of all the elements on both sides of the equation.

A simple look at the equation reveals that only Ba atoms are equal on both sides. The rest of the atoms are to be balanced. It is done as follows :
Step II: In order to equate the number of Al atoms, put coefficient 2 before AlCl₃ on the product side.
BaCl₂ + Al₂(SO₄)₃ ———-> 2AICl₃ + BaSO₄
Step III: In order to balance Cl atoms, put coefficient 3 before BaCl₂ on the reactant side.
3BaCl₂ + Al₂(SO₄)₃ ———–> 2AlCl₃ + BaSO₄
Step IV: To balance Ba atoms, put coefficient 3 before BaSO₄ on the product side.
3BaCl₂ + Al₂(SIO₄)₃ ———-> 2AlCl₃ + 3BaSO₄
Step V: On inspection, the number of S and O atoms on both sides of the equation are also found to be equal. Thus, the equation is in balanced form.
(d) The symbol equation for the reaction is :
K + H₂O ———–> KOH + H₂
The balancing of the equation is done in the following steps :
Step I. Let us count the number of atoms of all the elements on both sides.

A simple look at the equation reveals that the number of K and O atoms on both sides of the equation are equal. At the same time, the number of H atoms are not equal.
Step II: To balance the number of H atoms, put coefficient 2 before KOH on the product side and 2 before H₂O on the reactant side.
K + 2H₂O ———> 2KOH + H₂
Step III: To balance the number of K atoms in the above equation, put coefficient 2 before K atom on the reactant side.
2K + 2H₂O ———> 2KOH + H₂
Step IV: On inspection, the number of atoms of all the elements are found to be equal on both sides of the equation. It is balanced.

Question 6.
Balance the following chemical equations :

Answer:
(a) The symbol equation as given for the reaction is :
HNO₃ + Ca(OH)₂ ——–> Ca(NO₃)₂ + H₂O
The balancing of the equation is done in the following steps :
Step I: Let us count the number of atoms of all the elements on both sides of the equation.

A simple look at the equation reveals that the number of Ca atoms are equal on both sides.
Step II: In order to equate the number of N atoms, put coefficient 2 before HNO₃ on the reactant side.
2HNO₃ + Ca(OH)₂ ———-> Ca(NO₃)₂ + H₂O
Step III: In order to equate the number of H atoms, put coefficient 2 before H₂O on the product side.
2HNO₃ + Ca(OH)₂ ———-> Ca(NO₃)₂ + 2H₂O
Step IV: On inspection the number of O atoms on both sides of the equation is the same i.e., 8. Therefore, the equation is balanced.
(b) The symbol equation as given for the reaction is :
NaOH + H₂SO₄ ———-> Na₂SO₄ + H₂O
Step I: Let us count the number of atoms of all the elements on both sides of the equation.

A simple look at the equation reveals that the number of O and S atoms are equal on both sides.
Step II: In order to equate the number of Na atoms, put coefficient 2 before NaOH on the reactant side.
2NaOH + H₂SO₄ ———-> Na₂SO₄ + H₂O
Step III: In order to equate the number of H atoms, put coefficient 2 before H₂O on the product side.
2NaOH + H₂SO₄ ———> Na₂SO₄ + 2H₂O
Step IV: On inspection, the number of O atoms on both sides of the equation is the same i.e., 6. Therefore, the equation is balanced.
(c) The symbol equation as given for the reaction is already balanced.
NaCl + AgNO₃ ———-> AgCl + NaNO₃
On inspection, the number of atoms of all the elements are found to be equal on both sides of the equation. It is in the balanced form.
(d) The symbol equation as given for the reaction is :
BaCl₂ + H₂SO₄ ———-> BaSO₄ + HCl
Step I: Let us count the number of atoms of all the elements on both sides of the equation.

A simple look at the equation reveals that the number of Ba, S and O atoms are equal on both sides.
Step II: In order to equate the number of Cl atoms, put coefficient 2 before HCl on the product side.
BaCl₂ + H₂SO₄ ———> BaSO₄ + 2HCl
Step III: On inspection, the number of H atoms on both sides of the equation is the same i. e., 2. Therefore, the equation is balanced.

Question 7.
Write the balanced 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

Question 8.
Write the balanced chemical equations for the following reactions and identify the type of reaction :
(a) Potassium brormide (aq) + Barium iodide (aq) ————> Potassium iodide(aq) + Barium bromide (aq)
(b) Zinc carbonate (aq) ————> Zinc oxide (aq) + Carbon dioxide (aq)
(c) Hydrogen (aq) + Chlorine (aq) ———> Hydrogen chloride (aq)
(d) Magnesium (aq) + Hydrochloric acid (aq) ———–> Magnesium chloride (aq) + Hydrogen(aq).
Answer:
(a)          2KBr (aq) + BaI₂ (aq) ——–> 2Kl (aq) + BaBr₂ (aq)
The reaction is known as double displacement reaction.
(b)          ZnCO₃(s) ———-> ZnO(s) + CO₂(g)
The reaction is known as decomposition reaction. ‘
(c)          H₂(g) + Cl₂(g) ———–> 2HCl (g)
The reaction is known as combination reaction.
(d)          Mg (aq) + 2HCl (aq) ———–> MgCl₂ (aq) + H₂(g)
The reaction is known as displacement reaction.

Question 9.
What do you mean by exothermic and endothermic reactions ? Give examples.
Answer:
A chemical reaction is said to be exothermic in which a certain amount of heat energy is released. The container in which reaction is carried gets heated up. For example,
N₂ (g) + 3H₂(g) ———> 2NH₃ (g) + 92 kj
CH₄(g) + 2O₂(g) ———> CO₂ (g) + 2H₂O (l) + 890 kj
NaOH (aq) + HCl (aq) ———> NaCl (aq)+ H₂O (aq) + 57.5 kj
A chemical reaction is said to be endothermic in which a certain amount of heat energy is absorbed. The container in which the reaction is carried becomes cold. For example,
N₂(g) + O₂ (g) ———-> 2NO (g) – 180 kj
C(s) + H₂O (aq) ———-> CO (g) + H₂(g) – 130 kj

Question 10.
Why is respiration considered an exothermic reaction ? (CBSE 2011)
Answer:
We all know that we need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration, and hence, respiration is an exothermic reaction

Question 11.
Why are decomposition reactions called opposite of combination reactions ? Write equations for these reactions.
Answer:
Combination reaction may be defined as the reaction in which two or more substances combine under suitable conditions to form a new substance. For example,

A decomposition reaction may be defined as the reaction in which a single substance decomposes or splits into two or more substances under suitable conditions.
For example,

It may be concluded that a certain substance is formed or synthesised in combination reaction and it breaks or splits in decomposition reaction. Therefore, the two reactions oppose each other.

Question 12.
Write equations for each decomposition reaction, where energy is supplied in the form of heat, light and electricity.
Answer:

Question 13.
What is the difference between displacement and double displacement reactions ? Write equations for these reactions. (CBSE 2011)
Answer:
In a displacement reaction, one element takes the place of another in a compound dissolved in a solution. For example,
Fe(s) + CuSO₄ (aq) ———> FeSO₄ (aq) + Cu(s)
In a double displacement reaction, one component each of both the reacting molecules get exchanged to form the products. For example,
HCl (aq) + NaOH(aq) ———> NaCl (aq) + H₂O (aq)

Question 14.
In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write chemical equation involved.
Answer:
The chemical equation for the displacement reaction is :
Cu(s) + 2AgNO₃ (aq) ———-> Cu(NO₃)₂ (aq) + 2Ag (s).

Question 15.
What do you mean by precipitation reaction ? Explain giving examples.
Answer:
In a precipitation reaction, one of the products formed during the reaction does not dissolve in solution and gets settled at the surface of the container (beaker or tube). It is known as a precipitate. For example,

Question 16.
Explain the following in terms of gain or loss of oxygen with two examples of each,
(a) oxidation
(b) reduction.
Answer:
(a) Oxidation involves the gain of oxygen by a substance in a chemical reaction. For example,

(b) Reduction involves the loss of oxygen from a substance in a chemical reaction. For example,

Question 17.
A shining brown coloured element ‘X’ on heating in’ air becomes black in colour. Name the element ‘X’ and the black coloured compound formed. (CBSE 2013)
Answer:
The element ‘X’ is copper and the black coloured compound is copper (II) oxide also known as cupric oxide.

Question 18.
Why do you apply paint on iron articles ?
Answer:
Paint forms a protective coating on the surface of iron. Both oxygen and moisture (present in air) cannot have a direct contact with the surface of the iron metal. Therefore, the surface gets protected against rusting.

Question 19.
Oil and fat containing food items are flushed with nitrogen. Why ?
Answer:
Oil and fat containing food items or etables get rancid due to oxidation by air or oxygen. In case the container or bag is flushed with nitrogen, then oxidation or rancidity will be checked.

Question 20.
Explain the following terms wih one example of 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.

In-Text Questions

Question 1.
Why should magnesium ribbon be cleaned before burning in air ?
Answer:
Magnesium is a reactive metal. When it is exposed to atmosphere for longer time, the oxygen present in the atmosphere combines and MgO is formed on the metal surface. This oxide layer does not burn in the flame so magnesium ribbon has to be cleaned with sandpaper before burning in air.

Question 2.
Write the balanced equations 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)

Question 3.
Write the balanced equations with state symbols for the following reactions :
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride (in water) and water.
Answer:
The symbol equations in balanced form for the reactions are :

Question 4.
A solution of the substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ with water.
Answer:
(i) The substance 'X’ is calcium oxide (also called quick lime). Its formula is CaO

Question 5.
Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Answer:
Water contains two parts of hydrogen and one part oxygen. Therefore, during the electrolysis of water the amount of hydrogen gas collected in one of the test tubes is double than that of the oxygen produced and collected in the other test tube.

Question 6.
Why does the colour of copper sulphate change when an iron nail is dipped in it ?
Answer:
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.