Explore topic-wise InterviewSolutions in .

• Sometimes during rainy season the power supply to our home from the electric pole will be interrupted due to the formation of the metal oxide layer on the electric wire. 
• This metal oxide is an electrical insulator.

a) The substance is calcium carbonate. 

b) The gas liberated is carbon dioxide. 

c) The solution taken in the second test tube was solution of slaked lime. 

d) Two types of reactions took place in this experiment, i.e. decomposition and double displacement.

CaCO_3(s) → CaO_(s) . CO₂ ↑

Ca(OH)_2(aq) + CO_2(g) → CaCO_3(s) + H₂O

a) It is a decomposition reaction carried out in the presence of heat. So it is thermal decomposition reaction. 

b) It is a decomposition reaction carried out in the presence of light. So it is photochemical reaction. 

c) It is a decomposition taking place in the presence of electricity. So it is electrolysis reaction. 

d) Lead is more reactive than copper. So lead displaces copper from salt solution. So the reaction is chemical displacement reaction.

Antioxidants : 

The spoilage of food can be prevented by adding preservatives which prevent oxidation. The substances which prevent oxidation are called antioxidants.

(OR)

 The substances which prevent oxidation added to food containing fats and oil are called antioxidants.

1.  Mg(OH)_2(aq) + 2 HNO_3(aq) → Mg(NO₃)_2(aq) + 2 H₂O_(l)

It is both double displacement and neutralisation reaction.

2.  Mg_(s) + CO_(g) → MgO_(s) + C_(s)

It is a redox reaction in which magnesium is oxidised and carbon monoxide is reduced.

3.  BaCl_2(aq) + Na₂SO_4(aq) → BaSO_4(s) ↓ + 2 NaCl_(aq)

It is both precipitation and double displacement reaction.

4.  2 NaNO_3(s) → 2 NaNO_2(s) + O_2(g)

It is both endothermic and decomposition reaction.

1. Souring of milk 

2. Formation of curd from milk 

3. Cooking of food 

4. Digestion of food in our body 

5. Fermentation of grapes 

6. Rusting of iron 

7. Burning of fuels 

8. Burning of candle wax 

9. Ripening of fruits.

Oxygen molecules interact with different substance from metal to living tissue which may come into contact with it. The above changes occur. These are all the examples of the process of oxidation.

Endothermic reaction. 66 KJ of heat energy is observed in this reaction.

When one or more substances undergo a complete change with the release of absorption of energy to form one or more new substances called products. It is characterised by one or more characters like evolution of gas, change of colour, formation of precipitate, energy changes and change of state.

Aluminium is used to wrap food material because it reacts with oxygen and forms a protective layer of aluminium oxide which prevents further oxidation.

The balanced equation is 

CaCO_3(s) → CaO_(s) + CO_2(g) 

(40 + 12 + 3 x 16) U → (40 + 16) U + (12 + 2 x 16) U 

100 U → 56 U + 44 U 

100 g → 56g + 44g 

100 g calcium carbonate on decomposition produces 56 g of calcium oxide. 

Amount of calcium oxide formed due to decomposition of 2 kg of calcium carbonate

=  \(\frac{56}{100}\,\times\,2000 = 1120\,g.\)

Acid change blue litmus to Red.

Copper (Cu) + Oxygen (O₂) → CuO

1. addition reaction
2. salt and water
3. oxidation
4. an exothermic reaction

Chemical Change : The process in which the change is affecting the identity of a molecule by a change in chemical composition is called a “chemical change”.

Physical Change: The process where a change is occurring only in physical properties, without affecting the identity of the molecules is called a “physical change”.

The term ‘Exo’ means outside, ‘Endo’ means inside and ‘Thermo’ means heat.

Displacement reaction.

Chemical reactions are defined as changes in which substances undergo an irreversible change, resulting in the formation of a new substance.

Biocatalysts are enzymes which help our body to perform various metabolic functions, e.g. amylase, lipase etc.

Homogeneous reactions are those in which the reactants and products are in the same physical state and are miscible with each other.

A precipitate is an insoluble substance formed when two substances or chemicals react in solution form. The precipitate formed is indicated by writing an arrow pointing downwards (↓) beside the precipitate.

NaCl + AgNO₃ → AgCl ↓ + NaNO₃

A precipitate is a solid product’ which separates out from the solution during a chemical reaction.

New substances are formed in chemical reaction.

It is a method of protecting iron from rusting by coating them a thin layer of zinc.

During the respiration reaction energy releases, so it is an exothermic reaction.

The process of burning a substance in the presence of oxygen is called “combustion”.

• Oxidation reactions in food material that were left for a long period are responsible for spoiling of food. This process is called “rancidity”. 
• When these processes occur in food, undesirable odours and flavours can result. 
• Rancidity is an oxidation reaction.

Food materials containing fat / oil are exposed to air, for a long time they react with atmospheric oxygen and it is responsible for spoiling of food. This process is called “rancidity”.

1) In displacement reaction one element displaces another element from its compound and takes its place there in. 

2) Eg : Zinc pieces react with dilute hydrochloric acid and liberate Hydrogen gas.

 Zn_(s)+ 2HCl_(aq) → ZnCl_2(aq)+ H_2(g)

In the reaction Fe₂O₃ + 2Al → Al₂O₃ + 2Fe, 

Aluminium (Al) is oxidised and formed as Al₂O₃ .

i) Zn_(s) + Ag NO_3(aq) → Zn (NO₃)_2(aq)+ Ag_(s) 

Balanced equation : Zn_(s) + 2 AgNO_3(aq) → Zn (NO₃)_2(aq) + 2Ag_(s)

ii) Fe₂O + C_(s) → Fe_(s) + CO_2(g) 

Balanced equation : 2Fe₂O_3(s) + 3C_(s) → 4Fe_(s) + 3CO_2(g) 

iii) Ag_(s) + H₂S_(g) → Ag₂S_(s) + H₂O_(I) 

Balanced equation : We cannot balance the equation because atom ‘O’ does not exist in the reactants. 

iv) Cu_(s) + O_2(g) → CuO_(s) 

Balanced equation : 2Cu_(s) + O_2(g) → 2CuO_(s)

Zn + 2HCl → ZnCl₂ + H₂ 

• In the reaction 2 moles of HC/ produce 1 mole of H₂ . 
• If 1 mole of HCl participate in reaction, Vi mole of hydrogen will be produced. 
• 1 mole of H₂ gas contains 6.023 × 10²³ molecules at STP. Number of molecules in 1/2 mole of H₂ gas = 6.023 × 10²³ × ½ = 3.011 × 10²³

The type of reaction observed here is chemical decomposition the leaves that fall on the earth during the winter season get decomposed the carbohydrates present in the leaves are decomposed into simple sugar which can be absorbed by the plants where as plastic are not decomposed so it add to pollution.

The advantages for writing a chemical equation using formula and symbols are 

1. Qualitative : Helps in knowing the reactants and products 

2. Qualitative : Helps in knowing the relation between the quantities of reactants consumed and -products formed 

3. can represent in short scientific way and saves time and space 

4. Molecular formula helps to find out the exact number of atoms of different elements present in a molecule of the compound.

Zinc reacts with oxygen and forms layer of zinc oxide which prevents further oxidation. So a layer of zinc protects iron from rusting.

Fe + CuSO₄ → FeSO₄ + Cu

1) No reaction takes place because zinc is more reactive than iron. So iron cannot displace zinc from its salt solution. 

2) No reaction takes place because in solid state silver nitrate as well as sodium chloride is unable to dissociate into constituent ions. So exchange of ions is not possible.

A) Hydrogen gas and iron chloride are produced.

1. 4 Na + O₂ → 2 Na₂ O 

2. 2 H₂O₂ → 2 H₂O + O₂ 

3. Mg(OH)₂ + 2 HCl → MgCl₂ + 2 H₂O 

4. 4 Fe + 3 O₂ → 2 Fe₂O₃

a) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

b) Al₄C₃ + 12H₂O → 3CH₄ + 4Al(OH)₃

c) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

d) Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Reaction will not takes place if an iron piece is placed in solid state CuSO₄ crystals because there will not exist separate Cu⁺² , SO₄^-2 ions in CuSO₄ Crystals. In aqueous solution, they exists. So iron can not displace copper.

2Na + 2H₂O → 2NaOH + H₂

(b) combination reaction

Reaction:
(a) H₂ + Cl₂ → 2HCl
Explanation:
Two elements H₂ and Cl₂ combine to form a single compound, HCl. Hence, it is a combination reaction. As two elements combine to form a compound, it is an element – element combination.
(b) CaO + H₂O → Ca(OH)₂
Explanation:
Two compound CaO and HaO combine to form a single compound, Ca (OH)₂. Hence, it is a combination reaction. As two compounds combine to form a new compound, it is a compound – compound combination.
(c) CaCO₂ → CaO + CO₂
Explanation:
The compound CaCO₃ splits (decompose) into two compounds, CaO and
COO. Hence, it is a compound-com¬pound combination.
(d) FeSO₄ + Mg → MgSO₄ + Fe
Explanation:
Here magnesium (Mg) is displacing, iron (Fe) from its salt solution (FeSO₄). Hence, it is a displacement reaction.
(e) 2KI + Pb (NO₃)₂ → 2KNO₃ + Pbl₂
Explanation:
The reactants potassium iodide and lead nitrate react to form potassium nitrate and lead iodide. From this it is clear that, radicals are intercharged by the decomposition of reactants. Hence, it is a double decomposition reaction.

(a) addition

Reduction is a chemical reaction that involves addition of hydrogen or removal of oxygen from a substance. When hydrogen is passed over heated black copper oxide, the latter changes to reddish-brown copper metal. Here copper oxide is said to be reduced to copper by loss of oxygen.
H₂ + CuO → Cu + H₂O
Chlorine reacts with hydrogen in the presence of diffused light to form hydrogen chloride. Chlorine is said to be reduced to hydrogen chloride by gain of hydrogen.
Cl₂ + H₂ → 2HCl
A reducing agent is one that can reduce other substances by removing oxygen from them or by supplying hydrogen to them. In the reaction between copper oxide and hydrogen, hydrogen is the reducing agent as it removes oxygen from copper oxide. In the reaction between chlorine and hydrogen, hydrogen is the reducing agent as it supplies hydrogen to chlorine.

(b) chemical double decomposition reaction