DELHI PUBLIC SCHOOL BANGALORE EAST

CHEMISTRY-- Class X
CHAPTER-1

CHEMICAL REACTIONS AND EQUATIONS (NOTES)

Physical and chemical changes

PHYSICAL CHANGE – Process in which change in colour or state occurs but no new
substance is formed.
Example: Melting of ice, Evaporation of water, dissolving sugar in water etc.

CHEMICAL CHANGE – Process in which the original substances lose their nature and
identity and form one or more new chemical substances with different properties.
Example: Cooking of food, digestion of food in our body, respiration, burning of fuels,
fermentation of grapes etc.

Observations that help determine a chemical reaction

A chemical reaction can be determined with the help of one or more of the following
observations:
a) Evolution of a gas
b) Change in temperature
c) Formation of a precipitate
d) Change in colour
e) Change of state

During a chemical change, a chemical reaction takes place. This can be represented in the form of a
Chemical equation.

Chemical reaction - Chemical reactions are chemical changes in which reactants transform into
products. The atoms of one element do not change into those of another element, nor do atoms
disappear or appear from somewhere. Actually, chemical reactions involve the breaking and
making of bonds (or both) between atoms to produce new substances.

Chemical Equations:
Word equation
A word equation is a chemical reaction expressed in words rather than chemicalformulas. It helps
identify the reactants and products in a chemical reaction.
For example,
Zinc + dil. Sulphuric acid → Zinc Sulphate + Hydrogen

Writing chemical equations

Representation of a chemical reaction in terms of symbols and chemical formulae of the
 reactants and products is known as a chemical equation.

Zn(s) + dil.H2SO4(aq) → ZnSO4(aq) + H2(↑)

(Reactants) (Products)

Skeletal or Unbalanced chemical equation

An equation in which the number of atoms of each element on the two sides of the equation
(Reactants and products) is not equal.

Balanced chemical equation

The chemical equation in which the number of atoms of each element in the reactants side is
equal to that of the products side is called a balanced chemical equation. It follows the Law
of conservation of mass.

Balancing of a Chemical Equation: Refer textbook Pg 3,4,5

Making chemical equations more informative: Refer page 5

Types of chemical reactions:

1. Combination reaction
A reaction in which a single product is formed from two or more reactants.

• Burning of Magnesium ribbon

Mg + O2 → 2MgO
Magnesium Oxygen Magnesium oxide

• Reaction of water with Quick lime

CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat (Activity 1.4)
Quick lime Slaked lime

Exothermic Reactions
Reactions in which heat is evolved along with formation of products.
Eg
• CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
• CH4 + 2O2 → CO2 + 2H2O + Heat (Combustion)
• C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (Respiration)
• Decomposition of vegetable matter into compost

Endothermic Reactions
Reactions in which heat is absorbed during the reaction. Eg -
• N2(g) + O2(g) + Heat → 2NO(g)
nitric oxide
• CaCO3 + Heat → CaO + CO2

• Photosynthesis in plants

2. Decomposition reactions

Reactions in which a single reactant decomposes on the application of heat or light or

electricity to give two or more simpler products.

Types of decomposition reactions:

a. Thermal decomposition
These reactions take place by absorption of heat.

• Heating of Lead nitrate powder -

heat
2Pb(NO3)2 (s) → 2PbO(s) + 4NO2 (g) + O2 (g)
Lead nitrate Lead oxide Nitrogen dioxide Oxygen
(white) (yellow) (brown fumes)

• Heating of Ferrous Sulphate crystals -

heat
FeSO4 .7H2O (s) → FeSO4 + 7H2O
Lead nitrate (white) Anhydrous ferrous sulphate (white)
heat
2FeSO4 → Fe2O3 (s) + SO2( g) + SO3 (g)
ferrous sulphate Ferric oxide sulphur dioxide sulphur trioxide
(reddish brown)

• Decomposition of Limestone on heating -

CaCO3 (s) → CaO (s) + CO2 (g)

quick lime

b. Electrolytic decomposition
These reactions take place when electric current is passed through the compound in molten
state or in aqueous solution.
El current
2H2O(l) → 2H2 (g) + O2 (g)

c. Photolytic decomposition
These reactions take place on absorption of light.
sunlight
2 AgCl (s) → 2Ag(s) + Cl2 (g)

sunlight
2 AgBr(s) → 2Ag(s) + Br2 (g)
 • Black and White photography is based on these reactions as photographic films are
coated with these salts

NOTE: Most of the Decomposition Reactions are Endothermic reactions

Decomposition Reactions in our body: Digestion of food – Starch (from wheat, rice,
potatoes etc.) decomposes into simple sugars called glucose; proteins decompose into
simpler substances called amino acids

3. Displacement reactions
Reactions in which a more reactive element displaces a less reactive element from its
compound in aqueous/ molten form

i) Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

ii) Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

Activity series from Book page no. 45

All displacement reactions are usually exothermic in nature.

4. Double displacement reaction

Reactions in which an exchange of ions between the reactants takes place to give new
products. For example

Al2(SO4)3(aq) + 3Ca(OH)2(aq) → 2Al(OH)3(aq) + 3CaSO4(s)

Aluminium Calcium Aluminium Calcium
Sulphate hydroxide hydroxide sulphate

Pb(NO3)2(aq) + 2KI(aq) → 2KNO3(aq) + PbI2(↓)(s)

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

 Precipitation reaction

Reactions in which aqueous solutions of two compounds on mixing react to form an

insoluble compound which separates out as a solid (called precipitate). Eg:

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

white ppt

Pb(NO3)2(aq) + 2KI(aq) → 2KNO3(aq) + PbI2(↓)(s)

Yellow ppt

Passing CO2 through lime water:

Ca(OH)2 (aq) + CO2 (g) → CaCO3(s) + H2O(l)

milky suspension

Neutralization reaction

A reaction in which an acid reacts with a base to form salt and water.
These are also double displacement reactions

Acid + Base → Salt + Water

Eg:
NaOH + HCl → NaCl + H2O

2NH4OH + H2SO4 → (NH4)2 SO4 + 2H2O

NaOH + CH3COOH → CH3COONa + H2O

5. REDOX Reactions (Oxidation and Reduction)

• OXIDATION is defined as a process which involves

➢ Gain of oxygen or
➢ Loss of hydrogen
➢ Loss of electrons
• REDUCTION is defined as a process which involves
➢ Gain of hydrogen or
➢ Loss of oxygen
➢ Gain of electrons
• Reduction is the opposite of Oxidation
Eg :
Heat
2Cu + O2 → 2CuO
Reddish brown black

Heat
CuO + H2 → Cu + H2O

• Reduction and Oxidation go on side by side – occur simultaneously (REDOX

 REACTION) – one substance loses oxygen , another substance gains that oxygen

Oxidising agent – a substance which helps another substance to undergo oxidation – it

gives oxygen or gains hydrogen (it undergoes reduction)
Reducing agent – a substance which helps another substance to undergo reduction – it
gives hydrogen or gains oxygen (it undergoes oxidation)

Heat
CuO + H2 → Cu + H2O

Substance oxidized - H2
Substance reduced – CuO
Oxidising agent – CuO
Reducing agent – H2

Effects of Oxidation Reactions in Everyday life

Corrosion
The process of slowly eating up (deterioration) of a metal by the action of atmospheric
gases like O2 , CO2, H2S etc as well as moisture, acids etc. on the surface of metals; oxides,
carbonates, sulphides of metal are formed.

❖ Rusting of Iron:

4Fe(s) + 3O2 + xH2O → 2Fe2O3.xH2O

(from air) (rust)

Prevention –
Painting, greasing or oiling, galvanization, alloying etc

❖ Corrosion of copper:

Cu(s) + H2O + O2 + CO2 → CuCO3.Cu(OH)2

(from air) (green coating of basic
Copper carbonate)

❖ Corrosion (Tarnishing) of silver:

Ag(s) + H2 S → Ag2S + H2(g)

(from air) (black silver sulphide)

Rancidity
It refers to oxidation of fats and oils in food that is kept for a long time resulting in bad taste and
smell. Rancid food can cause stomach infection on consumption.
Prevention:
(i) Vacuum packing or storing in air-tight containers
(ii) Packaging with nitrogen
(iii) Refrigeration
(iv) Addition of antioxidants or preservatives
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Chemical Reactions and Equations

Question 1: Why should a magnesium ribbon be cleaned before burning in air?

Answer: Magnesium is very reactive metal. When stored it reacts with oxygen to form a layer
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 to
remove this layer so that the underlying metal can be exposed into air.

Question 2: 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)

(ii)

(iii)
Question 3: 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)

(ii)

Question 4: 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: The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

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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 (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.

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

Answer: When an iron nail is placed in a copper sulphate solution, iron displaces copper from
copper sulphate solution forming iron sulphate, which is green in colour.

Therefore, the blue colour of copper sulphate solution fades and green colour appears.

Question 7: Give an example of a double displacement reaction other than the one given in
Activity 1.10. Sodium carbonate reacts with calcium chloride to form calcium carbonate and
sodium chloride.

Answer:

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

Question 8: Identify the substances that are oxidised and the substances that are reduced in the
following reactions.

(i)

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

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

Question 9: Which of the statements about the reaction 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

Solutions:-

(i) (a) and (b)

Question 10:

The above reaction is an example of a

(a) combination reaction.

(b) double displacement reaction.

(c) decomposition reaction.

(d) displacement reaction

Solution:-

(d) The given reaction is an example of a displacement reaction.

Question 11: 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.

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(d) Iron salt and water are produced.

Solution:-
(a) Hydrogen gas and iron chloride are produced. The reaction is as follows:

Question 12: What is a balanced chemical equation? Why should chemical equations be
balanced?

Answer: 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.

Question 13: 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 )

(b)

(c )

(d)

Question 14: Balance the following chemical equations.

(a )

(b)

(c )

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(d)

Solutions:-

(a )

(b)

(c )

(d)

Question 15: 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 )

(b)

(c )

(d)

Question 16: 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:

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(a ) Double displace ment rea ction

(b) ; Dec omposition reac tion

(c ) Combination reac tion

(d) Displacement reac tion

Question 17: What does one mean by exothermic and endothermic reactions? Give examples.

Answer: 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

In other words, combination reactions are exothermic.

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.

Question 18: Why is respiration considered an exothermic reaction? Explain.

Answer: Energy is required to support life. Energy in our body is obtained from the food we eat.
During digestion, large molecules of food are broken down into simpler substances such as
glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this
combustion reaction is respiration. Since energy is released in the whole process, it is an
exothermic process.

Question 19: Why are decomposition reactions called the opposite of combination reactions?
Write equations for these reactions.

Answer: 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.

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Dec omposition reac tion:

Combination reac tion:

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

(a) Thermal decomposition:

(b) Decomposition by light:

(c) Decomposition by electricity:

Question 20: What is the difference between displacement and double displacement reactions?
Write equations for these reactions.

Answer: In a displacement reaction, a more reactive element replaces a less reactive element from
a compound.
where A is more reactive than
B

In a double displacement reaction, two atoms or a group of atoms switch places to form new
compounds.

For example:

Displacement reaction:

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Double displacement reaction:

Question 21: In the refining of silver, the recovery of silver from silver nitrate solution involved
displacement by copper metal. Write down the reaction involved.

Answer:

Question 22: What do you mean by a precipitation reaction? Explain by giving examples.

Answer: A reaction in which an insoluble solid (called precipitate) is formed is called a

precipitation reaction.

For example:

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

Another example of precipitation reaction is:

In this reaction, barium sulphate is obtained as a precipitate.

Question 23: Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation

(b) Reduction

(a) Oxidation is the gain of oxygen.

Answer:

For example:

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(i)

(ii)

In equation (i), H2 is oxidized to H2O and in equation (ii), Cu is oxidised to CuO.

(b) Reduction is the loss of oxygen.

For example:

(i)

(ii)

In equation (i), CO2 is reduced to CO and in equation (ii), CuO is reduced to Cu.

Question 24: A shiny brown-coloured element ‘X’ on heating in air becomes black in colour.
Name the element ‘X’ and the black coloured compound formed.

Answer: ‘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.

Question 25: Why do we apply paint on iron articles?

Answer: 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 their presence
is essential for rusting to take place.

Question 26: Oil and fat containing food items are flushed with nitrogen. Why?

Answer: Nitrogen is an inert gas and does not easily react with these substances. On the other
hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing
food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not
present inside the pack, rancidity of oil and fat containing food items is avoided.

Question 27: Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

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(a) Corrosion:

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.

This hydrated iron oxide is rust.

(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.

Rancidity can be avoided by:

1. Storing food in air tight containers

2. Storing food in refrigerators

3. Adding antioxidants

4. Storing food in an environment of nitrogen

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