Class 10 ncert text book question answers of chapter first

Q1.Which of the statement abut 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

ANS :- (a) and (b)
O2:- Fe2 O3 + 2Al -----> Al2 O3+2Fe
     The above reaction is an example of a :
(a) combination reaction

(b) double displacement reaction

(c) decomposition reaction

(d) displacement reaction

ANS:- (d) displacement reaction
Q3-- what happen when dilute hydrochloric acid added to iron fillings ? Ticj the correct answer

(a) hydrogen gas and iron chloride are produced

(b) chlorine gas and iron hydroxide are produced

(c) no reaction are takes place

(D) iron salt and water are produced

ANS:--- (a) hydrogen gas and iron chloride are produced
Q4--- what is a balanced chemical equation ? Why should chemical equation be balanced ?

ANS---> (I) balanced chemical equation

The chemical equation in which the number of atoms of different elements are equal in reactant side and product side
According to law of conservation of mass the total mass of products must be equal to the total mass of reactants 
So this could be possible only if the number of atoms if any element on reactant side of an equation are equal to the number of atoms of the same element on the product side
Q5. Translate the following statements into chemical equation and then balance them :

(a) hydrogen gas combined with nitrogen to form ammonia

(B) hydrogen sulphide gas burns in air to give water and sulphur dioxide

(c) barium chloride reacts with aluminum 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) 3H2 (g) + N2(g) ----->


(B) 2h2S(g) + 3O2(g) ------> 2h2O(l) + 2so2(g)

(C) 3BaCi2(s) + AL2 (so4) 3 -----> 2Alcl3 (AQ) + 3BaSo4

(D) 2K(s)+2H2O(l) -------> 2koh(AQ) + H2(g)
Q6---> balance the following chemical equations:

(a) HNO3 + Ca(OH)2 -----> ca(No3)2+ h2o

(B) NaOH+ H2So4------>Na2so4+ h2o

(C) NaCI + AgNo3-----> AgCi+ NaNo3

(D) Bacl2+ H2so4 ------> Baso4 + HcI

Ans----> (a) 2HNO3 + Can(OH)2 -----> can(No3)2 + 2H2O

(B) 2NaoH + Agno3 -----> AgCI + NaNo3

(D) BacI2 + H2SO4 ------> baso4 + 2HCI
O7-> write a balanced chemical equation for the following chemical reaction :

(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 .

ANS--- (a) Ca(OH)2 + Co2 -----> co2 ----> CaCo3 + H2O

(B) Zn + 2AgNo3 ------> Zn(No3)2 + 2Ag

(C) 2AI + 3CuCI2 -------> 2AICI3 + 3CU

(D) BacI2 + k2sO4 -----> baSo4 + 2kCI
Q8-> write the balanced chemical equation for the following and identify the type of reaction in each case

(a) ---- pattassium bromide(AQ) + barium iodide(AQ) ------> potassium iodide (AQ) + barium bromide

(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)


(A) 2kbr (AQ) + Bal2(AQ) ----> 2Ki(AQ)+babr2 (AQ)
                  (Double displacement reaction)

(B) ZnCo3 (s) ----> Zno(s) + Co2(g)
                           (Decomposition reaction )

(C) H2(g) + Cl2 (g) ------> 2HCI (g)           
                                (Combination reaction)

(D) Mg(s) + 2HcI (AQ) ----> Mgci2(AQ) + H2(g)
                              (Displacement reaction )
Q9--> what does one mean by exothermic and endothermic reaction ?

Ans--->[1] EXOTHERMIC REACTION----> the reaction in which heat is given out are called exothermic reaction  energy of reactants > energy of products  .

Examples :
(I) C6H12O6(s)+ 6O2(g)----> Co2+ 2H2o + heat

(II) CH4(s) + 2O2(g) -----> CO2+ 2H2O + Heat

(III) C(S) + O2(g) ----> Co2 (g) + heat

ENDOTHERMIC REACTION----> the reaction in which heat energy is absorbed  is called endothermic reaction
Energy of the reactants < energy of the products


(I) 2HgO (s) + heat ------> 2hg(l) + O2(g)

(II) N2(g) + O2(g) + heat ----> 2NO(g)

(III) c(s)+h2O(l)+heat----> CO(g) + H2(g)
Q10--> why is respiration considered an exothermic reaction ?

Ans---> the food that we eat like rice , potato, bread , etc contains carbohydrates . on digestion carbohydrates converted  to glucose during respiration the air we breath in oxidizes the glucose into carbon dioxide , water and heat  which provides our body sufficient energy .
Q11---> why are decomposition reaction called opposite of combination reaction ? Write equations for these reaction

Ans---> in decomposition reaction one substance splites to give two or more simpler substances whereas in a combination two or more substances combine from a single substance due to this reason decomposition reaction opposite to the combination reaction

EXAMPLE OF---> combination reaction

(I) CaCO3(s) ------> CaO(s) + CaO(s) + Co2

(II) 2feSO4 (s) ------> Fe2O3 (s) + So2 (g) + So3 (g)

EXAMPLE OF ----> decomposition reaction

(I) CaO(s)+ Co2 (g) -----> CaCo3 (s)

(II) 2H2(g) + O2 (g) ----> 2h2 O(l)
Q12---> write one equation each for decomposition reaction where energy is supplied in the form of heat , light or electricity.             

(1)CaCO3 (s) --------> CaO(s) + Co2(g)

(II) 2AgBr(s) -------> 2Ag (s) + Br2(g)

(III)2h2O(l)-----> 2H2(g)+ O2(g)
Q13---> what is the difference between displacement reaction and double displacement reaction write equations for these reactions


displacement reaction -> in single displacement reaction more active element displaces less active element from its salt solution


(I) CuSo4 (aQ)+ Fe(s) -----> feso4 (AQ) + cu(s)

(II) cuso4 (AQ) + Zn(s) ----> znso4 (AQ) + cu(s)

Double displacement reaction-----> in the double displacement reaction the solution of two ionic compounds react and there is a mutual exchange of ions


(I) AgNo3 (AQ) + NaCI (AQ) ------> AgCi (s) + NaNo3(AQ)

(II) 2AgNo3 (AQ) + cuci2(AQ) ----> 2agCi + Cu+ cu( No3) 2
Q14----> in the refining of silver the recovery of silver form silver nitrate solution involves displacement by copper metal write down the reaction involved .

Ans----> cu(s) + 2AgNo3(AQ) -----> cu(No3)2(AQ) + 2Ag(s)
Q15---> what do you mean by precipitation reaction explain by giving examples
Precipitation reaction are those reactions in which reactants reacts to form a product which is insoluble and this product separates out in the form of a precipitate

(I) Agno3(AQ) +NaCi (s) + NaNO3(AQ)

(II) Na2SO4(aq) + BacI 2 (AQ) ----> Baso4 (s) + 2NaCi ( AQ )
Q16---> Explain the following in terms of gain or loss of oxygen with two examples each :

(a) Oxidation ---> it is a process which involves gain of oxygen

(B) Reduction---> it is a process which involves loss of oxygen


(I) zno(s) + c(s) -----> Zn(s) +co(g)

(ii) pbO+c-----> pb+ CO
Q17---> a shiny brown coloured element 'X' on heating in air becomes black in colour name the element 'X' and the black coloured compound formed ??

Ans---> Copper is a shiny brown coloured element 'X' which on heating on air forms bleck copper oxide

Example :
2Cu(s)+ O2(g) -------> 2Cuo(s)
Q18---> why do we apply paint on iron articles
Ans---> by applying paint on iron articles they can be prevented from corrosion since paint does not allow oxygen and water to come in contact with the surface of iron

Q19----> oil and fats containing food items are flushed with nitrogen . why ?
Ans---> the microorganisms present in the oxygen of the air oxidise the fat and oil in food items to make such compunds which have bad smell and taste thus they become rancid flushing with nitrogen cutts off oxygen which protects the food from rancidity

When food kept in lower teamprature then the activity of microorganisms  gets slowed down and thus oxidation of food much slower
Q20---> Explain the following terms with an example
(A) corrosion (b) Rancidity

Ans---> (a) corrosion ----> the process of slowly eating away of the metals by the action of atmospheric gases on the surface of the metals into its compounds is called corrosion

(I) corrosion of iron is called rusting

(II) copper object acquire a green coating of basic copper carbonate

(iii) silver metals get tarnished on exposure to air because of the presence of hydrogen sulphide .

(b) Rancidity---> the microorganisms oxide the fats and oils present in them and give some unpleasant smell and taste or make food rancid
      The oxidation of oils or fats in a food resulting into a bad smell and bad taste is called rancidity
Example A food containing oil and fat if left for a long time develops bad taste and bad smells and get spoiled

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