Q1. A solution of slaked lime produced by the reaction is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO3.

On the basis of above paragraph answer the following questions:

1.) Give the reaction for the formation of calcium carbonate with physical states.

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

2.) Write the formulas of slaked lime, quick lime.

 Slaked lime: Calcium hydroxide is called as slaked lime with a formula of : Ca(OH)2  ,whereas calcium oxide is called as quick lime with a formula of : Ca0.

3.) Explain why calcium carbonate is used for white washing and not any other substance.

 Calcium carbonate is used for whitewashing as it produces a shiny film whilst the production of carbon dioxide and act as hard coating for the walls.

4.) Explain the importance of writing the physical states in a chemical equation.

 In any chemical reaction, physical states mention the nature of the reaction and their practical aspects which are necessary for lab uses.Physical states also explains whether reaction is exothermic or endothermic.

5.) Write any one application of calcium carbonate other than white washing.

Calcium carbonate is also used in the production of antacids and can also be used to increase the levels of calcium in body.

Q2. You must have observed that iron articles are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron. Some other metals also get tarnished in this manner. Have you noticed the colour of the coating formed on copper and silver? When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. The black coating on silver and the green coating on copper are other examples of corrosion.

On the basis of above paragraph answer the following questions:

1.) Explain one benefit of corrosion.

Ans. Corrosion occurring in red blood cells of iron makes the colour of cells red which is highly useful in transportation of oxygen.

2.) Write the formula of corrosion of iron.

Ans. The general formula of corrosion of iron is Fe2 O3.

3.) If corrosion occurs in the case of iron articles, why is the iron pillar at Qutub Minar not effected?

Ans. The iron pillar at Qutub Minar is an ancient piece of metal knowledge in which the metal is very very pure( wrought iron) with low sulphur content and high phosphorus making it resistive to the outer environment.

4.) Write any two ways to prevent rusting.

Ans. The two ways to prevent rusting are:

A) Galvanization.

B) Painting of iron articles.

5.) What doe you mean by galvanization?

Ans. Galvanization is the process of applying a protective layer of Zn coating on materials which are prone to rust making it resistive and long lasting.

Q3.We have seen that the decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Reactions in which energy is absorbed are known as endothermic reactions.

1.) Write the definition of exothermic reaction.

Ans. Reactions in which heat is evolved during the process are called as exothermic reactions, such as mixing of calcium oxide with water.

2.) What do you mean endothermic reactions?

Ans. Reactions in which heat is trapped or absorbed are called as endothermic reactions, such as glucose mixed with water.

3.) Write a reaction which falls under endothermic reaction.

Ans. Melting of ice, evaporation are considered under endothermic reactions. A chemical equation for these types of reaction is:

N2  + O2  + heat → 2 NO

4.) What is decomposition reaction?

Ans. Reactions in which a substance or reactant fragments to give one or many products.

5.) Explain photolysis.

Ans. Reactions in which reactant id decomposed with the help os sunlight are called as photolytic decomposition.

Ex: 2AgCl + sunlight → 2Ag + Cl2

Q4.Oxidation is the process of gaining of oxygen, or losing of hydrogen. Reduction is the process of losing of oxygen or gaining of hydrogen. The substance which undergoes oxidation is the reducing agent while the substance which undergoes reduction is known as the oxidising agent. Oxidation and reduction always take place together and these type of reactions are known as redox reactions. Some of the examples of redox reactions are given below:

(i) Give two examples of oxidation reaction from your everyday life.

Answer: Corrosion and Rancidity

(ii) Write the oxidising agent in the reaction III and VI.

Answer: CuSO4 in (III) and CuO in (VI)

(iii) Which of the following is an oxidising agent?
(a) LiAlH4
(b) Alkaline KMnO4
(c) Acidified K2Cr2O7
(d) Both (b) and (c)

Answer: d

(iv) Out of oxidation and reduction, which reaction takes place at anode?

Answer: Oxidation takes place at anode.

Q5. A chemical reaction is a representation of chemical change in terms of symbols and formulae of reactants and products. There are various types of chemical reactions like combination, decomposition, displacement, double displacement, oxidation and reduction reactions. Reactions in which heat is released along with the formation of products are called exothermic chemical reactions. All combustion reactions are exothermic reactions.

(i) The chemical reaction in which a single substance breaks down into two or more simpler substances upon heating is known as
(a) thermal decomposition reaction
(b) photo decomposition reaction
(c) electric decomposition reaction
(d) both (a) and (c)

Answer: (a) The chemical reaction in which a single substance breaks down into two or more simpler substances upon heating is known as thermal decomposition reaction.

(ii) The massive force that pushes the rocket forward through space is generated due to the
(a) combination reaction
(b) decomposition reaction
(c) displacement reaction
(d) double displacement reaction

Answer: (b) The massive force that pushes the rocket forward through space is generated due to the decomposition reaction. Hydrogen peroxide decomposes and provides it with a considerable reaction force thrust.

(iii) A white salt on heating decomposes to give brown fumes and yellow residue is left behind. The yellow residue left is of
(a) lead nitrate
(b) nitrogen oxide
(c) lead oxide
(d) oxygen gas

Answer: (c) Lead nitrate decomposes to give brown fumes of nitrogen dioxide gas and yellow residue of lead oxide is left behind.

(iv) Which of the following reactions represents a combination reaction?
(a) CaO (s) + H2O (l) → Ca(OH)2 (aq)
(b) CaCO3 (s) → CaO (s) + CO2(g)
(c) Zn(s) + CuSO4 (aq) → ZnSO4 (aq) + Cu(s)
(d) 2FeSO4(s) → Fe2O3 (s) +SO2(g) + SO3(g)

Answer: (a) A reaction in which two or more reactants combine to form a single product is known as a combination reaction.

(v) Complete the following statements by choosing correct type of reaction for X and Y.
Statement 1: The heating of lead nitrate is an example of ‘X’ reaction.
Statement 2: The burning of magnesium is an example of ‘Y’ reaction.
(a) X- Combination, Y- Decomposition
(b) X- Decomposition, Y-Combination
(c) X- Combination, Y-Displacement
(d) X- Displacement, Y-Decomposition

Answer: (b) Heating of lead nitrate to form nitrogen dioxide and lead oxide is an example of thermal decomposition reaction and the burning of magnesium ribbon in the air to form magnesium oxide is an example of combination reaction.

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