The first chapter in the NCERT Class 10 Science Book Solutions revolves around Chemical Reactions and Chemical Equations. This chapter will teach you how to accurately write chemical equations and balance them. You will also learn about the various types of chemical reactions, including Combination, Decomposition,Displacement, Double Displacement, Oxidation, and Reduction Reactions. The chapter also explains Endothermic and Exothermic reactions and Precipitation Reaction. You will also become familiar with the common effects of oxidation reactions viz. Corrosion and Rancidity.
Chemical Reaction Definition:When two or more substances react and form some new substance, it is called a chemical reaction.
Access Answers of NCERT class 10 Science Chapter 1 – Chemical reactions and equations
Q.1: Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
Ans: Iron is more reactive than copper. So, when an iron nail is dipped in a copper sulphate solution, iron displaces copper from its solution to form iron sulphate, which is green in color.Fe (s) + CuSO4 (aq) --> FeSO4 (aq) + Cu (s) Hence, the blue color of copper sulphate solution changes into green color because of this displacement reaction.
Q.2: Give an example of the double displacement reaction other than the one given in Activity 1.10.
Ans: When lead (II) nitrate is mixed with potassium iodide, potassium nitrate and lead iodide are formed.
Pb(NO3)2 + 2KI -->2KNO3 + PbI2
Na 2CO 3(aq) + CaCl2 (aq) --> CaCO 3 (s) + 2NaCl (aq)
Here sodium carbonate and calcium chloride exchange ions to form two new compounds.Q.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(liq)
Ans:
(i) Sodium (Na) is oxidized to Na2O.
(ii) CuO (Copper oxide) is reduced to Cu, while H2 gas is oxidized to H2O
Q.4 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.
Ans:
(i) The substance 'X' is calcium oxide (lime). Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).
Q.5 Why is the amount of gas collected in one of the test tubes in Activity double of the amount collected in the other? Name this gas.
Ans:
Water (H2O) contains two parts of hydrogen and one part of oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen gas goes to another hence, the amount of gas collected in one of the test tubes is double the amount collected in the other. The gas collected in double the amount is hydrogen and the other gas is oxygen. Concept insight: The key to this answer is the electrical decomposition reaction of water.
Q.6 Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Ans:
When an iron nail is placed in a copper sulphate solution, iron displaces copper from the blue colored copper sulphate solution forming light green colored ferrous sulphate solution and copper metal.
Q.7 Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s)--> 2Pb(s) + CO2(g)
(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:
i) (a) and (b)
Q.8 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:
(d) displacement reaction
Q.9 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) Iron salt and water are produced
Ans:
(a) Hydrogen gas and iron chloride are produced.
Q.10 What is a balanced chemical equation? Why should chemical equations be balanced?
Ans:
A chemical equation is balanced when the numbers of atoms of each type involved in a chemical reaction are same on both the reactant and product sides of the equation.The chemical equations must always be balanced to satisfy the law of conservation of mass which states that "Mass can neither be created nor destroyed in a chemical reaction." This means that the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. Hence, the number of atoms of each element in the products must be equal to the number of atoms of these elements in the reactants.
Q.11 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 sulpur 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 + N2 --> 2NH3
(b) 2H2S + 3O2-->2H2O + 2SO2
(c) 3BaCl2 + Al2(SO4)3-->2AlCl3 + 3BaSO4
(d) 2K + 2H2O-->2KOH + H2
Q.12 Balance the following chemical equations.
(a) HNO3 + Ca(OH)2-->Ca(NO3)2 + H2O
(b) NaOH + H2SO2-->Na2SO2 + H2O
(c) NaCl + AgNO2 -->AgCl + NaNO2
(d) BaCl2 + H2SO4-->BaSO4 + HCl
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.13 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.14 Write the balanced chemical equation 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) + Bal2-->2Kl (aq) + BaBr2 (s), Double displacement reaction and precipitation 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 (s) + H2 (g), Displacement reaction
Q.15 What does one mean by exothermic and endothermic reactions? Give examples.
Ans:
Reactions in which heat is released along with the formation of products are called exothermic reactions. Example of exothermic reaction:Burning of natural gas: CH4(g) + 2O2 (g)-->CO2 (g) + 2H2O (g) + Heat Reactions in which energy is absorbed are known as endothermic reactions. Example of endothermic reaction: 2AgBr(s) 2Ag(s) + Br2(g)
Q.16 Why is respiration considered an exothermic reaction? Explain.
Ans:
Food that we eat includes carbohydrates, proteins, vitamins etc. During digestion, carbohydrates are broken down into simpler substances called glucose. Glucose combines with oxygen in the cells of our body to form carbon dioxide and water along with energy. This reaction is called respiration. Since energy is released during this process, respiration is an exothermic reaction.
C6H12O6(aq)+6O2(g)--> 6CO2(aq)+6H2O(l)+Energy
Q.17 Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Ans:
In a decomposition reaction, a single substance decomposes to give two or more substances. Whereas in a combination reaction two or more substances combine to form a new single substance. And hence, decomposition reactions are opposite of combination reactions.
Decomposition reaction: AB + Energy-->A + B
Example:CaCO3(s) CaO(s) + CO2(g)
Combination reaction: A + B-->AB + Energy
Example: Burning of coal : C(s) + O2(g)-->CO2(g)
Q.18 Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Ans:
Heat CaCO3(s) CaO(s) + CO2(g)
Light 2AgBr(s) 2Ag(s) + Br2(g)
Electricity 2H2O (l) 2H2 (g) + O2 (g)
Concept insight: For answering this question, you have to write a complete balanced chemical reaction for all the three types of reactions.
Q.19 What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Ans:
A displacementreaction is a chemical reaction in which a more reactive element displaces a less reactive element from its salt solution
Example:
Fe + CuSO4-->FeSO4 + Cu
In this reaction, one displacement is taking place. Fe is displacing Cu. Double displacement reaction is a chemical reaction in which there is an exchange of ions between the reactants to give new substances. There are two displacements taking place in a double displacement reaction.
Example:
3 BaCl2 + Al2(SO4)3-->2 AlCl3 + 3 BaSO4
In this reaction, two displacements are taking place. Ba is displacing Al and Al is displacing Ba.
Q.20 In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reactions involved.
Ans:
2AgNO3(aq) + Cu(s)-->Cu(NO3)2 + 2Ag(s)
Q.21 What do you mean by a precipitation reaction? Explain by giving examples.
Ans:
Any reaction that produces an insoluble solid (precipitate) can be called a precipitation reaction.
These insoluble salts separate out from the solution and settle down as precipitate.
For example: When aqueous sodium sulphate solution and aqueous barium chloride are reacted, aqueous solution of sodium chloride and white precipitate of barium sulphate are formed.
Q.22 Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Ans:
(a) Oxidation: It is the gain of oxygen by a substance in a reaction.Example 1: When magnesium is burned in air, magnesium oxide is formed. 2Mg + O2-->2MgO
Here, magnesium is oxidized to magnesium oxide. It has gained oxygen.
Example 2: When copper oxide is heated with hydrogen, copper metal and water are formed.CuO + H2-->Cu + H2O
Here, H2 is getting oxidised to H2O. It has gained oxygen.
(b) Reduction: It is the loss of oxygen by a substance in a reaction.
Example 1: When zinc oxide is heated with carbon, zinc metal and carbon monoxide are formed. ZnO + C-->Zn + CO
Here, ZnO is getting reduced to Zn. It has lost oxygen.
Example 2: When copper oxide is heated with hydrogen, copper metal and water are formed.
CuO + H2-->Cu + H2OHere, CuO is getting reduced to Cu. It has lost oxygen.
Q.23 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:
The shiny brown coloured element 'X' is copper and the black coloured compound is copper oxide (CuO).
The chemical reaction is: 2Cu + O2 2CuO
Q.24 Why do we apply paint on iron articles?
Ans:
We apply paint on iron articles to avoid their rusting. When a coat of paint is applied to the surface of an iron article, it stops the contact of air and moisture with the iron metal and hence no rusting takes place.
Q.25 Oil and fat containing food items are flushed with nitrogen. Why?
Ans:
When fats and oils are kept for a long time, they get oxidized by aerial oxidation and become rancid and their smell and taste change. Food items containing oil and fat are flushed with nitrogen to prevent rancidity of oil and fat as nitrogen is an inert gas and prevent the oxidation of oil and fats.
Q.26 Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Ans:
(a) Corrosion: Corrosion is the process of eating away of metals by the attack of substances such as moisture, acids, etc.
Example - the black coating on silver and the green coating on copper are other examples of corrosion.
(b) Rancidity: When food items containing fats and oils are kept for a long time, they get oxidized and their smell and taste change. This process is known as rancidity.
Example: Butter when kept in open for long time, tastes and smells bad because of rancidity.
Q.27 Why should a magnesium ribbon be cleaned before burning in air?
Ans:
When magnesium ribbon is stored, it reacts with oxygen of air to form a layer of magnesium oxide. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. Hence, a magnesium ribbon should be cleaned before burning in air to remove this layer of magnesium oxide.
Q.28 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
Ans:
(i) H2 + Cl2-->2HCl
(ii) 3BaCl2 + Al2 (SO4)3-->3BaSO4 + 2AlCl3
(iii) 2Na + 2H2O-->2NaOH + H2
Q.29 Write balanced chemical equations 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.
Ans:
(i) BaCl2 (aq) + Na2SO4(aq)-->BaSO4 (S) + 2NaCl (aq)
(ii) NaOH (aq) + HCl (aq)-->NaCl (aq) + H2O (l)