# Chemical Reactions : Complete Set of Questions

This set of questions contains all the possible concepts
which could be asked in the examination

Introduction to Chemical Equation

Q.1 What is meant by a chemical reaction?

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another.

Example rusting of iron, burning of coal and digestion of food etc.

Q.2 Is burning of a candle wax a physical change or a chemical change?

Chemical change(candle wax is a hydrocarbon,which burns to produce co2, soot etc.)

Q.3 State one difference between a physical change and a chemical change?

In a physical change, no new substance is formed while in a chemical change, a new substance is formed.

Q.4 A magnesium ribbon should be cleaned before burning it in air. Why?

It should be cleaned to remove the dust particles and its oxides that may be present on  its upper surface, so that it comes directly in contact with air.

Q.5 What do you mean by balanced chemical reaction?

The equations in which atoms of various elements on the reactants and the products side are equal.

Eg: - $Zn + {H_2}S{O_4} \to ZnS{O_4} + {H_2}$

In this equation the number of atoms of Zn,H,S  and O are equal on both sides, this means equation is balanced.

Q.6 Which one is a chemical change—rusting of iron or melting of iron?

Rusting of iron.

Q.7 What does symbol (aq) represent in a chemical equation?

The symbol (aq) represents aqueous solution. When it is written after a chemical formula in a chemical equation, it represents that the element or compound is a aqueous solution i.e water is added into the solution.

Q.8 Complete and balance the following equations :

(a) $NaOH + ....... \to N{a_2}S{O_4} + {H_2}O$

(b) $Ca{(OH)_2} + ......... \to CaC{O_3} + {H_2}O$

(a) $2NaOH + {H_2}S{O_4} \to N{a_2}S{O_4} +2 {H_2}O$

(b) $Ca{(OH)_2} + C{O_2} \to CaC{O_3} + {H_2}O$

Q.9 Correct and balance the following equation :

(a) $Ca + {H_2}O \to CaOH + H$

(b) $N + H \to N{H_3}$

The corrected and balanced equations are as follows :

(a) $Ca + 2{H_2}O \to Ca{\left( {OH} \right)_2} + {H_2}$

(b) ${N_2} + 3{H_2} \to 2N{H_3}$

Q.10 Fill in the following blanks with suitable words : -

(a) Chemical equations are balanced to satisfy the law of __________.

(b) A solution made in water is known as an ___________ solution and indicated by the symbol _________.

(a) Conservation of mass

(b) Aqueous, (aq)

Q.11 Write the balance of chemical equation for the following reaction.

Zinc + Silver Nitrate $\to$ Zinc nitrate + Silver ?

The balanced chemical equation is as follows : -

$Zn\left( s \right) + 2AgN{O_3}\left( {aq} \right) \to Zn{\left( {N{O_3}} \right)_2}\left( {aq} \right) + 2Ag\left( s \right)$

Q.12 Write the essential condition for following reaction to take place.

$2AgCl \to 2Ag + C{l_2}$ and write one use of this reaction.

To carry out the reaction, presence of sunlight is essential.

This reaction is used in black and white photography.

Q.13 List any four observations that help us to determine whether a chemical reaction has taken place ?

(i) Change in state

(ii) Change in colour

(iii) Evolution of a gas

(iv) Change in temperature.

Q.14 On what basis is a chemical equation balanced?

A chemical equation is balanced on the basis of law of conservation of mass. According to this law mass can neither be created nor be destroyed.

Q.15 Why is respiration considered an exothermic process?

In respiration process, glucose undergoes slow combustion reaction , it combines with oxygen to form carbon-dioxide, water and energy. The reaction in which energy is released is called as exothermic reaction, hence respiration is a exothermic process.

${C_6}{H_{12}}{O_6}\left( {aq} \right) + 6{O_2}\left( g \right) \to 6C{O_2}\left( g \right) + 6{H_2}O\left( l \right) + Energy$

Q.16 What happens chemically when quicklime is added to water filled in a bucket?

Quicklime is chemically a calcium oxide, when it added to water filled in a bucket, it reacts with water chemically and form slaked lime as a product and produce heat energy. So, water filled in the bucket becomes hot. Q.17 Why is photosynthesis considered an endothermic reaction?

In the photosynthesis  process light energy from sunlight is absorbed to produce oxygen and glucose from carbon-dioxide and water in the presence of chlorophyll, and the process in which energy is absorbed is called as endothermic reaction, hence photosynthesis is a endothermic reaction.

Q.18 How will you indicate the following effects in a chemical equation?

(a) A solution made in water

(b) Exothermic reaction

(c) Endothermic reaction

(a) A solution made in water is represented by writing (aq) after its formula in a chemical equation (b) Exothermic reaction is represented by writing ‘heat’ or ‘energy’ on the products side of a chemical reaction. (c) Endothermic reaction is indicated by writing ‘heat’ or ‘energy’ on the reactants side of a chemical reaction. Q.19 Write complete balanced equations for the following reactions:

(a) Calcium(solid)+water(liquid)  →  Calcium hydroxide(solution)+Hydrogen(gas)

(b) Sulphur dioxide(gas)+oxygen(gas) → Sulphur trioxide(gas)

(a) $Ca\left( s \right) + 2{H_2}O\left( l \right) \to Ca{\left( {OH} \right)_2}\left( {aq} \right) + {H_2}\left( g \right)$

(b) $2S{O_2}\left( g \right) + {O_2}\left( g \right) \to 2S{O_3}\left( g \right)$

Q.20 Write the rules for writing a word equation for a chemical reaction. Give one example ?

(a) The reactants are written on the left hand slide with the plus sign (+) in between them.

(b) The products are written on the right hand side with a plus sign between them.

(c) An arrowhead pointed towards the products is marked between the reactant and the products.

E.g. : - Magnesium + Oxygen – Magnesium oxide.

Q.21 Balance the following chemical reactions : -

(a) $Mn{O_2} + HCl \to MnC{l_2} + C{l_2} + {H_2}O$

(b) $Fe\left( s \right) + {H_2}O\left( g \right) \to F{e_3}{O_4}\left( s \right) + {H_2}\left( g \right)$

(c) $BaC{l_2}\left( {aq} \right) + A{l_2}{\left( {S{O_4}} \right)_3}\left( {aq} \right) \to BaS{O_4}\left( s \right) + AlC{l_3}\left( {aq} \right)$

(a) $Mn{O_2} + 4HCl \to MnC{l_2} + C{l_2} + 2{H_2}O$

(b) $3Fe\left( s \right) + 4{H_2}O\left( g \right) \to F{e_3}{O_4}\left( s \right) + 4{H_2}\left( g \right)$

(c) $3BaC{l_2}\left( {aq} \right) + A{l_2}{\left( {S{O_4}} \right)_3}\left( {aq} \right) \to 3BaS{O_4}\left( s \right) + 2AlC{l_3}\left( {aq} \right)$

Q.22 (a) Give one example of a chemical reaction.

(b) State two characteristics of the chemical reaction which takes place when dilute sulphuric acid is poured over zinc granules.

(c) Give two characteristics of the chemical reaction occurs on adding potassium iodide solution to lead nitrate solution ?

(a) Example of a chemical reaction : -

Respiration is an exothermic chemical reaction.

${C_6}{H_{12}}{O_6}\left( {aq} \right) + C{O_2}\left( {aq} \right) \to 6C{O_2} + 6{H_2}O + Energy$

(b) When dilute sulphuric acid is poured over Zinc granules : -

(i) Hydrogen gas is evolved.

(ii) Mixture becomes hot.

Reaction : $Zn + {H_2}S{O_4} \to ZnS{O_4} + {H_2} + Heat$

(c) On adding potassium iodide solution to lead nitrate solution : -

(i) Yellow precipitate of lead iodide are formed.

(ii) Colour of the solution changes from colourless to yellow.

$2KI + Pb{\left( {N{O_3}} \right)_2} \to Pb{I_2} + 2KN{O_3}$

Q.23 (a) What is a chemical equation ? Explain with the help of an example ?

(b) Giving examples, state the difference between balanced and unbalanced chemical equations.

(c) Balance the following chemical equation : -

(i) $N{H_3} \to {N_2} + {H_2}$

(ii) $C + C{O_2} \to CO$

(a) The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as chemical equation.

For e.g. : - Burning of magnesium ribbon in air to give white powder of magnesium oxide. (b) (c)

(i) $2N{H_3} \to {N_2} + 3{H_2}$

(ii) $C + C{O_2} \to 2CO$

Q.24 What happens when silver chloride is exposed to sunlight ? write a chemical equation for this reaction. Also give one use of such a reaction ?

When silver chloride is exposed to sunlight, decomposition of silver chloride takes place.

- The chemical reaction is given as :

$2AgCl\left( s \right) \to 2Ag\left( s \right) + C{l_2}$

Use : -

This reaction is used in black and white photography

Q.25 Balance the following chemical reactions : -

(a) $Al + CuC{l_2} \to 2AlC{l_3} + 3Cu$

(b) $PbO + HCl \to PbC{l_2} + {H_2}O$

(c) $N{H_3} + {O_2} \to NO + {H_2}O$

(a) $2Al + 3CuC{l_2} \to 2AlC{l_3} + 3Cu$

(b) $PbO + 2HCl \to PbC{l_2} + {H_2}O$

(c) $4N{H_3} + 5{O_2} \to 4NO + 6{H_2}O$

Q.26 What is meant by a skeletal chemical equation ? What does it represent ? Using the equation for electrolytic decomposition of water, differentiate between a skeletal chemical equation and a balanced chemical equation ?

Skeletal chemical equation is an unbalanced chemical equation.

It represents a chemical reaction. Q.27 Write balanced chemical equations for the following reactions : -

(a) Sodium carbonate on reaction with hydrochloric acid in equimolar concentrations gives sodium chloride and sodium hydrogen carbonate.

(b) Calcium carbonate on reaction with hydrochloric acid gives calcium chloride, water and carbon dioxide.

(c) Nitrogen reacts with hydrogen under pressure to form ammonia. Q.28 Write the balanced chemical equation for the following reactions.

(a) Sodium carbonate solution on reaction with hydrochloric acid in equinolar cancentration gives sodium chloride and sodium hydrogen carbonate.

(b) When cold water is added to sodium violent exothermic reaction takes place and sodium hydroxide and hydrogen are produced.

(c) Potassium metal reacts with water to give potassium hydroxide and hydeogin is released.

(a) $N{a_2}C{O_3}\left( {aq} \right) + HCl\left( {aq} \right) \to NaCl\left( {aq} \right) + NaHC{O_3}\left( {aq} \right)$

(b) $2Na\left( s \right) + 2{H_2}O\left( l \right) \to 2NaOH\left( {aq} \right) + {H_2} + Heat$

(c) $2K\left( s \right) + 2{H_2}O\left( l \right) \to 2KOH\left( {aq} \right) + {H_2}$

Q.29 (a) What is meant by a chemical reaction ? Explain with the help of an example ?

(b) Give one example each of a chemical reaction characterised by :

(i) evolution of a gas

(ii) change in colour

(iii) formation of a precipitate

(iv) change in temperature

(v) change in state ?

(a) Chemical reaction is the process in which new substances with new properties are formed. During a chemical reaction, atoms of one element do not change into those of another element only a rearrangement of atoms takes place in a chemical reaction.
eg : - The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction. (b) (i) Evolution of a gas : When zinc granules react with dilute sulphuric acid in a flask, hydrogen gas evolves. (ii) Change in colour : The chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution is characterised by a change in colour from orange to green.

(iii) Formation of a precipitate : The chemical reaction between potassium iodide and lead nitrate is characterised by the formation of a yellow precipitate of lead iodide.

(iv) Change in temperature : - The chemical reaction between quicklime and water to form slaked lime is characterised by change in temperature. (v) Change in state : The combustion reaction of candle wax is characterised by change in state from solid to liquid and gas.

Q.30 (a) What do you understand by exothermic and endothermic reactions.

(b) Give one example of an exothermic reaction and endothermic reaction.

(c) Find out endothermic & exothermic reactions from the following ?

(i) Respiration

(ii) Photosynthesis

(iii) Burning of natural gas

(iv) Digestion of food

(a) Exothermic reactions: Those reactions in which heat is evolved are known as exothermic reactions.

Endothermic reactions: Those reactions in which heat is absorbed are known as endothermic reactions.

(b) Example of exothermic reaction : When natural gas burns in the oxygen of air, it forms CO2 and water vapour and large amount of heat is evolved.

$C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O + Heat$

Example of endothermic reaction : When nitrogen and oxygen are heated to a very high temperature, they combine to form nitrogen monoxide.

${N_2} + {O_2} + Heat \to 2NO$

(c) Exothermic reactions : -

(i) Respiration

(ii) Burning of natural gas

(iii) Digestion of food

Endothermic reactions

(iv) Photosynthesis

Q.31 (a) What happens when an  aqueous solution of sodium of sodium sulphate reacts with an aqueous solution of barium chloride ?

(b) Write the balanced chemical equation for the reaction which takes place.

(c) State the physical conditions of reactants in which the reaction will not take place.

(d) Name the type of chemical reaction which occurs ?

(a) When an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride, exchange of ions between both the reactants takes place and precipitate of barium sulphate forms. It is a precipitation reaction

(b) The balanced chemical reaction, which takes place is as : -

$N{a_2}S{O_4} + BaC{l_2} \to BaS{O_4} + 2NaCl$

(c) Reaction will not takes place . If physical state of reactants are solid state

(d) Double displacement reaction takes place

Types of Chemical Reaction

Q.32 What type of chemical reactions is represented by this equation ?

$A + BC \to ACB$

Displacement Reaction.

Q.33 Which of the following is a combination reaction and which is a displacement reaction ?

(a) $C{l_2} + 2KI \to 2KCl + {I_2}$

(b) $2K + C{l_2} \to 2KCl$

(a) Displacement reaction

(b) Combination reaction

Q.34 Identify the component oxidised in the following reaction : -

${H_2}S + C{l_2} \to S + 2HCl$

In the above reaction, H2S is oxidised to S.

Q.35 What type of chemical reactions are represented by the following equations ?

(a) $A + B \to C$

(b) $X \to Y + Z$

(a) Combination reaction

(b) Decomposition reaction

Q.36 What type of chemical reaction take place when

(a) lime-stone is heated

(b) a magnesium wire is burnt in air ?

(a) Decomposition reaction by the action of heat.

(b) Combination reaction.

Q.37 Name the type of reaction for the following:-

(i) Vegetable matter changing into compost

(ii) Burning of natural gas

(iii) Silver items turning black when left exposed to air.

(iv) Change in colour of copper sulphate solution for blue and green on dipping a nail in it?

(i) Decomposition reaction

(ii) Oxidation reaction

(iii) Oxidation reaction

(iv) Displacement reaction

Q.38 Identify the type of each of the following reactions:

(a) A reaction in which a single product is formed from two or more reactants.

(b) The reaction mixture becomes warm

(c) An insoluble substance is formed

(d) External surface of the container in which reaction takes place becomes freezing cold?

(a) Combination reaction

(b) Exothermic reaction

(c) Precipitation reaction

(d) Endothermic reaction

Q.39 What type of reaction is represented by digestion of food in our body?

Decomposition Reaction is represented by digestion of food in our body. In this reaction, glucose undergoes combustion process and combines with oxygen to produce carbon-dioxide and water with evolution of heat. Q.40 Name the various types of chemical reactions?

The various types of chemical reactions are as follows:

1- Combination reaction

2- Decomposition reaction

3- Displacement reaction

4- Double displacement reaction

5- Oxidation reaction

6- Reduction reaction.

Q.41 Why does the colour of copper sulphate solution change when an iron nail is kept immersed in it?

The displacement reaction occurs between copper sulphate and iron to produce ferrous sulphate solution, which is green in colour, That’s why colour of copper sulphate changes when iron nail is immersed in it. Q.42 What are exothermic and endothermic reactions? Give one examples of each?

Exothermic reaction:-The chemical reaction in which heat is released is called an exothermic reaction.

$e.g.,C\left( s \right) + {O_2}\left( g \right) \to C{O_2}\left( g \right) + Heat$

Endothermic reaction:- The  chemical reaction in which heat is absorbed is called an endothermic reaction. Q.43 State an important use of decomposition reactions?

An important use of decomposition reaction is digestion of food in our body. In this reaction, glucose undergoes combustion process and combines with oxygen to produce carbon-dioxide and water with evolution of heat. Q.44 What type of chemical reaction is used to extract metals from their naturally occurring compounds like oxides and chlorides?

Decomposition reaction, Which is carried out by electricity is used to extract metals from their naturallly occuring compounds like oxides or chlorides.

Q.45 An aluminium can is used to store ferrous sulphate solution. It is observed that in few days, holes appeared in the can. Explain the observation and write chemical equation to support your answer ?

Due to displacement reaction holes appeared in the can because aluminium is more reactive then iron. Hence aluminium sulphate is formed and iron is precipated out.

Equation : - $3FeS{O_4} + 2Al \to A{l_2}{\left( {S{O_4}} \right)_3} + 3Fe$

Q.46 In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the chemical equation of the reaction involved?

Here the more electropositive Copper displaces lesser electropositive silver. The chemical reaction involved is as:

$Cu\left( s \right) + 2AgN{O_3}\left( {aq} \right) \to 2Ag\left( s \right) \downarrow + Cu{\left( {N{O_3}} \right)_2}\left( {aq} \right)$

Q.47 What type of reactions are represented by the following equations ?

(i) $CaC{O_3} \to CaO + C{O_2}$

(ii) $CaO + {H_2}O \to Ca{\left( {OH} \right)_2}$

(iii) $2FeS{O_4} \to F{e_2}{O_3} + S{O_2} + S{O_3}$

(iv) $N{H_4}Cl \to N{H_3} + HCl$

(i) Decomposition reaction

(ii) Combination reaction

(iii) Decomposition reaction

(iv) Decomposition reaction

Q.48 What type of chemical reactions take place when :

(a) A magnesium wire is burnt in air ?

(b) Lime stone is heated ?

(c) Silver bromide is exposed to sunlight ?

(d) Electricity is passed through water ?

(a) Combination reaction

(b) Decomposition reaction by action of heat

(c) Decomposition reaction by the action of light

(d) Decomposition reaction by the action of electricity

Q.49 Which of the following is a combination and which is a displacement reaction ?

(a) $C{l_2} + 2KI \to 2KCl + {I_2}$

(b) $2K + C{l_2} \to 2KCl$

(a) Displacement reaction

(b) Combination reaction

Q.50 In the following reaction between lead sulphide and hydrogen peroxide.

$PbS\left( s \right) + 4{H_2}{O_2}\left( {aq} \right) \to PbS{O_4}\left( s \right) + 4{H_2}O\left( l \right)$

(a) Which substance is reduced

(b) Which substance is oxidized ?

(a) Hydrogen peroxide (H2O2) is reduced

(b) Lead sulphide (PbS) is oxidised.

Q.51 Identify the component oxidised in the following reaction :

${H_2}S + C{l_2} \to S + 2HCl$

In this reaction, H2S is oxidised to S.

Q.52 When SO2 gas is passed through saturated solution of H2S, the following reaction occurs.

$S{O_2} + 2{H_2}S \to 2{H_2}O + 3S$

In this reaction, which substance is oxidised and which one is reduced ?

In the given reaction, SO2 is reduced to S, it loses oxygen, whereas H2S which gains oxygen gets oxidised to H2O.

Q.53 What is decomposition reaction ? Explain it with suitable example ?

A reaction in which a single reactant decomposes to give simple products is known as decomposition reaction. Q.54 Why are decomposition reactions called the opposite of combination reaction ? Write equations for these reactions ?

In a combination reaction, two substances combine to form one compound while in a decomposition reaction, a compound breaks down into two or more substances. So, these two reactions are opposite to each other.

E.g. : - Combination reaction : - $C + {O_2} \to C{O_2}$

Decomposition reaction : - $CaC{O_3} \to CaO + C{O_2}$

Q.55 When a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube : -

(a) What is observed in the above reaction ?

(b) What type of reaction in this ?

(c) Write a balanced chemical equation to represent the above reaction.

(a) On adding a solution of potassium iodide to a solution of lead nitrate, it is observed that yellow precipitate of lead iodide (PbI2) is formed in the test tube.

(b) This is a double displacement or precipitaton reaction.

(c) $Pb{\left( {N{O_3}} \right)_2}\left( {aq} \right) + 2KI\left( {aq} \right) \to Pb{I_2}\left( s \right) + 2KN{O_3}\left( {aq} \right)$  (Yellow ppt.)

Q.56 Identify the type of chemical reaction taking place in each of the following and write balanced chemical equation.

(a) Barium chloride solution is mixed with copper sulphate solution and a white precipitate is observed.

(b) On heating copper power in air in a china dish, the surface of copper powder turns black ?

(a) $CuS{O_4}\left( {aq} \right) + BaC{l_2}\left( {aq} \right) \to BaS{O_4} + CuC{l_2}$

It is a double displacement or precipitation reaction. It is an oxidation reaction.

Q.57 Give three differences between displacement and double displacement reactions ? Q.58 Write one example of a  decomposition reaction which is carried out

(a) With electricity

(b) By applying heat ?

Examples of the decomposition reaction :-

(a) with electricity

$2{H_2}O \to 2{H_2} + O$ (Electrolysis of water)

(b) by applying heat :

$CaC{O_3}\left( s \right) \to CaO\left( s \right) + C{O_2}\left( g \right)$

Q.59 What type of reactions are represented by the following equations ?

(a) $CaO + C{O_2} \to CaC{O_3}$

(b) $2Na + 2{H_2}O \to 2NaOH + {H_2}$

(c) $Mg + CuS{O_4} \to MgS{O_4} + Cu$

(a) Combination reaction

(b) Displacement reaction

(c) Displacement reaction

Q.60 Which of the following is a combination and which is a displacement reaction ?

(a) $C{l_2} + 2KI \to 2KCl + {I_2}$

(b) $2K + C{l_2} \to 2KCl$

(a) Displacement reaction

(b) Combination reaction

Q.61 Fill in the following blanks with suitable words ?

(a) The addition of oxygen to a substance is called ______ whereas removal of oxygen is called    ____

(b) The addition of hydrogen to a substance is called ______ whereas removal of hydrogen is called ______.

(c) Anti – oxidants are often added to fat containing foods to prevent ________ due to oxidation.

(a) Oxidation, reduction

(b) Reduction, oxidation

(c) Rancidity

Q.62 (a) Define a combination reaction

(b) Give one example of a combination reaction which is also exothermic

(c) Give one example of a combination reaction which is also endothermic ?

(a) Combination reactions are those in which a single product is formed form two or more reactions

(b) Carbon burns in air to form carbon dioxide.

$C\left( s \right) + {O_2} \to C{O_2} + Heat$

(c) Nitrogen burns in air to form nitric oxide.

$Heat + {N_2}\left( g \right) + {O_2}\left( g \right) \to 2NO\left( g \right)$

Heat is absorbed, So, it is a endothermic reaction.

Q.63 (a) Balance the chemical equation : -

$Fe\left( s \right) + {H_2}O\left( g \right) \to F{e_3}{O_4}\left( s \right) + {H_2}\left( g \right)$

(b) Identify the type of reaction in the equation given below :

$N{a_2}S{O_4}\left( {aq} \right) + BaC{l_2}\left( {aq} \right) \to BaS{O_4} + NaCl\left( {aq} \right)$

(c) You should have noted that when copper powder is heated in a china dish, the surface of copper powder becomes coated with black colour substance.

(i) Why has this black coloured substance formed ?

(ii) What is this black substance ?

(iii) Write the chemical equation of the reaction that takes place ?

(a) $3Fe + 4{H_2}O \to F{e_3}{O_4} + 4{H_2}$

(b) Double displacement reaction

(c) (i) Black coloured substance is copper oxide. It is formed because oxygen is added to copper.

(ii) Copper (II) oxide (CuO). Harmful Effects of Oxidation

Q.64 Explain the term rancidity?

The process of oxidizing fatty and oily substances is called as rancidity.

Q.65 How can we prevent fried food from turning rancid?

Fried food can be prevented from turning rancid by packaging them in air tight containers or packages.

Q.66 To prevent rancidity of foods containing fats and oils, some substances are added to them.

What are these substances?

Antioxidants are added to prevent rancidity of foods containing fats and oils. They prevent oxidation of fats and oils.

Q.67 Why do silver ornaments lose their shine when kept for some time?

Silver ornaments get tarnished by reacting with atmospheric gases to form silver sulphide.

Q.68 (a)List  any two changes which takes place when oily foods gets oxidized?

(b) Mention a measure which prevents or slows down its oxidation?

(b) Antioxidants are added to prevent or slow down oxidation.

Q.69 An Aluminium can is used to store ferrous sulphate solution. it is observed that in few days, holes appeared in the can. Explain the observation and write chemical equation to support your answer?

Displacement reaction

Reason: Due to displacement reaction holes appeared in the can because aluminium is more reactive than iron. Hence Aluminium sulphate is formed and iron is precipitated out, which results into the appearance of holes in the can.

Equation : - $3FeS{O_4} + 2Al \to Al{\left( {S{O_4}} \right)_3} + 3Fe$

Q.70 Which term is used to indicate the development of unpleasant smell and taste in fat and oil containing foods due to aerial oxidation (when they are kept exposed for a considerable time)?

Rancidity is the term used to indicate the development of unpleasant smell and taste in fat and oil containing foods due to aerial oxidation (when they are kept exposed for a considerable time).

Q.71 What are anti-oxidants? Why are they added to fat and oil containing foods?

Antioxidants are the general name of chemicals that are added to foods containing fats and oils to prevent rancidity which is responsible for bad taste and bad smell in food. They are reducing agents that prevent oxidation of fats and oils.

Q.72 Name two anti-oxidants which are usually added to fat and oil containing foods to prevent rancidity?

The two anti-oxidants which are usually added to fat and oil containing foods to prevent rancidity are:-

1-Butylated hydroxyanisole (BHA)

2-Butylated hydroxytoluene(BHT)

Q.73 You must have tasted or smelt the fat containing food materials left for a long time. Such foods taste and smell bad. What is the reason for this and the name given to the phenomenon ? List two measures taken at home to prevent it ?

It happens due to oxidation of fatty matters contained in the food and this phenomenon is known as rancidity measures of prevention : -

(1) Keeping such food materials in air tight containers.

(2) Refrigeration of such food materials.

(3) Flushing inactive gases such as nitrogen.

Q.74 List three effects of oxidation in air daily life ? Are these effects useful or harmful ? Justify ?

(i) Corrosion : It is  harmful, as it loses shine of some metals such as copper and silver. Corrosion of iron called rusting is a serious problem as enormous amount of money is spent to replace damaged iron.

(ii) Rancidity : - It is also harmful, because when fats and oil are oxidised, they becoe rancid and their smell and taste change.

(iii) Burning of Fuels : - It is useful as these are exothermic reactions, large amount of heat energy is evolved.

Q.75 (a) Potato chips manufactures fill the packet of chips with nitrogen gas. Why ?

(b) Why do we apply point on iron articles ?

(a) Nitrogen is an inert gas. It prevents chips from rancidity or oxidation. Thus its taste remain intact.

(b) Iron articles are painted to prevent them from getting rusted. Coating is done to prevent it from coming in the contact of moist air, which is the cause of corrosion.

Q.76 (a) Explain the term “corrosion“ with an  example. Write a chemical equation to show the process of corrosion of iron ?

(b) What special name is given to the corrosion of iron ?

(c) What type of chemical reactions is involved in the corrosion of iron ?

(d) Name any three objects which are gradually damaged by the corrosion of iron and steel ?

(a) Corrosion is the process in which metals degrades gradually by the action of air, moisture or a chemical on their surface for examples, red – brown coating on iron

Process of Corrosion : -

(i) Iron metal is oxidesed  by the oxygen   of air in the presence of moisture : -

Iron oxygen water hydrated iron oxide

(b) Corrosion of iron is named as

Rusting

(c)  Redox reaction is involved in the corrosion of iron.

(d) Following are three objects that gradually degrades by corrosion of iron & steel : -

(i) Iron railings

(ii) Iron nails

(iii) Ships

Q.77 (a) Explain the term “ rancidity “. What damage is caused by rancidity ?

(b) What type of chemical reaction is responsible for causing rancidity ?

(c) State and explain the various methods for preventing rancidity of food ?

(a) Rancidity : The condition produced by aerial oxidation of fats and oils in food marked by unpleasant smell and taste is called rancidity
It damages food items due to the development of unpleasant small , generally in fatty and oily foods.

(b) Oxidation reaction is responsible for causing rancidity.

(c) We can apply following methods for preventing rancidity of food : -

(i) use of antioxidants

(ii) use of nitrogen gas, it is an unreactive gas which prevents oxidation of fat and oil.

(iii) use of airtight containers which reduces, exposure to oxygen and slows down the oxidation process.

Value Based Questions : -

Q.1 Manoj was asked whether respiration is an exothermic reaction or not. He replied that no, respiration is not exothermic giving the reason that in respiration carbondioxide  is released instead of energy. His classmate shabana told him how respiration is an exothermic reaction Manoj thanked shabana  for this.

(i) Write the explanation of shabana

(ii) What value does shabana show ?

(i) In respiration, glucose combines with oxygen in the cells of our body and provides energy .

So, respiration is an exothermic reaction

(ii) Value : Co – operative learning among students promoting self – confidence among students

${C_6}{H_{12}}{O_6} + 6{O_2} \to 6C{O_2} + 6{H_2}O + Energy$

Q.2 When the solution of substance X is added to a solution of potassium iodide, then a yellow solid separates out from the solution ?

(a) What do you think substance X is likely to be ?

(b) Name the substance which the Yellow solid consists of.

(c) Which characteristics of chemical reaction is illustrated by this example ?

(d) Write a balanced chemical equation for the reaction which takes place. Mention the physical states of all the reactants and products involved in the chemical equation ?

(a) According to me substance X is likely to be lead nitrate, because on reacting with potassium iodide it forms a yellow precipital.

(b) The substance which the yellow solid consists of is lead iodide.

(c) This chemical reaction is characterized by the formation of precipitate.

(d) $2KI\left( s \right) + Pb{\left( {N{O_3}} \right)_2}\left( {aq} \right) \to Pb{I_2}\left( s \right) + 2KN{O_3}\left( {aq} \right)$

Q.3 A metal salt MX when exposed to light splits up to form metal M and a gas X2. Metals M is used in making ornaments whereas gas X2 is used in making bleaching powder. The salt MX is itself used in black and white photography.

(a) What do you think metal M is ?

(b) What could be gas X2 ?

(c) Name the metal salt MX ?

(d) Name any two salt solutions which on mixing together can produce a precipitate of salt MX.

(e) What type of chemical reaction takes place when salt MX is exposed to light ? Write the equation of the reaction ?

(a) Metal M is silver (Ag)

(b) Gas X2 is chlorine (Cl2)

(c) Metal salt MX is AgCl

(d) Solution of sodium chloride and silver nitrate on mixing together can produce a precipitate of salt MX.

(e) Decomposition reaction by light takes place.

$2AgCl\left( s \right) \to 2Ag\left( s \right) + C{l_2}(g)$

Q.4 When a green iron salt is heated strongly, its colour finally changes to brown and colour of burning Sulphur is given out :

(a) Name the Iron salt

(b) Name the type of reaction that takes place during the heating of Iron salt.

(c) Write a chemical equation for the reaction involved.

(a) The iron salt is ferrous sulphate crystal

(b) Decomposition reaction takes place during the heating of iron salt.

(c) The chemical reaction involved is : -

$2FeS{O_4} \to F{e_2}{O_3} + S{O_2} + S{O_3}$

Q.5 When a strip of red-brown metal X is placed in a colourless salt solution an YNO3 then metal Y is set free and a blue coloured salt solution X(NO3)2 is formed. The liberated metal Y forms a shining white deposit on the strip of metal X.

(a) What do you think metal X is ?

(b) Name the salt YNO3 ?

(c) What could be metal Y ?

(d) Name the salt X (NO3)2 ?

(e) What type of reaction takes place between metal X and salt solution YNO3 ?

(a) I think metal X is copper (Cu).

(b) Salt YNO3 is AgNO3.

(c) Metal Y is silver (Ag)

(d) The salt X(NO3)2 is Cu(NO3)2.

(e) Displacement reaction takes place between metal X and salt solution YNO3.

Q.6 Shilpa was very happy to know that her maternal uncle along with  his family have been arriving to her house. Her mother cooked a lot of delicious food for them. After sometime her maternal uncle rang up and told, they postponed  the programme for an urgent work, Her mother got worried to know this. She the thinks that the food items will spoil shilpa helps her mother to prevent the cooked food from getting spoil.

(i) State the reason for food item becoming spoil when kept for a very long time.

(ii) What method did the shilpa suggest her mother to preserve the food ?

(iii) What value does shilpa show ?

(i) When food items are kept for a very long time, they get oxidised and become rancid.

(ii) Shilpa suggests her mother to keep the oily food in air tight container

(iii) Shilpa is an intelligent girl .She applies her knowledge and intelligence to overcome the problem that comes suddenly.

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