Answer the following questions :

1.  What is the insoluble substance formed in a chemical reaction called ?
Ans :  Precipitate is the insoluble substance formed in a chemical reaction .

2.  Name the gas evolved when lead nitrate is heated .
Ans :  Nitrogen dioxide and oxygen
are the gas evolved when lead nitrate is heated .

3.  Give one use of quick lime .
Ans :  It is used in the manufacture of cement .

4.  What is rancidity ? What is the general name of chemicals which are added to fat and oil containing food so as to prevent the rancidity ?
Ans :  The oil and fat containing food when left exposed to air reacts with oxygen and gets oxidized and become rancid , this process is called rancidity . In this process their smell and taste changes . The general name of the chemicals that are added to prevent this oxidation are called as antioxidants                                              eg:  nitrogen is an antioxidant.

5.  Why is photosynthesis considered as endothermic reaction ?
Ans :  Photosynthesis is a reaction in which energy is required to form glucose from carbon-dioxide and water. Energy in the form of sunlight is required to break the bonds of hydrogen and oxygen. Therefore it is known as endothermic reaction .

6.  On what basis is a chemical equation balanced ?
Ans :  The number of atoms on left hand side of the reaction (i.e., reactants) is equal to the number of atoms on the right hand side of the equation (i.e., product). Law of conservation of mass:Mass of atoms in chemical reaction can neither be created nor destroyed , it remains the same .  

7.  Give an example of photochemical reaction .
Ans :  In black and white photography AgBr gets decomposed due to light and forms Silver metal and Bromine gas .

8.  Why is respiration considered as an exothermic reaction ?
Ans :  Respiration is considered as an exothermic reaction because heat is evolved during respiration .

9.  State one advantage and one disadvantage of corrosion .
Ans :  Advantage – In some metals protective layer is formed in its surface to prevent it from further corrosion .The layer prevents further corrosion 
E.g. Aluminium (Al) forms a layer of Aluminium oxide (Al2O3) by corrosion .  Disadvantage Loss of metal .

10.  What is meant by a chemical reaction ?
Ans :  The reaction representing a chemical change is called a chemical reaction .

11.  Name the type of reaction seen in the following cases :
i) Garbage producing a foul smell . ii) Black and white photograph film when exposed to sunlight . iii) Carbon dioxide gas passed through lime water .
Ans : 
i) Decomposition reaction . ii) Decomposition reaction . iii) Combination reaction.

12.  Explain and name the type of reaction seen when iron reacts with hydrochloric acid.
Ans: Iron reacts with HCl to form hydrogen gas and iron chloride. This reaction is called as displacement reaction.

13.  What is electrolytic decomposition? Give two uses of electrolytic decomposition reaction .
Ans: When electricity is passed through a molten compound which is ionic in nature, then the ions of the compound separate into its component thereby decomposing the compound. It is used in separating hydrogen and oxygen gas from water. It is also used to decompose sodium chloride to sodium metal and chlorine gas.

14.  What is a redox reaction? When a magnesium ribbon burns in air with a dazzling flame and forms a white ash, is magnesium oxidisied or reduced? Why ?
Ans: The reaction which shows both oxidation and reduction reaction in it is called redox reaction. 2Mg + O2 → 2MgO  In this case, magnesium is oxidised as oxygen combines with it.

15.  What happens chemically when quick lime is added to water ?
Ans: Quick lime is CaO. On adding water to it, combination reaction takes place to give calcium hydroxide with release of large amount of heat with a hissing sound. CaO + H2O → Ca(OH)2  + Δ

16.  Identify the type of reaction in the following example . Na2SO4(aq) + BaCl2 (aq) → BaSO4(s) + 2NaCl(aq)
Ans: It is double displacement reaction.

17.  Balance the following chemical reaction . Pb(NO3)2(s) → heat → PbO(s) + NO2(g) + O2(g)
Ans: 2Pb(NO3)2 → heat → PbO(s) + 4NO2(g) + O(g)

18.  Name a reducing agent that may be used to obtain manganese from manganese dioxide.
Ans: Coke and carbon is used as reducing agent.

19.  (a) A solution of substance ‘X’ is used for white washing. What is the substance ‘X’ ?State the chemical reaction of ‘X’ with water.
Ans: ‘X’ is Calcium oxide (CaO). CaO(s) + H2O(l) → Ca(OH)2(aq) + heat.

(b) Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
Ans: It  is because iron displaces copper from CuSO to form FeSO which is pale green. Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)Blue Pale green .

20.  Write the balanced equation for the following reaction and identify the type of reaction in each case.
(i) Potassium bromide + Barium iodide → Potassium iodide + Barium bromide. (ii) Hydrogen(g) + Chloride(g) → Hydrogen chloride(g)
Ans: (i) 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(aq) It is double displacement reaction. (ii) H2(g) + Cl2(g) → 2HCl(g) It is a combination reaction.

21.  A white salt on heating decomposes to give brown fumes and a residue is left behind .
(i) Name the salt.
(ii) Write the equation for the decomposition reaction.
Ans: (i) Lead nitrate is white salt. (ii) 2Pb(NO3)2(s) → heat → 2PbO(s) + 4NO2(g) + O2(g) 

22.  When a solution of potassium iodide is added to a solution of lead nitrate in a test tube, a reaction take place. (a) What type of reaction takes place ? (b) Write a balanced chemical equation equation to represent the above reaction.
Ans: (a) Double displacement as well as precipitation reaction. (b) Pb(NO3)2(aq) + 2KI(as) → PbI2(s) + 2KNO3(aq) Yellow .

23.  You might have noted that when copper powder is heated in a china dish, the surface of copper powder becomes coated with a black colour substance. (i) How has this black coloured substance is formed ? (ii) What is that black substance ? (iii) Write the chemical equation of the reaction that takes place. Ans:  (i) Copper reacts with oxygen to form copper oxide which is black (oxidation of copper takes place). (ii) Copper oxide. (iii) 2Cu(s) + O2(g) → 2CuO (s) .

24.  Write the balanced chemical equations for the following reactions and identify the type of reaction in each case. Thermite reaction, iron(III) oxide reacts with aluminium and gives molten iron and aluminium oxide.
Ans: Fe2O3(s) + 2Al(s) → Al2O3(s) + 2Fe(l) + heat. It is a displacement reaction because aluminium is more reactive then iron and displaces it. The iron melted in here is used in joining the railway tracks.

25.  Distinguish between a displacement reaction and double displacement reaction.

        Displacement reaction    Double displacement reaction
The reaction in which more reactive metal displaces less reactive metal. The reaction in which two different atoms or group of atoms exchange for each other.

26.  Consider the following chemical reaction. X + Barium chloride gives y + sodium chloride (i) Identify X and y (ii) Type of reaction
Ans: (i) ‘X’ is NaSO4 and ‘Y’ is BaSO4. (ii) The type of reaction Na2SO4+ BaCl2→ BaSO4 + 2NaCl The reaction is a precipitation reaction. It is also a double displacement reaction.

27.  We need to balance a skeletal chemical equation. Give reason to justify the statement.
Ans: Skeletal chemical equations are unbalanced. We need to balance chemical equation because of law of conservation of mass. It states that matter can neither be created nor be destroyed. Therefore each and every atom involved in chemical equation must be balanced.