NCERT Class 10 Science First Chapter Chemical Reactions and Equations Solution

NCERT Class 10 Science First Chapter Chemical Reactions and Equations Solution

NCERT Class 10 Science Chapter 1 Solution: Dear students Here in this post we are posted First Chapter Chemical Reactions and Equations full exercise questions & answer by our Science teacher. This Solution is helpful for CBSE English medium students. Download this 1st Chapter Science solution & prepare your homework.

Questions (Text book page no:6)

1) Why should a magnesium ribbon be cleaned before burning in air?

Ans: Magnesium get oxidised when kept in air for long time and forms a layer of magnesium oxide on it. This layer does not allow magnesium to burn properly. Hence it is to be cleaned before burning in air.

2.) Write the balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine → Hydrogen chloride

H2 + Cl2 —> 2HCl

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

3BaCl2+ Al2(SO4)3 —> 3BaSO4 (↓)  + 2AlCl3

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

2Na+ 2H2 —> 2NaOH+ H2 (↑)

(3) Write a balanced chemical equation with state symbols for the following reactions.

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

BaCl2 (aq) + NaSO4 (aq) —> BaSO4 (↓) + 2NaCl (aq)

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

NaOH (aq) + HCl (aq) —> NaCl (aq) + H2O(l)

Questions (Text book page no:10)

1.) A solution of a substance ‘X’ is used for whitewashing

(i) Name the substance ‘X’ and write its formula.

Ans: Calcium oxide is a substance used for white washing. It is also   known as Quick lime. Its formula is CaO.

(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Ans:  CaO (l) + H2O —>  Ca(OH)2 (aq)

 (Calcium oxide)        (Calcium hydroxide)

(Calcium hydroxide is known as slaked lime)


2) Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Ans: In Activity 1.7, water is electrolysed to give hydrogen gas at cathode and oxygen gas at anode.

The reaction is as follows:

2 H2O (l) —> 2 H2(g) + O2 (g)

Thus, two molecules of water on electrolysis gives two molecule of hydrogen gas and one molecule of oxygen gas. So, we can say that the amount of gas collected in one test tube is the double of the amount collected in the other.

Questions (Text book page no:13)

1.) Why does the colour of copper sulphate solution changes when an iron nail is dipped in it ?

Ans: As compared to copper, the iron has more reactivity and present in upper part or above the copper. Therefore the iron called as more reactive metal, it’ll displace the copper which is less reactive metal, so the colour of copper sulphate solution changes when an iron nail is dipped in it.

2.) Give an example of double displacement reaction other than the one given in activity 1.10.

Ans: Double displacement reaction can be defined as when there is presence of two reactants, these two reactants will share the ions between them and leads to formation of new product. The example can be taken as when there is reaction between two substrates such as lead nitrate and potassium iodide. The resultant product will potassium nitrate with yellow precipitate formation due to lead iodide.

Pb(NO3)2 (aq) + KI (aq)     —>       PbI2 (s) + KNO3 (aq)

3.) Identify the substances that are oxidized and the substances that are reduced in the following reactions.

(i) 4Na(s) + O2 (g)        —>           2Na2O (s)

(ii) CuO(s)  + H2 (g)       —>       Cu(s) + H2O (l)

Ans :

(i) In the first reaction Na will get oxidized and O2 will be reduced substance.

(ii) In the second reaction, H2 is oxidizing and copper will be oxidizing substance.


(Q1) Which of the statements about the reaction below are incorrect?

2PbO (s) + C (s) → 2Pb (s) + Co2 (g)

Ans: (i) (a) & (b).

Reason: Carbon is oxidised by formation of Co2 & PbO is reduced by formation of Pb.

(Q2) Fe2 O3+2Al → Al2O3+2Fe

The above reaction is an example of

Answer: (d) Displacement reaction.

Reason: Al displaces Fe, due to higher reactivity of Fe.

(Q3) What happens when dilute hydro chloric acid is added to iron fillings? Tick the correct answer.

Answer: (a) Hydrogen gas & iron chloride are produced

Reason: Fe(s) + dil HCL → FeCl2 + H2 (g) ↑


Fe (s) + 2HCL → FeCl2 + H2 (g) ↑

(Q4) What is balanced chemical equation? Why should chemical equations be balanced?

Answer: The total number of atoms of each element is the same on both sides of the arrow is called as balanced chemical equation.

– The chemical equations should be balanced because, according to the law of conservation of mass, mass neither be created not destroyed in a chemical reaction.

– Hence, the total mass of reactant & product in the reaction is same.

– Thus the number of atoms of each elements remains same, before & after a chemical reaction.

(Q5) Translate the following statements into the chemical equations & then balance them.

(a) Hydrogen gas combine with nitrogen to form ammonia.

Ans: H2 (g) + N2 (g) → NH3 (g)

Balanced chemical equations

3H2 (g) + N2 (g) → 2 NH3 (g)

(b) Hydrogen Sulphide gas burns in air to give water & sulfur dioxide.

Ans: H2 S (g) + O2 (g) → 2H2O (l) + SO2 (g)

Balanced chemical equation

2H2 S (g) + 3O2 (g) → 2H2O (l) + 2So2 (g)

(c) Barium chloride reacts with aluminum sulphate to give aluminum chloride & a precipitate of barium sulphate.

Ans: BaCl2 + Al2 (SO4)3 → 2AlCl3 + 3BaSO4

Balanced chemical equation

3BaCl2 + Al2 (So4)3 →  2AlCl3 + BaSo4

(d) Potassium metal reacts with water to give potassium & hydrogen gas.

Ans: K (s) + H2 O (l) → KoH + H2 (g)

Balanced chemical equation:

2K (s) + 2H2O (l) → 2KoH + H2 (g)

(Q6) Balance the following chemical equations:

(a) HNo3 + Ca(OH)2 Ca(No3)2 + H2o

Ans: 2HNo3 + Ca(OH)2 → Ca(NO3)2 + 2H2O

(b) NaoH + H2SO4 Na2SO4 + H2O

Ans: 2NaOH + H2SO4 → Na2SO4 + 2H2O

(c) Nacl + AgNo3 Agcl + NaNo3

Ans: Nacl + AgNo3 Agcl + NaNo3

Already balanced chemical reaction

(d) Bacl2 + H2So4 BaSo4 + HCL

Ans: Bacl2 + H2So4 BaSo4 + 2HCL

(Q7) Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + carbon dioxide → Calcium carbonate + water

Ans: Ca (OH)2 + Co2 CaCo3 + H2O

(b) Zinc + Silver nitrate → Zinc nitrate + silver

Ans: Zn + 2 AgNo3 Zn (NO3)2 + 2Ag

(c) Aluminum + Copper chloride → Aluminium chloride + copper

Ans: 2Al + 3CuCl2 2AlCl3 + 3Cu

(d) Barium chloride + potassium sulphate → Barium sulphate + potassium chloride

Ans: BaCl2 + K2So4 BaSo4 + 2KCL

(Q8) Write the balanced chemical equation for the following & identify the type of reaction in each case.

(a) Potassium bromide (aq) + barium iodide (aq) → Potassium iodide (aq) + Barium bromide (s)

Answer: 2KBr (aq) + BaI2 (aq) 2KI (aq) + BaBr2 (Aq)

  • This reaction is double displacement type

(b) Zinc Carbonate (s) → Zinc oxide (s) + carbon dioxide (g)

Answer: ZnCo3 → Zno + Co2

  • This reaction is decomposition type

(c) Hydrogen + chlorine (g) → Hydrogen chloride (g)

Ans: H2 (g) + Cl2 (g) 2HCL (g)

  • This reaction is combination type.

(d) Magnesium (s) + hydrochloric acid (aq) → Magnesium chloride (aq) + hydrogen (g)

Ans: Mg (s) + 2HCL (aq) Mg Cl2 (aq) + H2 (g)

This reaction is displacement type.

(Q9) What does one mean by exothermic & endothermic reactions? Give examples.

Ans: Exothermic reactions:

The reactions in which heat is released along with formation of product is called as exothermic reaction.

Ex:- (1) Burning of natural gas

CH4 (g) + 2O2 (g) Co2 (g) + 2H2O (g)

(2) Reaction between hydrochloric acid & sodium hydroxide produce sodium chloride & water.

HCL + NaOH NaCl + H2O + heat

Endothermic reaction:

The reactions in which energy is absorbed in the form of heat or sunlight is called as endothermic reaction

Ex: In photosynthesis process, plants converts carbon dioxide & water into glucose & oxygen in presence of sunlight.

6Co2 (g) + 6H2O (l) sunlight> C6H12O6 + 6O2 (g) + Glucose

(Q10) Why is respiration considered an exothermic reaction? Explain

Ans: Energy in our body is obtained from food that we cat. During digestion the complex substances are broken down into simple substances like glucose.

In respiration process this glucose combines with oxygen in the cells & it release energy. This released energy is used for various functions of our body.

In the respiration process energy is released, Hence it is considered as exothermic reaction.

Ex: C6H12O6 + 6O2 (g) 6 Co2 (g) + 6H2O (l)

       Glucose         Oxygen            Carbon dioxide + Energy

(Q11) Why are decomposition reactions called the opposite of combination reaction? Write equations for these reactions

Ans: Decomposition reactions are those reactions in which a single substance break into two or more than two products.

– These reactions are endothermic reactions it requires energy. It absorbs energy in the form of heat.

  • Whereas, its exact opposite combination reactions. Combination reactions are those in which two or more substances combine to form a single product. These reactions are exothermic energy is released.


* Decomposition reactions:

(Q12) Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Ans: Decomposition reactions requires energy in the form of heat known as thermal decomposition, light known as photolysis & electricity known as electrolysis.

The examples of these reactions are as follows

Q13) What is the difference between displacement & double displacement reaction? Write equations for these reactions.

Displacement reaction

Double displacement reaction
(1) In these reactions the ions of more active elements displaces the ions of less active element.

(1) In these reactions, both the reactants exchanges their ions between them.

(2) During displacement reactions change in color takes place.

(2) During double displacement reaction precipitation is formed

(3) These reactions are usually slow and require more time for complete the reaction.

(3) These reactions are usually fast & require very less time.

Ex: – Zn (s) + Cuso4 (Aq)

ZnSo4 (Aq) + Cu(s)

In this reaction Zinc is more reactive than Cu. Hence it displace copper from its compound .

Ex: Na2 So4 (Aq) + Bacl2 (Aq)

(Sodium  sulphate)  ↓ (Barium chloride)

         BaSo4 (s)          +          2 NaCl

            (Barium sulphate       (Sodium chloride)

In this reaction sodium sulphate  & barium chloride exchange their ions by the formation of barium sulphate (ppt) & sodium chloride.

(Q14) In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Ans: The reaction involved is as follows

Cu (s) + 2 AgNo3 (Aq) Cu (No3)2 + 2 Ag (s)

Copper    Silver nitrate   copper nitrate   Silver

(Q15) What do you mean by a precipitation reaction is called as precipitation reaction.

Ans: The reaction in which precipitate is produced is called as precipitation reaction.

Example: Double displacement reactions are precipitation reactions.

When Sodium Sulphate reacts with barium chloride it produce white precipitate of barium sulphate along with sodium chloride.

Na2So4 (Aq) + Bacl2 (Aq) BaSo4 (s) + 2Nacl (Aq)

(Sodium sulphate)  (Barium chloride) (Barium sulphate) sodium chloride

White ppt

(16) Explain the following in terms of gain or loss of oxygen with two examples each.



If a substance gains oxygen during a reaction, such reactions are known as oxidation reaction.


In example Cu (Copper) gains oxygen & convert it into copper oxide


If a substance loses a oxygen during a reaction, such reactions are known as reduction reaction.


In above reaction Cuo is converted into Cu with the removal of oxygen in the form of water.

(Q17) A shiny brown coloured element ‘x’ on heating in air becomes black in colour. Name the element ‘x’ & the black coloured compound formed.

In above reaction a shiny brown coloured element ‘X’ is Cu (Copper) & the black coloured compound is Cuo (Copper Oxide)

(Q18) Why do we apply paint on iron articles?

Ans: We apply paint on iron articles to prevent corrosion of iron. Due to various components of atmosphere like moisture, oxygen, the oxidation of iron takes place which results oxidation of iron takes place which results iron rusts that is known as corrosion. Corrosion damages the iron articles.

If we apply paint on iron articles iron does not comes in direct contact with moisture & oxygen. Hence corrosion does not occurs.

(Q19) Oil & fats containing food items are flushed with nitrogen why?

Ans: Oil & fats containing food items are flushed with nitrogen because, oil & fats are easily oxidised. They becomes food rancid, their smell & taste changes.

Nitrogen is antioxidant, it prevents the food from getting oxidise, as a result which rancidity does not takes place & food remains safe.

(Q20) Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Ans: (a) Corrosion: When the surface of metal comes in contact with air, acid moisture, the metal get corrodes this process is known as corrosion.


Ag + H2S + O2 Ag2S + H2 O

Silver        (Black colour)

When silver react with H2S & O2 from atmosphere it corrodes silver (Ag) & forms black colour coating on them.

(b) Rancidity: The process of slow oxidation of oil & fats present in the food resulting in changes of smell & taste is known as rancidity.

Example: When potato chips containing fats & oils are exposed for considerable time to a moisture, it starts giving unpleasant taste & odour.

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