Class 10 Science Chapter 1 Chemical reactions and equations
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NCERT Solutions For Class 10 Science Chapter 1 Chemical reactions and equations
Class 10 Science Chapter 1 Chemical reactions and equations
Q. 1. Which of the statements about the reaction below are incorrect ?
2PbO ( s ) + C ( s ) → 2Pb ( s ) + CO₂ ( g )
( i ) Lead is getting reduced .
( ii ) Carbon dioxide is getting oxidised .
( iii ) Carbon is getting oxidised .
( iv ) Lead oxide is getting reduced .
( a ) ( i ) and ( ii )
( b ) ( i ) and ( iii )
( c ) ( i ) , ( ii ) and ( iii )
( d ) All
Ans . Correct option : ( a )
Explanation : Lead oxide is getting reduced to lead and Carbon is getting oxidised .
Q. 2. Fe₂O3 + 2Al → Al₂O3 + 2Fe
The above reaction is an example of a :
( a ) combination reaction .
( b ) double displacement reaction .
( c ) decomposition reaction .
( d ) displacement reaction
Ans . Correct option : ( d )
Explanation : The given reaction is an example of a displacement reaction .
Q. 3 . What happens when dilute hydrochloric acid is added to iron filings ? 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 ) Iron salt and water are produced .
Ans . Correct option : ( a )
Explanation : When dilute hydrochloric acid is added to iron filings , hydrogen gas and iron chloride are produced . The reaction is as follows :
Fe ( s ) + 2HCl ( aq ) → FeCl₂ ( aq ) + H₂
Q. 4. What is a balanced chemical equation ? Why should chemical equations be balanced ?
Ans . The chemical equation in which the number of atoms of each element on both the sides is equal is called the balanced equation . A chemical equation should be balanced because there is no loss or gain of any matter during a chemical reaction , i.e. , the law of conservation of mass must hold good for the reaction .
Q. 5. Translate the following statements into chemical equations and then balance them .
( a ) Hydrogen gas combines with nitrogen to form ammonia .
( b ) Hydrogen sulphide gas burns in air to give water and sulphur dioxide .
( c ) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate .
( d ) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas .
Ans . ( a ) 3H₂ ( g ) + N₂ ( g ) → 2NH3 ( g )
( b ) 2H₂ S ( g ) + 3O₂ ( g ) → 2H₂O ( l ) + 2SO₂ ( g )
( c ) 3BaCl₂ ( aq ) + Al₂ ( SO4 )3 ( aq ) → 2AlCl3 ( aq ) + 3BaSO4 ( ppt )
( d ) 2K ( s ) + 2H₂O ( l ) → 2KOH ( aq ) + H₂ ( g )
Q. 6. Balance the following chemical equations .
( a ) HNO3 + Ca ( OH )2 → Ca ( NO3 )2 + H₂O
( b ) NaOH + H₂SO4 → Na₂SO4 + H₂O
( c ) NaCl + AgNO3 → AgCl + NaNO3
( d ) BaCl₂ + H₂SO4 → BaSO4 + HCI
Ans . ( a ) 2HNO3 + Ca ( OH )2 → Ca ( NO3 )2 + 2H₂O
( b ) 2NaOH + H₂SO4 → Na₂SO4 + 2H₂O
( c ) NaCl + AgNO3 → AgCl + NaNO3
( d ) BaCl₂ + H₂SO4 → BaSO4 + 2HCl
Q. 7. Write the balanced chemical equations for the following reactions .
( 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 → CaCO3 + H₂O
( b ) Zn + 2AgNO3 → Zn ( NO3 )2 + 2Ag
( c ) 2Al + 3CuCl₂ → 2AlCl3 + 3Cu
( d ) BaCl2 + K2SO4 → BaSO4 + 2KCl
Q. 8. Write the balanced chemical equations for the following and identify the type of reaction in each case .
( a ) Potassium bromide ( aq ) + Barium iodide ( aq ) → Potassium iodide ( aq ) + Barium bromide ( s )
( 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 )
Ans . ( a ) 2KBr ( aq ) + BaI₂ ( aq ) → 2Kl ( aq ) + BaBr₂ ( s ) ; Double displacement reaction
( b ) ZnCO3 ( s ) → ZnO ( s ) + CO₂ ( g ) ; Decomposition reaction
( c ) H₂ ( g ) + Cl₂ ( g ) → 2HCl ( g ) ; Combination reaction
( d ) Mg ( s ) + 2HCl ( aq ) → MgCl₂ ( aq ) + H₂ ( g ) ; Displacement reaction
Q. 9. What does one mean by exothermic and endothermic reactions ? Give examples .
Ans . Exothermic reaction : Reactions in which energy is released in the form of heat , light , or sound along with the formation of products are called exothermic reactions .
Mixture of sodium and chlorine to yield table salt :
2Na + Cl₂ → 2NaCl + 411 kJ of energy
Burning of natural gas :
CH₂ ( g ) + 2O₂ ( g ) → CO₂ ( g ) + 2H₂O ( g )
We get energy to stay alive from the food we eat . During digestion , food is broken down into simpler substances . For example , rice , potatoes and bread contain carbohydrates . These carbohydrates are broken down to form glucose . This glucose combines with oxygen in the cells of our body and provides energy . The special name of this reaction is respiration .
C6H₁2O6 ( aq ) + 6O₂ ( aq ) → 6CO₂ ( aq ) + 6H₂O ( l ) + ( Glucose ) Energy
The decomposition of vegetable matter into compost is also an example of an exothermic reaction .
Endothermic reaction : Reactions in which energy is absorbed or required energy either in the form of heat , light or electricity for breaking down the reactants are known as endothermic reactions .
In the process of photosynthesis , plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen .
Q. 10 . Why is respiration considered an exothermic reaction ? Explain .
Ans . Energy is required to support life . Energy in our body is obtained from the food we eat . During digestion , large molecules of food are broken down into simpler substances such as glucose . Glucose combines with oxygen in the cells and provides energy . The special name of this combustion reaction is respiration . Since energy is released in the whole process , it is an exothermic process .
C6H₁2O6 ( aq ) + 6O₂ ( g ) → 6CO₂ ( g ) + 6H₂O ( l ) + Energy
Q. 11. Why are decomposition reactions called the opposite of combination reactions ? Write equations for these reactions .
Ans . Decomposition reactions are those in which a compound breaks down to form two or more substances . These reactions require a source of energy to proceed . Thus , they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy .
Decomposition reaction :
AB + Energy → A + B
Combination reaction :
A + B → AB + Energy
2CO + O₂ → 2CO₂ + Energy
Q. 12. Write one equation , each for decomposition reactions , where energy is supplied in the form of heat , light or electricity .
Ans . ( 1 ) Thermal decomposition :
( 2 ) Decomposition by light :
( 3 ) Decomposition by electricity :
Q. 13. What is the difference between displacement and double displacement reactions ? Write equations for these reactions .
Ans . Difference between displacement and double displacement reactions are given ahead :
Q. 14. In the refining of silver , the recovery of silver from silver nitrate solution involved displacement by copper metal . Write down the reaction involved .
Ans . 2AgNO3 ( aq ) + Cu ( s ) → Cu ( NO3 )2 ( aq ) + 2Ag ( s )
Silver nitrate Copper Copper nitrate Silver
Q. 15. What do you mean by a precipitation reaction ? Explain by giving examples .
Ans . A reaction in which an insoluble solid ( called precipitate ) is formed is called a precipitation reaction . The salts formed are precipitates and are the products of precipitation reaction . The precipitation reactions are the double displacement reactions involving the production of a solid form residue called the precipitate . The reaction also occurs when two or more solutions with different salts are combined , resulting in the formation of insoluble salts that precipitates out of the solution .
( i ) Na2SO4 ( aq ) + BaCl2 ( aq ) → BaSO4 ( s ) + 2NaCl ( aq )
The white precipitate of BaSO4 is formed by the reaction of SO4²– and Ba²+ . The other product formed is sodium chloride which remains in the solution .
( ii ) Na₂CO3 ( aq ) + CaCl₂ ( aq ) → CaCO3 ( s ) + 2NaCl ( aq )
In this reaction , calcium carbonate is obtained as a precipitate . Hence , it is a precipitation reaction
( iii ) AgNO3 ( aq ) + KCl ( aq ) → AgCl ( ppt . ) + KNO3 ( aq )
In this reaction , a white precipitate called as silver chloride or AgCl is formed which is in a solid state .
Q. 16. Explain the following in terms of gain or loss of oxygen with two examples each .
( a ) Oxidation ( b ) Reduction
Ans . ( a ) Oxidation is the gain of oxygen .
For example :
In equation ( i ) , CO₂ is reduced to CO and in equation ( ii ) , CuO is reduced to Cu .
Q. 17. 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 . X ‘ is copper ( Cu ) and the black – coloured compound formed is copper oxide ( CuO ) . The equation of the reaction involved on heating copper is given below .
Q. 18. Why do we apply paint on iron articles ?
Ans . Iron articles are painted because it prevents them from rusting . When painted , the contact of iron articles from moisture and air is cut off . Hence , rusting is prevented . Their presence is essential for rusting to take place .
Q. 19. Oil and fat containing food items are flushed with nitrogen . Why ?
Ans . Nitrogen is an inert gas and does not easily react with these substances . On the other hand , oxygen reacts with food substances and makes them rancid . Thus , bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack . When oxygen is not present inside the pack , rancidity of oil and fat containing food items is avoided .
Q. 20. Explain the following terms with one example each .
( a ) Corrosion ( b ) Rancidity
Ans . ( a ) Corrosion : A process where materials , usually metals , deteriorate on interaction with air , moisture , chemicals , etc. , as a result of chemical reactions between them is known as corrosion .
The black coating on silver and the green coating on copper are other examples of corrosion . Corrosion causes damage to car bodies , bridges , iron railings , ships and to all objects made of metals , especially those of iron . Corrosion of iron is a serious problem .
Corrosion makes the metals scaly and decreases their envisioned usage period . The strength , appearance and permeability are also degraded . A considerable amount of damage is done by corrosion every year , which includes damaging of cars , grills , buses , railings , sinking of ships , falling off bridges , deterioration of statues , etc.
For example , iron , in the presence of moisture , reacts with oxygen to form hydrated iron oxide .
4Fe + 3O2 + nH2O → 2Fe2O3.nH2O
This hydrated iron oxide is rust .
( b ) Rancidity : The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity . It spoils food materials that makes difficult consumption .
For example , the taste and smell of butter changes when kept for long .
Rancidity can be avoided by :
( i ) Storing food in air tight containers .
( ii ) Storing food in refrigerators or at very low temperatures .
( iii ) Adding antioxidants .
( iv ) Storing food in an environment of nitrogen .
Q. 1. Why should a magnesium ribbon be cleaned before burning in air ? [ NCERT Q. 1 , Page 6 ]
Ans . Magnesium is a very reactive metal like Na , Ca , etc .. When exposed to air it reacts with oxygen to form a layer of magnesium oxide ( MgO ) on its surface .
2Mg + O₂ → 2MgO
This layer of magnesium oxide is pretty stable and prevents additional reaction of magnesium with oxygen . The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be used for the reaction .
Q. 2. Write the balanced equation for the following chemical reactions .
( i ) Hydrogen + Chlorine → Hydrogen chloride
( ii ) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
( iii ) Sodium + Water Sodium hydroxide + Hydrogen [ NCERT Q. 2 , Page 6 ]
Ans . ( i ) H₂ + Cl₂ → 2HCI
( ii ) 3BaCl₂ + Al₂ ( SO4 )3 → 3BaSO4 + 2AICI3
( iii ) 2Na + 2H₂O → 2NaOH + H₂
Q. 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 .
( ii ) Sodium hydroxide solution ( in water ) reacts with hydrochloric acid solution ( in water ) to produce sodium chloride solution and water . [ NCERT Q. 3 , Page 6 ]
Ans . ( i ) BaCl₂ ( aq ) + Na₂SO4 ( aq ) → BaSO4 ( s ) + 2NaCl ( aq )
( ii ) NaOH ( aq ) + HCl ( aq ) → NaCl ( aq ) + H₂O ( l )
Q. 4. A solution of a substance ‘ X ‘ is used for white washing .
( i ) Name the substance ‘ X ‘ and write its formula .
( ii ) Write the reaction of the substance ‘ X ‘ named in ( i ) above with water . [ NCERT Q. 1 , Page 10 ]
Ans . ( i ) The substance ‘ X ‘ is calcium oxide . Its chemical formula is CaO .
( ii ) Calcium oxide reacts vigorously with water to form calcium hydroxide ( slaked lime )
CaO ( s ) + H2O ( l ) → Ca ( OH ) ₂ ( aq )
Calcium oxide or Calcium Hydroxide
Quicklime or Slaked lime
Q. 5. Why is the amount of gas collected in one of the test tubes during electrolysis water , double of the amount collected in the other ? Name this gas . [ NCERT Q. 2 , Page 10 ]
Ans . During the electrolysis of water , hydrogen and oxygen get separated by the electricity . Water ( H₂O ) contains two parts hydrogen and one part of oxygen . Since hydrogen goes to one test tube and oxygen goes to another , the amount of gas collected in one of the test tubes is double of the amount collected in the other .
Q. 6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it ? [ NCERT Q. 1 , Page 13 ]
Ans . When an iron nail is dipped in a copper sulphate solution , iron displaces copper from copper sulphate solution forming iron sulphate , which is green in colour . This is because iron is more reactive than copper .
CuSO4 + Fe → FeSO4 + Cu .
Therefore , the blue colour of copper sulphate solution fades and green colour appears .
Q.7 . Give an example of a double displacement reaction other than formation of barium sulphate and sodium chloride . [ NCERT Q. 2 , Page 13 ]
Ans . Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride .
Na₂CO3 + CaCl₂ → CaCO3 + 2NaCl
In this reaction , sodium carbonate and calcium chloride exchange ions to form two new compounds . Hence , it is a double displacement reaction .
Q. 8. Identify the substances that are oxidised 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 ) [ NCERT Q.3 , Page 13 ]
Ans . ( i ) Sodium ( Na ) is oxidised as it gains oxygen and oxygen gets reduced .
( ii ) Copper oxide ( CuO ) is reduced to copper ( Cu ) while hydrogen ( H₂ ) gets oxidised to water ( H₂O ) .
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