Chemical Reactions and Equations Questions and Answers Class 10

NCERT Solutions for Class 10 Science Chemical Reactions and Equations Questions and Answers

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In Text Questions Page no-6

Q1. Why should magnesium ribbon be cleaned Before burning in air ?

Ans. Magnesium is very reactive Metal. when magnesium ribbon remains exposed to moist air, A white layer of magnesium oxide is formed On its surface. This hinders the burning of magnesium. Hence, this layer is first removed by rubbing with Sand paper before burning.

Q2. Write the balanced equation for the following chemical reactions.

(i).Hydrogen + Chlorine –> Hydrogen Chloride

   Ans. H2+ Cl2 –> 2HCl

(ii).Barium chloride + Aluminium Sulphate –> Barium Sulphate + Aluminium Chloride.

Ans. 3BaCl2 + Al(SO4)3 –> 3BaSO4 + 2AlCl3

(iii).Sodium + Water –> Sodium hydroxide + Hydrogen.

Ans. 2Na + 2H2O –> 2NaOH + H2

Q3. Write a balanced chemical equation on with state symbols for the following reactions.

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

Ans. BaCl2 + Na2SO4 –> BaSO4 + 2NaCl

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

Ans. NaOH + HCl –> NaCl + H₂O.

Intext Question Page no-10

Q 1. A solution of a substance X Is used for white washing.

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

Ans.The substance ‘X’ used to white washing is Quicklime (Calcium oxide). The chemical formula is CaO.

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

Ans. When quick lime is mixed with water, The following reaction takes place :

CaO (s) + H2O(l) –> Ca(OH)2 (aq)

Quick lime (Calcium oxide) + water à Calcium hydroxide (Slaked lime)

Q2. Why is the amount of gas collected in one of the test tubes double of the amount collected in the other on electrolysis of water ? Name this gas.

Ans. Water contains two parts of hydrogen and one part oxygen. Therefore, during the electrolysis of water the amount of hydrogen gas collected in one of the test tubes is double than that of the produced and collected in the other test tube.

In text Question Page no-13

Q1. Why does the colour of copper sulphate change when an iron nail is dipped in it?

Ans. When an iron nail dipped in the copper sulphate solution than iron displaces copper from the Copper Sulphate solution because Iron is more reactive than copper , according to the following reason-

Fe (s) + CuSO4 –> FeSO4 (aq) + Cu (s)

Iron + Copper Sulphate –> Iron(ii) Sulphate + Copper

Thus, Copper Sulphate reacts to form iron (ii) Sulphate, the blue colour of copper sulphate Solution fades.

Q2. Give example of a double displacement Reaction other than the one in Activity 1.10.

Ans. 2KBr (aq) + BaI2 (aq) –> 2KI (aq) + BaBr2 (aq)

Q3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(i).4 Na (s) + O2 (g) –> 2Na2O (s)

Ans. Sodium (Na) has gained oxygen to form Na2O. Hence, Na has been oxidized to Na2. Obviously therefore O2 has been reduced

(ii).CuO(s) + H2(g) –>Cu(s) + H₂O(l)

Ans. Copper oxide (CuO) has Lost oxygen to form Cu. Hence, CuO has been reduced to Cu. H2 has gained oxygen to form H2O. Hence, H2 has been oxidised to form  H2O.


Q.1 Which of the statements about the Reaction below are incorrect ?

2PbO (s) + C(s) -> 2Pb(g) + CO2 (g)

  1. Lead is getting reduced.
  2. Carbon dioxide is getting oxidized.
  3. Carbon is getting oxidized.
  4. Lead oxide is getting reduced.
  5. a and b
  6. a and c
  7. a, b and c
  8. all

Ans. Statements a and b are incorrect. Hence, (i) is the correct option.

Q2. Fe2O3 + 2Al -> Al2O3 + 2Fe

The above reaction is an example of:

  1. Combination reaction
  2. Double displacement reaction
  3.  Decomposition reaction
  4. Displacement reaction

Ans. The given equation is a displacement Reaction in which Fe of Fe2O3 has been displaced by Al. Hence, (d) is the correct option.

Q3.  What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced.

(b) Chlorine gas and iron hydroxide are produced.

(c) No Reaction takes place.

(d) lron salt and water are produced.

Ans. Hydrogen gas and iron chloride are produced. Therefore, (a) is the correct option.

Q4. What is a balanced chemical equation? why should the chemical equation be balanced?

Ans. An equation in which the numbers of atoms of each element on the two sides of the equation is equal is called balanced chemical equation. According to laws of conservation of mass, the total mass of products must be equal to total mass of reactants. This is possible only if the number of Atoms of each element is same on the two sides of the equation.

Q5.Translate the following statements into chemical equations and then balance them.

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

Ans. 3H(g) + N2 (g) -> 2NH3 (g)

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide..

Ans. 2H2S (g) + 3O2(g) -> 2H2O (l) + 2SO2 (g)

(c)Barium chloride reacts with aluminium sulphate to give aluminium chloride and precipitate of barium sulphate.

Ans.3BaCl2 (aq) + Al2(SO4)3(aq) -> 2AICI3 (aq) + 3BaSO4(s)

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

Ans. 2K(s) + 2H2O (l) -> 2KOH (aq) +H2 (g)

Q6. Balance the following chemical equations:

(i) HNO3 + Ca(OH)2 -> Ca(NO3)2 + H2O

Ans. 2HNO3 + Ca(OH)2 -> Ca(NO3)2 + 2H2O

Nitric acid + Calcium hydroxide -> Calcium nitrate + water

(ii) NaOH + H2SO4 -> Na2SO4 + H2O

Ans. 2NaOH + H2SO4 -> Na2S04 +2H2O

Sodium hydroxide + Sulphuric acid -> Sodium Sulphate + Water

(iii) NaCl + AgNO3 -> AgCl + NaNO3

Ans. NaCl + AgNO3 -> AgCl + NaNO3

Sodium Chloride + Silver nitrate -> Silver Chloride + Sodium nitrate

(iv) BaCl2 + H2SO4 -> BaSO4 + HCl

Ans. BaCl2 + H2SO4 -> BaSO4 + 2HCl

Barium chloride + Sulphuric acid -> Barium Sulphate + Hydrochloric acid

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 + 2AgNO3 -> Zn (NO3)2 + 2Ag

(c)   Aluminium + Copper Chloride -> Aluminium chloride + Copper.

Ans. 2Al + 3CuCl2 -> 2AICI3 + 3 Cu

(d) Barium chloride + Potassium sulphate ->  Barium Sulphate + Potassium chloride.

Ans. BaCl2 + K2SO4 -> BaSO4 + 2KCl.

Q8. Write the balanced chemical equation to the following and identity the type of reaction in each case.

(a) Potassium bromide (aq) + Barium iodide (aq) -> Potassium iodide (aq) + Barium bromide (s)

Ans. 2KBr (aq) +Bal2 (aq) -> 2KI (aq) + BaBr2 (s)

Type: Double Displacement reaction

(b) Zinc carbonate (s) -> Zinc oxide (s) + Carbon dioxide (g)

Ans. ZnCO3 (s) -> Zn O (s) + CO2 (g)

Type: Thermal decomposition reaction

(c) Hydrogen (g) + Chlorine (g) -> Hydrogen chloride (g)

Ans. H2(g) +Cl2 (g) -> 2HCl(g)

Type : Combination reaction

(d) Magnesium(s) + Hydrochloric acid (aq) ->Magnesium chloride (aq) + Hydrogen (g).

Ans.(a) 2KBr (aq) + Bal2 (aq)

Type: Double Displacement reason

(b) Zinc carbonate (s) -> Zinc oxide (s) + Carbon dioxide (g)

Ans. ZnCO3 (s) ->  ZnO (s) + CO2 (g)

Type: Thermal decomposition medium

(c) Hydrogen(g) + Chlorine (g) -> Hydrogen chloride (g)

Ans. H2(g) + Cl2 (g) -> 2HCl(g)

Type : Combination reaction

(d) Magnesium +Hydrochloric acid (aq) -> Magnesium chloride (qa) + Hydrogen (g).

Ans. Mg (s) + 2HCI(aq) -> MgCl2(aq) + H2(g)

Type: Displacement reaction

Q9. What does one mean exothermic and endothermic reactions? Give examples.

Ans. Exothermic reactions: Those reaction in which heat is evolved during the reaction are called exothermic reaction.


(i) Burning of coke.

C (s) + O2(g) -> CO2(g) + Heat

Coke + Oxygen (from air) -> Carbon Dioxide

(ii) Burning of natural gas (methane)

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

Methane + Oxygen -> Carbon dioxide + Water

Endothermic reactions: Those reactions In which heat is absorbed during the reaction are called endothermic reactions.


(i)Reaction of coke with steam.

C(s) + H₂O (g) + Heat -> CO(g) + H2(g)

Coke + Steam -> Carbon monoxide. + Hydrogen

(ii)Decomposition of mercuric oxide.

2HgO (s) + Heat -> 2Hg(l) + O2(g)

Mercuric oxide  ->  Mercury  Oxygen

Q10. Why is respiration considered as an exothermic reaction? Explain.

Ans. The food taken by the living beings is ultimately ultimately broken down to glucose by the digestive system. The glucose so formed is slowly oxidised to carbon dioxide and water with the release of heat energy. Thus, respiration is an exothermic reaction.

Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Ans. In a decomposition reaction, single reactant breaks down to produce two or more products, where as  in a combination reaction,  two or more reactions combine to give a single product. Thus, these reactions are supposed to be opposite of each other.


(i) Ca CO3 (s) -> CaO (s) + CO₂(g)

(Decomposition reactions)

(ii) 2FeSO4(s) -> Fe2O3 + SO2(g) + SO3(g)

(Decomposition reactions)


(i)2H2 + O2 -> 2H2O

(combination reaction)

(ii)2Mg(s)+O2(g) -> 2Mg2O(s)

(combination reaction)

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

Ans. (i) Decomposition reaction involving absorption of heat.

CaCO3 (s) -> CaO (s) + CO(g)

(ii)Decomposition reaction involving description of heat list

2 AgCl(s) -> 2Ag(s) + Cl2(g)

(iii)Decomposition reaction involving absorption of electrical energy.

2H2O(l) -> 2H2O(g) + O2(g)

Q13. What is the difference between the displacement and double displacement reaction? Write equation for these reactions.

Ans. In a displacement reaction, a more reactive element displaces a less reactive element from its solution. But in a double displacement, two or more atoms groups from different compounds displace each other.

Chemical equation for Single displacement

Zn (s) t CuSO4(aq) -> ZnSO4(aq) + Cu(s)

Here, Zn Displaces Cu from its Solutions (CuSO4).

Chemical equation for double displacement

BaCl2 (aq) + K2SO4 (aq) -> BaSO4 + 2KCI

Here,  Ba and K displaces each other.

Q14. In the sieve of silver, the recovery of nitrate solution involved displacement by copper metal. Write down the reaction involved.

Ans. Cu (S) + 2AgNO3(aq) -> Cu (NO3)2(aq) + 2Ag (s)

Copper + Silver nitrate ->  Copper (ii) nitrate + Silver

Q15. What do you mean by precipitation reactions? Explain giving examples.

Ans. When on mixing the clear solution of two ionic compounds, an insoluble solid is formed that separates out the solid formed is called a precipitate and the reaction is called precipitation reaction.

For example:

(1) When barium chloride solution is added to an aqueous solution of sodium sulphate, a white precipitate of barium sulphate is obtained.

BaCl2(aq) + Na2SO4(aq) -> BaSO4 (s)+2NaCl (aq)

(White ppt.)

(ii) When silver nitrate is added to an aqueous solution of sodium chloride, a white precipitate of silver chloride (AgCl), which is soluble in NH4OH is obtained.

AgNO3(aq) + NaCl(aq) -> AgCl(s) (White ppt) + NaNO3(aq)

Q16. Explain the following in terms of gain or loss of oxygen with two examples each.

(i) Oxidation

(ii) Reduction

Sol. (i) Oxidation It is a process in which a substance gains oxygen, e.g

(a) 4Na + O2 -> 2Na2O

Sodium Oxygen Sodium oxide

(b) 2H2 + O2 -> 2H2O

Hydrogen Oxygen-> Water

In the above reactions, Na and H2 gains oxygen to form Na2O and H2O respectively.

(ii) Reduction: It is a process in which a substance loses oxygen.

e.g. (a) CuO+H2-> Cu +H2O

CuO loses oxygen to form Cu.

(b) 2KClO3(s) -> 2KCl(s) + 3O2(g)

KCIO3 loses oxygen to form KCl and O2.

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. Element X is copper and the black coloured compound is copper (II) oxide.

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 (rusting). Paint does not allow oxygen (from air) and water (moisture) to come in contact with the surface of iron.

Q19. Oil and fat containing food items are flushed with nitrogen. Why?

Ans. Nitrogen is unreactive gas as compared to oxygen. Oil and fat present in the food items get oxidised and become rancid in the presence of air or oxygen. But such reaction is prevented in the presence of nitrogen. Therefore, food items like potato chips etc., are packed with nitrogen gas to prevent them from rancidity for a

long time.

Q20. Explain the following terms with one example of each:

  1. Corrosion
  2. Rancidity

Ans. (i) The phenomenon due to which metals are slowly eaten away by the reaction of air, water and chemicals present in the atmosphere is called corrosion.

(ii) It is the process of slow oxidation of oil and fat present in the food materials resulting in the change of smell and taste in them.

Some other extra questions :

Q1. What is a chemical reaction ?

Ans. A process in which the original substance lose their nature and identify to form a new chemical substances with different properties is called a chemical reaction.

Q2. What do you mean by balanced equation ?

Ans. An equation in which the number of atoms of different elements on the two sides of the equation are equal is called a balanced equation.

Q3.   Why should the equation be balanced ?

Ans. This is because according to the law of conservation of mass , mass can neither be created nor destroyed. Hence , the number of atoms of each element on the two sides of the equation are equal is called a balanced equation.

Q4. What is a chemical equation ?

Ans. The representation of a chemical reaction by using the symbols and formulae of the reactants and the products is called a chemical equation.

Q5. What do you mean by ‘ activity series of metals ‘ and also name the most reactive metal and least reactive metal of the activity series of metals.

Ans. The arrangement of metals in order of decreasing reactivity is called activity series of metals. Potassium is the most reactive metal and platinum is the least reactive metal.

Q6. Why does zinc react with dilute sulphuric to give hydrogen gas but copper does not ?

Ans. This is because zinc is more reactive than hydrogen whereas copper is less reactive than hydrogen in the activity series.

Q7. What is the difference between oxidation and reduction ?

Ans. Oxidation : Oxidation is defined as a process which involves gain of oxygen or loss of hydrogen.

Reduction :  Reduction is defined as a process which involves gain of hydrogen or loss of oxygen.

Q8. Why is a copper vessel covered with a green coating in rainy season ?

Ans. This is due to attack of oxygen, carbon dioxide and water vapours of the air on copper forming green coloured basic copper carbonate.

Q9. What is a difference between a balanced and unbalanced chemical equation ?  Explain with suitable examples.

Ans. An equation in which the number of atoms of all the elements are equal on the two sides of the equation is called a balanced chemical equation. On the other hand , if the number of atoms  of one or more elements is not equal on the two sides of the equation , it is called unbalanced equation.

Q10. What do you mean by rancidity?

Ans. Oxidation of fatty foods forming compounds with bad smell and bad taste is called rancidity.

Q11. What reactions take place during the digestion of food in our body ?Paragraph

Ans. Our food mainly contains starch and proteins. In the presence of enzymes as catalyst in our body , starch decomposes into simple sugars whereas proteins decomposes to form simpler molecules called amino acids.

Q12. Why do gold and silver not corrode in moist air ?

Ans. Gold and silver lie very low on the activity series of metals. Thus , they have very low reactivity and hence do not corrode in moist air.

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