NCERT Solutions for Class 10 Science Chapter 1 Chemical Reactions And Equations

Q1 ) Which of the statements about the reaction below are incorrect?

$$2PbO(s)+C(s)$$

$$?$$

$$2Pb(s)+C{O}_2$$

• (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

Answer :

(i) a and b

Reason -
Oxidation is a reaction in which oxygen combines with another substance.
Reduction is a reaction in which a compound is deprived of all, or part, of the oxygen it contains.
Thus in mentioned reaction:
Carbon is getting oxidised(since oxygen has been added to it)
Lead Oxide is getting reduced (since oxygen has been removed from it)

Q2 ) The below reaction is an example of a

$$F{e}_2{O}_3+2Al?A{l}_2{O}_3+2Fe$$

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

Answer :

Option d
Displacement reaction
A Displacement reaction is a reaction in which a more active element displaces a less active element from its salt solution.
In the mentioned reaction:
Aluminium is more active metal than Iron.
Therefore Iron is displaced by aluminium.

Q3 )

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 :

Option - a
As we all know

$$Acid+Metal?Salt+Hydrogen$$

Therefore when Dilute Hydrochloric acid (Acid) is added to Iron Fillings (Metal)
Iron Chloride (Salt) is formed with Hydrogen (Gas).

$$2HCl+Fe?FeC{l}_2+H_2$$

Q4 )

What is a balanced chemical equation? Why should chemical equations be balanced?

Answer :

Ans:
Balanced Chemical Equation is an equation in which the total number of atoms of each element in the reactants,on the left side of the equation,is the same as the number of atoms in the products formed, on the right side of the equation.

Why should Chemical Equation be balanced?

An equation must be balanced in order to comply with the "Law of Conservation of Matter",which states that matter is created nor destroyed in the course of a chemical reaction.
An unbalanced equation would imply that atoms have been created or destroyed.
Example :

$$Zn+H_{2}SO4?ZnS{O}_4+H_2$$

Element Name	In Reactants	In Products
No. of Zn atoms	1	1
No.of H atoms	2	2
No.of S atoms	1	1
No.of O atoms	4	4

Thus above reaction between zinc and dilute sulphuric acid is a balanced equation.

Q5 )

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 sulpur 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 :

$$(a)H_2+N_2?N{H}_3$$
Here on reactants side Hydrogen and Nitrogen both elements has 2 atoms and on products side Hydrogen and Nitrogen have 3 atoms each.
Equation is not balanced.
Balanced equation:
$$3H_2+3N_2?2N{H}_3$$
Similarly,

$$(b)2H_{2}S+3O_2?2H_{2}O+2S{O}_2$$

$$(c)3BaC{l}_2+A{l}_2(S{O}_4{)}_3$$

$$?$$

$$2AlC{l}_3+3BaS{O}_4$$

$$(d)2K+2H_{2}O?2KOH+H_2$$

Q6 )

Balance the following chemical equations

$$(a)HN{O}_3+Ca(OH{)}_2$$

$$?$$

$$Ca(N{O}_3{)}_2+H_{2}O$$

$$(b)2NaOH+H_{2}S{O}_4$$

$$?$$

$$N{a}_{2}S{O}_4+2H_{2}O$$

$$(c)NaCl+AgN{O}_3$$

$$?$$

$$AgCl+NaN{O}_3$$

$$(d)BaC{l}_2+H_{2}S{O}_4$$

$$?$$

$$BaS{O}_4+2HCl$$

Answer :

a) Let's count the number of atoms of each element in reactants and products side.
On reactants side:
H:3,N:1,O:5,Ca:1
On products side:
H:2,N:2,O:7,Ca:1
By Hit and trial method
Let's try to balance N by multiplying 2 to $$HN{O}_3$$ on reactants side.
Now we have:
On reactants side: H:4,N:2,O:8,Ca:1
On products side: H:2,N:2,O:7,Ca:1
Now if we multiply 2 to $$H_{2}O$$ on products side
Equation so far :

$$2HN{O}_3+Ca(OH{)}_2$$

$$?$$

$$Ca(N{O}_3{)}_2+2H_{2}O$$

Since the numbers on the left equal the right, you now have a balanced equation, so this must be the answer:

SIMILARLY.

$$(b)2NaOH+H_{2}S{O}_4$$

$$?$$

$$N{a}_{2}S{O}_4+H_{2}O$$

$$(c)NaCl+AgN{O}_3$$

$$?$$

$$AgCl+NaN{O}_3$$

$$(d)BaC{l}_2+H_{2}S{O}_4$$

$$?$$

$$BaS{o}_4+2HCl$$

Q6). Balance the following chemical equations

$$(a)HN{O}_3+Ca(OH{)}_2$$

$$?$$

$$Ca(N{O}_3{)}_2+H_{2}O$$

$$(b)NaOH+H_{2}S{O}_4$$

$$?$$

$$N{a}_{2}S{O}_4+H_{2}O$$

$$(c)NaCl+AgN{O}_3$$

$$?$$

$$AgCl+NaN{O}_3$$

$$(d)BaC{l}_2+H_{2}S{O}_4$$

$$?$$

$$BaS{O}_4+HCl$$

Answer :

a) Let's count the number of atoms of each element in reactants and products side.
On reactants side:
H:3,N:1,O:5,Ca:1
On products side:
H:2,N:2,O:7,Ca:1
By Hit and trial method
Let's try to balance N by multiplying 2 to $$HN{O}_3$$ on reactants side.
Now we have:
On reactants side: H:4,N:2,O:8,Ca:1
On products side: H:2,N:2,O:7,Ca:1
Now if we multiply 2 to $$H_{2}O$$ on products side
Equation so far :

$$2HN{O}_3+Ca(OH{)}_2$$

$$?$$

$$Ca(N{O}_3{)}_2+2H_{2}O$$

Since the numbers on the left equal the right, you now have a balanced equation, so this must be the answer:

SIMILARLY.

$$(b)2NaOH+H_{2}S{O}_4$$

$$?$$

$$N{a}_{2}S{O}_4+H_{2}O$$

$$(c)NaCl+AgN{O}_3$$

$$?$$

$$AgCl+NaN{O}_3$$

$$(d)BaC{l}_2+H_{2}S{O}_4$$

$$?$$

$$BaS{o}_4+2HCl$$

Q7 )

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)
Calcium Hydroxide: $$Ca(OH{)}_2$$
Carbon Dioxide: $$C{O}_2$$
Calcium Carbonate: $$CaC{O}_3$$
Water: $$H_{2}O$$
$$Ca(OH{)}_2+C{O}_3?CaC{O}_3+H_{2}O$$

SIMILARLY

$$(b)2AL+3Cu(Cl{)}_2?AlC{l}_3+3Cu$$

$$(c)Zn(s)+2AgN{O}_3(s)$$

$$?$$

$$Zn(N{O}_3{)}_2+2Ag$$

$$(d)BaC{l}_2+K_{2}S{O}_4?BaS{O}_4+2KCl$$

Q8 )

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 :

$$(a)2KBr(aq)+Ba{I}_2(aq)$$

$$?$$

$$2KI(aq)+BaB{r}_2$$

It is a Double Decomposition Reaction.
This is a type of chemical change in which two compounds in a solution react to form two new compounds by mutual exchange of radicals.It is also called double displacement reaction.

$$(b)ZnC{O}_3(s)?ZnO(s)+C{O}_2(g)$$
It is Decomposition reaction.
The chemical reaction in which a compound splits into two or more simpler substances (elements or compounds) is called decomposition reaction.

$$(c)H_2(g)+C{l}_2(g)?2HCl$$
It is a Direct Combination reaction.
A reaction in which two or more substances combine together to form a single substance is called a combination reaction or synthesis.

$$(d)Mg(s)+2HCl(aq)$$

$$?$$

$$MgC{l}_2(aq)+H_2(g)$$

It is a Displacement reaction.
It is a chemical change in which a more active element displaces a less active element from its salt solution.

Q10 )

Why is respiration considered an exothermic reaction ? Explain.

Answer :

Exothermic reaction is a reaction in which energy is released in the form of heat or light.In exothermic reaction total energy of product is less than total energy of reactants.
In case of Respiration: Respiration is a chemical process in which cells of an organism obtain energy by combining glucose and oxygen forming carbon dioxide,water and energy.,
Respiration is called an exothermic reaction because energy is released in the process of forming carbon dioxide and water by combining glucose and oxygen.

$$C_6{H}_{12}{O}_6(aq)+6O_2(g)$$

$$?$$

$$6C{O}_2(aq)+6H_{2}O(L)+Energy$$

Q11 )

Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer :

Combination reaction is a reaction in which two or more substances combine together to form a single substance.
$$A+B?AB$$

Decomposition reaction is a reaction in which a compound splits into two or more simpler substances (elements or compounds).
$$AB?C+D+.....$$
This is why Decomposition Reaction is called opposite of Combination Reaction.
$$CaC{O}_3(S)?CaO(s)+C{O}_2(g)$$
This is a Decomposition reaction $$H_2(g)+C{l}_2?2HCl(g)$$
This is a Combination reaction

Q12 )

Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer :

Decomposition may occur in the presence of Heat or Light or by the passage of an Electric Current.
Heat light and electricity are different forms of energy which cause breaking of bonds in the molecule of reactants to yield simpler products.
(a)The compound calcium carbonate when heated, decomposes to form two elements, calcium oxide and carbon dioxide
$$CaC{O}_3(s)?CaO(s)+C{O}_2(g)$$
In this reaction energy is supplied in the form of heat.
(b)When silver chloride is exposed to sunlight it undergoes photochemical decomposition forming silver and chlorine.
$$2AgCl(s)+sunlight?2Ag(s)+C{l}_2(g)$$
In this reaction energy is supplied in the form of light,i.e.sunlight
(c)When electric current is passed through water,the latter decomposes into hydrogen and oxygen.
$$2H_{2}O+electricity?2H_2+O_2$$
In this reaction energy is supplied in the form of electricity.

Q13 )

What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer :

Displacement reaction:

(a) When a more reactive element displaces a less reactive element from its salt solution, In displacement reaction one displacement takes place. it is called displacement reaction.
(b) In displacement reaction one displacement takes place.

Double Displacement reaction:

(a) There is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction.
(b) In double displacement reaction two displacements takes place.
Following are examples of Displacement reaction and Double Displacement reaction.

Displacement reaction

$$Mg(s)+2HCl(aq)?MgC{l}_2+H_2(g)$$

Double Displacement reaction

$$2KBr(aq)+BaC{l}_2$$

$$?$$

$$2KCl(aq)+BaB{r}_2$$

Q14 )

In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer :

The silver nitrate is in solution and the metallic copper will dissolve to form copper nitrate; as it does, it will precipitate the silver in solution as metallic silver.
This is a Displacement reaction as copper is more active than Silver(referring to activity series),copper displaces silver from its nitrate solution.

$$2AgN{O}_3(aq)+Cu(s)$$

$$?$$

$$Cu(N{O}_3{)}_2+2Ag(s)$$

Q15 )

What do you mean by a precipitation reaction? Explain by giving examples.

Answer :

Certain chemical reactions are characterized by the formation of insoluble solid substances called precipitates.
Reactions in which precipitate is formed are called precipitation reactions.

$$N{a}_{2}S{O}_4(aq)+BaC{l}_2(aq)$$

$$?$$

$$2Nacl(aq)+BaS{O}_4(s)$$

In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.

Q16 )

Explain the following: (a) Oxidation (b) Reduction

Answer :

(a) Oxidation: When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation.
In the following reaction:
copper gets oxygen and hence is oxidized.
$$2Cu+O_2+Heat?2CuO$$

(b) Reduction: If a substance loses oxygen or gains hydrogen during a reaction, it is that the substance is reduced. This phenomenon is called reduction.
In the following reaction:
zinc loses oxygen, and hence called is reduced.
$$ZnO+C?ZnO$$

Q17 )

Shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer :

Copper is a brown coloured shiny element, when heated in air, it becomes black in colour.
$$2Cu+O_2+Heat?2CuO$$ Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide.

Q18 )

Why do we apply paint on iron articles?

Answer :

When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, to prevent an iron article to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted.

Q19 )

Oil and fat containing food items are flushed with nitrogen. Why?

Answer :

When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.

Q9 ) What is meant by exothermic and endothermic reactions? Give examples.

Answer :

Exothermic reaction:
Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.
Example: Mixture of sodium and chlorine to yield table salt

Endothermic reaction:
Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.
For example: In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

Q20 ) Explain the following terms with one example each. (a) Corrosion (b) Rancidity

Answer :

(a) Corrosion:
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.
(b) Rancidity:
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.

Q1 ) Why should a magnesium ribbon be cleaned before burning in air?

Answer :

Magnesium ribbon should be cleaned before burning in air because Magnesium metal reacts with oxygen present in the atmosphere and forms a very stable compound i.e, Magnesium Oxide(MgO). In order to prevent further reactions with Oxygen, it is therefore required to clean the ribbon to remove the layer of MgO.

Q2 ) Write a balanced equation for the following chemical reactions.
(i) Hydrogen+Chloride $?$ Hydrogen Chloride

(ii) Barium Chloride+Aluminium Sulphate $?$ Barium Sulphate+Aluminium Chloride

(iii) Sodium+Water $?$ Sodium hydroxide+Hydrogen

Answer :

(i)
$H_2+C{l}_2?2HCl$

(ii)

$3BaC{l}_2+A{l}_2(S{O}_4{)}_3$

$?$

$2AlC{l}_3+3BaS{O}_4$

(iii) $2Na+2H_{2}O?2NaOH+H_2$

Q6 ) 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 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)

$BaC{l}_2+N{a}_{2}S{O}_4$

$?$

$BaS{O}_4+2NaCl$

(ii)$NaOH+HCl?NaCl+H_{2}O$

Q4 ) A solution of a substance ‘X’ is used for whitewashing.
(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) Calcium Oxide or quick lime is the substance used for whitewashing.Hence, the substance ‘X’ is Calcium Oxide and it’s formula is $CaO$.
(ii) $CaO+H_{2}O?Ca(OH{)}_2$

Q5 ) 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 :

(Hydrolysis of water has been performed in the activity 1.7 in the text book.)
When electric current is passed through water it decomposes to release $H_{2}and{O}_2$ gas , so gas collected in one of the test tubes is double of the amount collected in the other tube.Here, after electrolysis two molecules of Hydrogen and one molecule of Oxygen gas is released, hence the amount of Hydrogen collected would be double than that of Oxygen.

Q6 ) Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer :

Iron is more reactive than copper so, when an iron nail dipped in the copper sulphate solution, iron displaces copper from the copper sulphate. Therefore, the colour of the copper sulphate solution changes. Reaction taking place :
$Fe+CuS{O}_4?FeS{O}_4+Cu$

Q7 ) Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer :

$AgN{O}_3+NaCl?AgCl+NaN{O}_3$ Here silver donates its ion to Sodium and accepts chloride ion to form Silver Chloride and Sodium Nitrate ,so the reaction between silver Nitrate $AgN{O}_3$and Sodium Chloride $NaCl$is an example of double displacement reaction.

Q8 ) Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) $4Na(s)+O_2(g)?2N{a}_{2}O(s)$

(ii) $CuO(s)+H_2(g)?Cu(s)+H_{2}O(l)$

Answer :

(i)In the given reaction, Sodium gains and combines with oxygen hence it is oxidised. Consequently Oxygen is getting reduced.

(ii)In this reaction Hydrogen gains Oxygen and forms water, hence it is getting oxidised. And on the other hand Copper is losing Oxygen hence, getting reduced.