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Lakhmir Singh Class X Chemistry Chapter 1- Chemical Reactions and Equations


Exercise : Solution of Questions on page Number :18


Answer 1:
In respiration, glucose undergoes slow combustion by combining with oxygen to form carbon dioxide, water and energy. Apart from other functions, this energy maintains our body heat.
C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy
Due to this production of energy, respiration is considered an exothermic process.


Answer 2:
A chemical equation is balanced on the basis of law of conservation of mass; it states that “matter can neither be created nor be destroyed”. If numbers of atoms of various elements are different in reactants’ and products’ side, it means they are either destroyed or created, which violate the law of conservation of mass.


Answer 3:
When quicklime (calcium oxide) is added to water filled in a bucket, they react chemically to form slaked lime (calcium hydroxide) and produce heat energy. So, water in the bucket becomes hot.
CaO (s)       +       H2(l)   →    Ca(OH)2 (aq)      +      Heat
Calcium oxide           Water          Calcium hydroxide


Answer 4:
Magnesium ribbon should be cleaned before burning in air to remove the basic magnesium carbonate coating on it. This layer should be removed so that the ribbon may readily combine with oxygen while burning in air. Basic magnesium carbonate layer consists of magnesium carbonate and magnesium hydroxide.


Answer 5:
False
Formula of a reactant or product cannot be altered, but chemical equation is balanced by multiplying the reactant or product with numbers like 2,3,4,5 and so on to balance the number of atoms in the chemical equation.


Answer 6:
In this reaction, oxygen is written as O, but it is a diatomic molecule and its formula is O2.
The correct balanced chemical equation read as follows:
2Mg (s) + O2 (g) → 2MgO (s)


Answer 7:
The symbol (aq) stands for aqueous solution. When it is written after a formula in a chemical equation, it represents that the element or compound is a solution made in water.


Answer 8:
Photosynthesis is considered an endothermic reaction because in this process light energy from sunlight is absorbed to produce oxygen and glucose from carbon dioxide and water in the presence of chlorophyll.


Answer 9:
(a) A solution made in water is indicated by writing (aq) after its formula in a chemical equation.
Example: 6CO2 (g) + 6H2O (l) →chlorophyllsunlight C6H12O6 (aq) + 6O2 (g)
(b) Exothermic reaction is indicated by writing “heat” or “heat energy” or “energy” on the products’ side of a chemical reaction.
Example: CH4 + 2O2 → CO2 + 2H2O + Heat
(c) Endothermic reaction is indicated by writing “heat” or “heat energy” or “energy” on the reactants’ side of a chemical reaction.
Example: N2 + O2 + Heat → 2NO


Answer 10:
(a) 2H2S (g) + 3O2 (g) → 2H2O (g) + 2SO2 (g)
(b) P4 + 5O2 → 2P2O5
(c) CS2 (g) + 3O2 (g) → CO2 (g) + 2SO2 (g)
(d) 2Al (s) + Fe2O3 (s) → Al2O3 (s) + 2Fe (s)
(e) BaCl2 + ZnSO4 → ZnCl2 + BaSO4


Answer 11:
(a) Ca(OH)2 + CO2 → Ca(CO3) + H2O
(b) 2Al + 3CuCl2 → 2AlCl3 + 3Cu


Exercise : Solution of Questions on page Number : 19


Answer 12:
(a) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(b) Ca(OH)2 + CO2 → CaCO3 + H2O


Answer 13:
The corrected and balanced equations read as follows:
(i) Ca + 2H2O → Ca(OH)2 + H2
(ii) N2 + 3H2 → 2NH3


Answer 14:
(a) Ca (s) + 2H2O (l) → Ca(OH)2 (aq) + H2 (g)
(b) 2SO2 (g) + O2 (g) → 2SO3 (g)


Answer 15:
(i) 4Na + O2 → 2Na2O
(ii) 2H2O2 → 2H2O + O2
(iii) Mg(OH)2 + 2HCl → MgCl2 + 2 H2O
(iv) 4Fe + 3O2 → 2Fe2O3
(v) 2Al(OH)3 → Al2O3 + 3H2O
(vi) 2NH3 + 3CuO →3Cu + N2 + 3H2
(vii) Al2(SO4)3 + 6NaOH → 2Al(OH)3 + 3Na2SO4
(viii) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(ix) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(x) BaCl2 + H2SO4 → BaSO4 + 2HCl


Answer 16:
(a) Chemical equations are balanced to satisfy the law of conservation of mass.
(b) A solution made in water is known as an aqueous solution and indicated by the symbol (aq).


Answer 17:
(a) Burning of magnesium ribbon in air to give white powder of magnesium oxide is an example of chemical reaction.
2Mg (s) + O2(g) →HEAT 2MgO (s)
(b) When dilute sulphuric acid is poured over zinc granules, hydrogen gas is evolved and the mixture becomes hot. Hence, “evolution of a gas” and “change in temperature” are two characteristics of this chemical reaction.
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g) + HEAT
(c) On adding potassium iodide solution to lead nitrate solution, yellow precipitates of lead iodide are formed and the colour of the solution changes from colourless to yellow. Hence, “formation of a precipitate” and “change in colour” are the characteristics of this chemical reaction.
2KI (aq) + Pb(NO3)2 (aq) ​→ PbI2 (s) + 2KNO3 (aq)


Answer 18:
(a) The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as chemical equation.
For example, zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
This can be represented by chemical equation as follows:
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
(b)
Balanced Chemical Equation Unbalanced Chemical Equation
A balanced chemical equation has an equal numbers of atoms of different elements in the side of reactants and products. An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reaction.
In a balanced chemical equation the sum of masses of the reactants is equal to the sum of the masses of the products. In an unbalanced chemical equation the sum of masses of the reactants is not equal to the sum of the masses of the products.
It obeys the law of conservation of mass It does not obey the law of conservation of mass
Example:
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
Here, number of Zn, H, S and O are same in both reactants’ and products’ side. Example:
H2 + O2 → H2O
Here, one oxygen atom is more in reactant’s side than in product’s side; it means one atom of oxygen is being destroyed, which is a violation of law of conservation of mass.
(c) (i) 2NH3 → N2 + 3H2
(ii) C + CO2 → 2CO


Answer 19:.
The balanced equation for the reaction of hydrogen gas and copper oxide is written as follows:
H2 (g) + CuO (s) → Cu (s) + H2O (g)
(i) Elements: Hydrogen (H2) in reactants’ side and copper (Cu) in products’ side are elements.
(ii) Compounds: Copper oxide (CuO) in reactants’ side and water (H2O) in products’ side are the compounds.
(iii) Reactants: H2 and CuO are the reactants.
(iv) Products: Cu and H2O are the products.
(v) Metals: Cu is a metal.
(vi) Non-metals: H2 is a the non-metal.


Answer 20:
(a) A chemical equation can be made more informative by:
Indicating physical states of reactants and products.
Indicating heat change in the reaction.
Indicating the reaction condition.
Example:
2KClO3 (s) + Heat → MnO2 2KCl (s) + 3O2 (g)
where,
(s) and (g) signifies solid and gaseous state of compounds, respectively.
Heat written on reactants’ side signifies that the reaction consumes heat.
MnO2 written above right-handed arrow signifies reaction takes place in the presence of MnO2.
(b) The chemical equation for the given reaction reads as follows:
Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l).


Answer 21:
(a) A balanced chemical equation has an equal number of atoms of different elements in the reactants’ and products’ sides.
Chemical equation should be balanced because if the number of atoms in reactants’ or products’ side is different, it means that the atoms are either created or destroyed, which is a clear violation of the law of conservation of mass.
(b) 2Al (s) + 3Cl2 (l) → 2AlCl3 (s)
(c) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)


Exercise : Solution of Questions on page Number : 22


Answer 22:
(a) Physical state of reactants and products can be shown by writing (s) for solid, (l) for liquid, (aq) for aqueous solution and (g) for gaseous substance after its formula or symbol in chemical reaction.
Example:
C (s) + O2 (g) → CO2 (g) + HEAT
Here, Carbon is taken in solid state and carbon dioxide and oxygen are in gaseous state.
(b) Zn (s) + 2HCl (aq) → ZnCl2 (aq) + H2 (g)
(c) FeSO4 (aq) + 2NaOH (aq) → Fe(OH)2 (s) + Na2SO4 (aq)


Answer 23:.
Two observations that may suggest that a chemical reaction has taken place are
(i) evolution of a gas and
(ii) change in temperature.
For example, zinc granules react with dilute sulphuric acid to evolve hydrogen gas, which is easily observed by the bubbles of hydrogen gas appearing around zinc granules. Also, the flask becomes hot, which shows heat is evolved during the reaction—that is, change in temperature.
Zn + H2SO4 → ZnSO4 + H2↑ + Heat


Answer 24:
(a) 2Al(OH)3 (aq) + 3H2SO4 (aq) → Al2(SO4)3 (aq) + 6H2O (l)
(b) MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O


Answer 25:
(a) MgCO3 (s) + 2HCl (aq) → MgCl2 (aq) + CO2 (g) + H2O (l)
(b) 2NaOH (aq) + H2SO4 (aq) → Na2SO4 (aq) + 2H2O (l)


Answer 26:
Carbon monoxide reacts with hydrogen under the following conditions:
(i) 300 atm. pressure
(ii) 300oC temperature
(iii) in the presence of ZnO and CrO3 as catalysts
The balanced chemical equation for the given reaction is as follows:
CO (g) + 2H2 (g) →ZnO +  CH3OH (l)


Answer 27:
(a) 2KClO3 (s) → MnO2 ∆ 2KCl (s) + 3O2 (g)↑
(b) 2Mg (s) + CO2 (g) → 2MgO (s) + C (s)


Answer 28:
(a) Ca(CO3) (s) + 2HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 (g)
(b) NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)


Answer 29:
4NH3 (g) + 3O2 (g) → 2N2 (g) + 6H2O (l)


Answer 30:
6CO2 (g) + 6H2O (l) →ChlorophyllSunlight C6H12O6 (aq) + 6O2 (g)


Answer 31:
3BaCl2 (aq) + Al2(SO4)3 (aq) → 3BaSO4 (s) + 2AlCl3 (aq)


Answer 32:
2KNO3 (s) → ∆ 2KNO2 (s) + O2 (g)


Answer 33:.
(a) Chemical reaction is the process in which atoms of some elements rearrange—that is, undergo a chemical change—to form a new product with different properties. This involves breaking of old bonds between the reacting atoms and making of new chemical bonds between the rearranged atoms.
For example, the burning of magnesium ribbon in air. In this reaction, magnesium ribbon, on heating, combines with oxygen to form a white powder of magnesium oxide, which is totally different in its properties from magnesium and oxygen.
2Mg(s) + O2(g) →heat 2MgO (s)
(b)
(i) Evolution of gas: When zinc granules react with dilute sulphuric acid in a flask, hydrogen gas evolves and zinc sulphate solution is formed.
Zn+ H2SO4→ ZnSO4+ H2
(ii) Change in colour: When sulphur dioxide gas is passed through acidified potassium dichromate solution, orange colour of potassium dichromate solution changes to green colour.
3SO2(g) + K2Cr2O7 (aq) + H2SO4(aq) → K2SO4(aq) + Cr2(SO4)3(aq) + H2O(l)
(iii) Formation of a precipitate: When potassium iodide is added to lead nitrate solution (colourless and clear), yellow precipitate of lead iodide solution is formed.
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
(iv) Change in temperature: When quicklime reacts with water, slaked lime is formed with a release of large amount of heat, causing change in temperature in the reaction mixture.
CaO +H2O →Ca(OH)2 + Heat
(v) Change in state: The combustion reaction of candle wax (solid) melts initially to produce molten wax (liquid) and then produces carbon dioxide (gas) and water vapour (liquid). So, in this reaction, state of matter gets changed from solid to liquid and gas.


Exercise : Solution of Questions on page Number : 49


Answer 34:
Following are the various characteristics of a chemical reaction:
Evolution of gas: Zinc and dilute sulphuric acid react to form zinc sulphate with the evolution of hydrogen gas.
Formation of precipitate: Sulphuric acid and barium chloride react to form white precipitates of barium sulphate.
Change in colour: Purple colour of potassium permanganate solution on addition of citric acid changes to colourless solution.
Change in temperature: Barium hydroxide and ammonium chloride react to form barium chloride, ammonia and water. This is an endothermic reaction in which heat is absorbed, causing the temperature to fall.
Change in state: Combustion of candle wax (solid) leads to the formation of water (liquid) and carbon dioxide (gas).
(b)
(i) When sodium carbonate is added to dilute hydrochloric acid, carbon dioxide gas evolves. Hence, this reaction is characterised by evolution of gas.
(ii) Lemon juice (contains citric acid) changes the purple colour of potassium permanganate solution to a colourless solution. Hence, this reaction is characterised by change in colour.
(iii) When dilute sulphuric acid is added to barium chloride solution, it forms white precipitates of barium sulphate. Hence, this reaction is characterised by formation of precipitate
(iv) When quicklime is treated with water, slaked lime is formed with the evolution of large amount of heat. Hence, this reaction is characterised by change in temperature.
(v) When wax (solid) is burnt in the form of candle, it produces water (liquid) and carbon dioxide (gas). Hence, this reaction is characterised by change in state.


Answer 35:
(a) Exothermic reaction is the chemical reaction that produces heat along with products.
Example: CaO + H2O → Ca(OH)2 + Heat
Endothermic reaction is the chemical reaction that consumes heat to form products.
Example: N2 + O2 + Heat → 2NO
(b) Example of exothermic reaction: Zinc granules on reacting with dilute sulphuric acid produce zinc sulphate along with heat energy.
Zn + H2SO4 → ZnSO4 + H2
Example of endothermic reaction: Barium hydroxide on reacting with ammonium chloride produces ammonia, water and barium chloride. It also results in decrease in temperature, as it consumes heat energy.
Ba(OH)2 + 2NH4Cl → BaCl2 +2NH3 + H2O
(c)
(i) Burning of natural gas: Exothermic reaction
Burning of natural gas releases a huge amount of heat energy.
(ii) Photosynthesis: Endothermic reaction
Photosynthesis occurs by absorbing light energy from sunlight.
(iii) Electrolysis of water: Endothermic reaction
Electrolysis of water needs energy to break water into hydrogen and oxygen.
(iv) Respiration: Exothermic reaction
Respiration is an exothermic reaction in which glucose undergoes combustion to produce carbon dioxide, water and energy.
(v) Decomposition of calcium carbonate: Endothermic reaction
Decomposition of calcium carbonate is an endothermic reaction because it needs energy to decompose into carbon dioxide and calcium oxide.


Answer 36:
(c) atoms of one element change into those of another element to form new products
It is not atoms but the bonds between these atoms break and form during chemical reactions. Atoms of elements can rearrange but cannot change into other element.


Answer 37:
(d) melting of candle wax on heating
On heating, wax changes only its physical state (solid to liquid), not its properties. Hence, only physical change takes place.


Answer 38:
(d) potassium iodide
Potassium iodide on reacting with lead nitrate gives yellow precipitate of lead iodide.


Answer 39:
(b) citric acid
Reaction of purple-coloured potassium permanganate with citric acid is characterised by change in colour from purple to colourless.


Answer 40:
(a) potassium dichromate solution and sulphur dioxide
Reaction between these two are characterised by change in colour. The orange colour of acidified potassium dichromate changes to green on passing sulphur dioxide.


Answer 41:
(c) change in temperature of mixture
Reaction of quicklime and water is highly exothermic, i.e., it evolves heat energy and changes the temperature of mixture.


Answer 42:
(b) combination of nitrogen and oxygen to form nitrogen monoxide
When nitrogen and oxygen are heated at very high temperature (3,000oC approx), they form nitrogen monoxide. Hence, it is an endothermic reaction.


Answer 43:
(d) C6H12O6 + 6O2 → 6CO2 + 6H2O
This is the reaction of respiration process in which glucose burns in oxygen to produce heat energy needed by our body. So, this reaction is an exothermic reaction


Exercise : Solution of Questions on page Number : 22


Answer 44:
(c) process of respiration
In the process of respiration, heat energy that maintains our body temperature is released. Therefore, this process is an exothermic reaction.


Answer 45:
(b) law of conservation of mass
Equations are balanced when the mass of atoms of different elements in reactants’ side equals the atoms in products’ side. If it is not so, it means that matter is either created or destroyed in reactions, which is impossible.


Answer 46:
(a) Substance X is likely to be lead nitrate, because on reacting with potassium iodide it forms a yellow precipitate.
(b) The yellow solid or precipitate consists of lead iodide, which is one of the products in this reaction.
(c) This chemical reaction is characterised by the formation of precipitate.
(d) 2KI (s) + Pb(NO3)2 (aq) → PbI2 (s) + 2KNO3 (aq)


Answer 47:.
(a) Solid X is calcium oxide, commonly known as quicklime. Its chemical formula is CaO.
(b) Product Y is calcium hydroxide, commonly known as slaked lime. Its chemical formula is Ca(OH)2.
(c) Common name of the solution of Y is “lime water”, which is used to test carbon dioxide gas.
(d) CaO (s) + H2O (l) → Ca(OH)2 (aq) + HEAT
(e) This chemical reaction is characterised by change in temperature because a huge amount of heat is evolved in this reaction, causing rise in temperature of the mixture.


Answer 48:
(a) Zinc (Zn) and magnesium (Mg) metals can behave like metal X.
(b) Sulphuric acid (H2SO4) and hydrochloric acid (HCl) can behave like acid Y.
(c) Gas Z is hydrogen gas (H2).
(d) Gas Z (i.e., hydrogen) is lighter than air because it is the lightest element in the periodic table.
(e) The reaction between metal X and dilute acid Y is exothermic because it produces huge amount of heat.
(f) If X is zinc and Y is sulphuric acid, then the equation can be written as follows:
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)


Answer 49:
(a) Substance P is calcium carbonate. Its common name is limestone and its chemical formula is CaCO3.
(b) Substance Q is calcium oxide (CaO).
(c) Gas R is carbon dioxide gas (CO2).
(d) Substance S is calcium hydroxide [Ca(OH)2]. Its clear solution is known as lime water.
(e) Substance T is calcium carbonate (CaCO3). Two natural forms of calcium carbonate are calcite and mollusc shell.


Exercise : Solution of Questions on page Number : 23


Answer 50:
(a) Metal X could be magnesium metal.
(b) Powder Y is magnesium oxide (MgO).
(c) Magnesium metal (X) combines with oxygen gas to form powdery magnesium oxide (Y).
(d) Substance Z is magnesium hydroxide [Mg(OH)2]. Suspension of magnesium hydroxide (Z) is used as an antacid.
(e) 2Mg (s) + O2 (g) → 2MgO (s)


Answer 51:
(a) Metal X could be copper (Cu).
(b) The salt XSO4 is copper sulphate; its formula is CuSO4 and it is blue in colour.
(c) Copper hydroxide [Cu(OH)2] is the blue precipitate.
(d) CuSO4 (aq) + 2NaOH (aq) → Cu(OH)2 (s) + Na2SO4 (aq)


Answer 52:
(a) Metal M is sodium (Na), which is used as a coolant in nuclear reactors.
(b) Solution S is sodium hydroxide solution (NaOH). It is alkaline, it turns red litmus to blue.
(c) Gas G is hydrogen gas, which is lighter than air.
(d) 2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g) + HEAT
(e) This reaction is exothermic because it releases excessive heat.


Answer 53:
(a) X is carbon monoxide gas (CO); it is formed when a fuel burns in limited supply of air. Y is hydrogen gas (H2); it is formed by the action of dilute acid on active metal. Z is methanol (CH3OH); it is a liquid organic compound that can react with sodium metal to produce hydrogen gas.
(b) CO (g) + 2H2 (g) →ZnO + CH3OH       (l)


Answer 54:
(a) Compound A is potassium chlorate (KClO3).
(b) Compound B is potassium chloride (KCl).
(c) Gas C is oxygen gas (O2), which does not burn itself but helps burn other things.
(d) Black substance X is manganese oxide (MnO2). Its function is to catalyse the reaction.
(e) General name of substance like X (MnO2) is catalyst.


Answer 55:
(a) Gas A is carbon dioxide (CO2), which is the main cause of global warming.
(b) Common name of B is water (H2O).
(c) C is the sunlight, an environmental factor that helps in photosynthesis process.
(d) Material D is chlorophyll. It is a green material found in leaves of plants.
(e) Organic compound E is glucose (C6H12O6).
(f) Gas F is oxygen (O2) gas, which is necessary for breathing. It is released naturally during photosynthesis.


Exercise : Solution of Questions on page Number : 45


Answer 1:
Digestion of food in our body represents decomposition reaction. In this reaction, glucose undergoes slow combustion and combines with oxygen to produce carbon dioxide and water with evolution of heat energy.
C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + HEAT
Glucose Oxygen Carbon Water
dioxide


Answer 2:
The various types of chemical reactions are as follows:
Combination reaction
Decomposition reaction
Displacement reaction
Double displacement reaction
Oxidation and reduction reaction


Answer 3:
The colour of copper sulphate solution changes when iron nail is kept immersed in it because displacement reaction occurs between copper sulphate and iron to produce ferrous sulphate solution, which is green in colour.
CuSO4 (aq) + Fe (s) → FeSO4 (aq) + Cu (s)
Copper sulphate Iron Ferrous sulphate Copper
(Blue solution) (Nail) (Greenish solution) (Reddish brown nail)


Answer 4:
The balanced chemical equation reads as follows:
Zn (s) + 2AgNO3 (aq) → Zn(NO3)2 (aq) + 2Ag (s)


Answer 5:
“Rancidity” is the term used to indicate the development of unpleasant smell and taste in fat and oil containing foods due to aerial oxidation (when they are kept exposed for a considerable time).


Answer 6:
Antioxidants is the general name of the chemicals that are added to fat and oil containing foods to prevent the development of rancidity. Antioxidants are reducing agents that prevent food from getting oxidised easily.


Answer 7:
An important use of decomposition reaction is digestion of food in our body. It is because the carbohydrates and proteins in the food we eat decompose to simpler sugars like glucose and amino acids, respectively. These further break down to provide us energy to do work.


Answer 8:
Antioxidants are reducing agents that prevent oxidation of fats and oils. They are added to fatty and oily foods to prevent them from turning rancid (i.e., change in taste and smell of food) due to oxidation. Example: Butylated hydroxyanisole (BHA).


Answer 9:
Food products containing fats and oils are packaged in nitrogen gas because it is unreactive; also, no oxidation occurs because of the absence of oxygen. Hence, there is no rancidity (unpleasant smell).


Answer 10:
Examples of the decomposition reaction are as follows:
(a) with electricity:
2H2O (l) → 2H2 (g) + O2 (g) (electrolysis of water)
(b) by applying heat:
CaCO3 (s) → CaO(s) + CO2 (g)
(Limestone) (Quicklime)


Answer 11:
Decomposition reaction, which is carried out by electricity, is used to extract metals from their naturally occurring compounds like oxides or chlorides.


Answer 12:
The two main antioxidants that are usually added to fatty and oily foods to prevent rancidity are:
1. Butylated hydroxyanisole (BHA)
2. Butylated hydroxytoluene (BHT)


Answer 13:
(a) Decomposition reaction where energy is supplied in the form of heat:
CaCO3 (s) → CaO (s) + CO2 (g)
(b) Decomposition reaction where energy is supplied in the form of light:
2AgCl (s) → 2Ag (s) + Cl2 (g)
(c) Decomposition reaction where energy is supplied in the form of electricity:
2H2O (l) → 2H2 (g) + O2 (g)


Answer 14:
Here, the more electropositive Cu displaces lesser electropositive Ag.
Cu (s) + 2AgNO3 (aq) → 2Ag (s)↓ + Cu(NO3)2 (aq)


Answer 15:
(i) Decomposition reaction by the action of heat
(ii) Combination reaction
(iii) Decomposition reaction by the action of heat
(iv) Decomposition reaction by the action of heat
(v) Combination reaction


Exercise : Solution of Questions on page Number : 49


Answer 16:
(a) Combination reaction
(b) Decomposition reaction by the action of heat
(c) Decomposition reaction by the action of light
(d) Decomposition reaction by the action of electricity
(e) Combination reaction


Answer 17:
(i) Displacement reaction
(ii) Combination reaction
(iii) Decomposition reaction
(iv) Double displacement reaction
(v) Displacement reaction


Answer 18: 
(a) 2FeSO4 →Heat Fe2O3 + SO2 +SO3
(b) 2Pb(NO3)2 (s) →Heat 2PbO (s) + 4NO2 (g) + O2 (g)


Answer 19:
(a) Displacement reaction
(b) Combination reaction


Answer 20:
(a) Combination reaction
(b) Displacement reaction
(c) Displacement reaction
(d) Decomposition reaction
(e) Double displacement reaction


Answer 21:
(a) Hydrogen peroxide (H2O2) is reduced.
(b) Lead sulphide (PbS) is oxidised.
This is because lead sulphide gains oxygen, while hydrogen peroxide loses oxygen.


Answer 22:
In the above reaction, H2S is oxidised to S, as hydrogen is being removed.


Answer 23:
In the above reaction, SO2 is reduced to S as it loses oxygen. On the other hand, H2S, which gains oxygen, gets oxidised to H2O.


Answer 24:
(a) The addition of oxygen to a substance is called oxidation, whereas removal of oxygen is called reduction.
(b) The addition of hydrogen to a substance is called reduction, whereas removal of hydrogen is called oxidation.
(c) Anti-oxidants are often added to fat-containing foods to prevent rancidity due to oxidation.


Answer 25:
Oxidation reaction is a reaction that involves addition of oxygen to a substance and removal of hydrogen from a substance.
(i) C is oxidised to CO because oxygen is added.
(ii) ZnO is reduced to Zn because oxygen is removed.


Answer 26:
(a) The reaction in which one substance gets oxidised and other gets reduced is known as redox reaction.
Example:
ZnO + C → Zn + CO
Here, C is oxidised to CO because oxygen is being added and ZnO is reduced to Zn because oxygen is being removed. Therefore, it is a redox reaction.
(b) Magnesium gets oxidised because oxygen is added to the substance (Mg). The chemical reaction is given as follows:
2Mg + O2 → 2MgO
(c) (i) HCl is oxidised to Cl2.
(ii) MnO2 is the oxidising agent.
(iii) MnO2 is reduced to MnCl2.
(iv) HCl is a reducing agent.


Exercise : Solution of Questions on page Number : 47


Answer 27:
(a) The reaction in which a single product is formed from two or more reactants is called a combination reaction.
(b) Carbon burns in air to form carbon dioxide.
C (s) + O2 (g) → CO2 (g)
Heat is released during this reaction.
(c) Nitrogen burns in air to form nitric oxide.
N2 (g) + O2 (g) → 2NO (g)
Heat is absorbed during this reaction.


Answer 28:
(a) Burning of magnesium ribbon in a dazzling flame of air (i.e., oxygen) is an oxidation reaction.
The chemical reaction is given below:
2Mg + O2 → 2MgO
(b) Most of the oxidation reactions are exothermic reactions because energy is released during oxidation.
Example: C (s) + O2 (g) → CO2 (g)
(c) It can be explained by the following example:
CuO + H2 → Cu + H2O
In this reaction, CuO gets reduced to Cu by losing oxygen and H2 gets oxidised to H2O by gaining oxygen. Hence, oxidation and reduction proceed side by side.


Answer 29:
(a) The colour of ferrous sulphate crystal is green. After heating, the ferrous sulphate crystal loses water molecules and forms anhydrous ferrous sulphate, which is white in colour. Subsequently, it decomposes to give ferric oxide, which is brown in colour, sulphur dioxide and sulphur trioxide.
(b) The products formed are ferric oxide, sulphur dioxide and sulphur trioxide. During this change, decomposition reaction occurs.
The chemical reaction is given as follows:
2FeSO4 (s) → Fe2O3 (s) + SO2 (g) + SO3 (g)


Answer 30:.
Decomposition reaction is a chemical reaction in which a single compound breaks down to simpler products. It is the opposite of combination reaction.
Example: Decomposition of ferrous sulphate crystals.
2FeSO4 (s) → Fe2O3 (s) + SO2 (g) + SO3 (g)
Heat the ferrous sulphate crystals; the colour of ferrous sulphate crystals is green. On heating, the ferrous sulphate crystals lose their water molecules and therefore the colour of the crystals changes. It is a decomposition reaction because they decompose to simpler molecules.


Answer 31:
The balanced chemical reaction is as follows:
ZnO + C → Zn + CO
(i) Oxidising agent: ZnO
It oxidises C to CO.
(ii) Reducing agent: C
It reduces ZnO to Zn.


Answer 32:
(a) 2Cu + O2 → 2CuO
It is a combination reaction, as well as a redox reaction, because two reactants combine to form a single compound.
(b) CuO + H2 → Cu + H2O
In this reaction, Cu is being displaced from the reaction, so it is a displacement reaction. It is also a redox reaction because CuO is reduced to Cu and H2 is oxidised to H2O.


Answer 33:
(a)

Displacement Reaction Double Displacement Reaction
Displacement reaction is a chemical reaction in which displacement of a metal from its solution takes place by the other metal that is more reactive than the metal being displaced.  Double displacement reaction is a chemical reaction in which exchange of ions between both the reactants takes place.
Example:
Zn + CuSO4 → ZnSO4 + Cu
Here, Zinc (Zn), which is more reactive than Cu, displaces copper from its solution.
 Example:
Na2SO4 + BaCl2 → BaSO4 + 2NaCl
Here, both Na2SO4 and BaCl2 exchange their ions.

(b) Precipitation reaction: It is a chemical reaction in which an insoluble solid (precipitate) is formed that is separate from the solution.

Example: Na2SO4 + BaCl2 → BaSO4 + 2NaCl
Here, white precipitate of barium sulphate is formed.


Answer 34:
(a) (i) Oxidation is the gain of oxygen.
Example:
2Mg + O2 → 2MgO
Here, Mg is oxidised to MgO by accepting oxygen.
(ii) Reduction is the loss of oxygen.
Example:
ZnO + C → Zn + CO
Here, ZnO is reduced to Zn by losing oxygen.
(b) Balanced chemical reaction is as follows:
2Cu + O2 → 2CuO
(i) The substance oxidised is copper.
(ii) The substance reduced is oxygen.


Answer 35:
(i) Oxidation is the loss of hydrogen.
Example:
MnO2+ 4HCl → MnCl2 + 2H2O + Cl2
Here, HCl oxidises to Cl2 by losing hydrogen.
(ii) Reduction is the gain of hydrogen.
Example:
CuO + H2 → Cu + H2O
Here, CuO reduces to Cu by gaining hydrogen.
(b) Balanced chemical reaction is given as follows:
2Mg + O2 → 2MgO
(i) The substance oxidised is magnesium.
(ii) The substance reduced is oxygen.


Answer 36:
(a) Displacement reaction is a chemical reaction in which displacement of a metal from its solution takes place by the other metal that is more reactive than the metal being displaced.
Example:
Zn + CuSO4 → ZnSO4 + Cu
Here, Zn, being more reactive than Cu, displaces copper from its solution.
(b)Double displacement reaction is a chemical reaction in which both the reactants exchange their ions to form two new compounds.
Example:
Na2SO4 + BaCl2 → BaSO4 + 2NaCl


Answer 37:
(a) In decomposition reaction, a compound breaks down to two or more simple products, whereas in combination reaction, two or more reactants combine to form a single product. Therefore, both the reactions are opposite to each other.
For example, in a combination reaction, simple molecules of hydrogen and oxygen combine to form a water molecule.
2H2 (g) + O2 (g) → 2H2O (l)
While in decomposition reaction, reverse process takes place—​i.e., water decomposes to hydrogen and oxygen.
2H2O (l) → 2H2 (g) + O2 (g)
(b) The balanced chemical equation is given as follows:
Cu (s) + 2AgNO3 (aq) → Cu(NO3)2 (aq) + 2Ag (s)


Answer 38:
(a) When a piece of iron is placed in copper sulphate solution, it becomes brown in colour and the blue colour of the copper sulphate solution fades. This is because iron, being more reactive metal than copper, displaces copper from its solution.
Fe (s) + CuSO4 (aq) → FeSO4(aq) + Cu(s)
Thus, the type of reaction involved is displacement reaction.
(b) The balance chemical reaction is given as follows:
BaCl2 (aq) + 2NaSO4 (aq) → BaSO4 (s) + 2NaCl (aq)


Answer 39:
(a) H2 gets oxidised to H2O.
(b) CuO gets reduced to Cu.
(c) CuO is the oxidising agent.
(d) H2 is the reducing agent.


Answer 40:
(a)The following reaction takes place:
AgNO3 + NaCl → NaNO3 + AgCl
(b) Exchange of ions takes place in the given reaction. Hence, it is a double displacement reaction.


Answer 41:
When silver chloride is exposed to sunlight, decomposition of silver chloride takes place.The chemical reaction is given as follows:
2AgCl (s) → 2Ag (s) + Cl2 (g)
This type of reaction is used in black and white photography.


Answer 42:
When zinc strip is dipped into copper sulphate solution, it displaces copper from its solution because zinc is more reactive than copper.
(a) Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
(b) It is a displacement reaction.


Answer 43:
(a) Corrosion is the process by which a metal corrodes when it comes in contact with its surroundings such as moisture, air and acids, for example, powdered (brown) coating on iron objects such as iron railings.
In corrosion, oxidation of iron takes place.
Fe (s) → Fe2+ (aq) + 2e
Iron supplies electrons at the edge of the droplet to reduce oxygen from the air.
O2 (g) + 2H2O (l) + 4e → 4OH (aq)
Within the droplet, the hydroxide ions react with the iron(II) ions and iron(II) hydroxide is precipitated.
Fe2+ (aq) + 2OH (aq) → Fe(OH)2 (s)
Rust is then quickly produced by the oxidation of the precipitate.
4Fe(OH)2 (s) + O2 (g) → 2Fe2O3 • H2O (s) + 2H2O (l)
(b) Corrosion of iron is called rust.
(c) Redox reaction is involved in the corrosion of iron.
(d) Following are the three objects that are damaged by corrosion:
(i) Iron railings
(ii) Ships
(iii) Bridges


Answer 44:
(a) Rancidity is the process of oxidation of fats and oils that spoil the food in such a way that it becomes unpleasant to taste and smell.
Rancidity damages food items due to the development of unpleasant smell, generally in fatty and oily foods.
(b) Oxidation is responsible for causing rancidity.
(c) For preventing or retarding rancidity of food, following methods can be applied:
(1) Use of antioxidants:
Antioxidants are those substances that prevent oxidation.
(2) Use of nitrogen gas:
Nitrogen gas is added during the packaging of the potato chips to prevent oxidation of fat and oil because it is an unreactive gas.
(3) Use of airtight containers:
Preserving food in airtight containers reduces its exposure to oxygen and thus slows down the oxidation process.
(4) Use of refrigerator:
Preserving food in the refrigerator slows down rancidity.


Answer 45:
 (a) Exchange of ions between both the reactants takes place; therefore, it is an example of double displacement reaction. Since, precipitate of barium sulphate forms, it is also a precipitation reaction.
(b) The balanced chemical reaction is given as follows:
Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)
(c) Reaction will not take place if the reactants are in solid state.
(d) Double displacement reaction takes place.
(e) Another reaction of same type is given as follows:
AgNO3 (aq) + NaCl (aq) →  AgCl (s) + NaNO(aq)


Answer 46:
(c) reduction
The removal of oxygen from the substance is called reduction.
Example: ZnO + C → Zn + CO
In this reaction, zinc loses oxygen and hence gets reduced. On the other hand, carbon gains oxygen and gets oxidised.


Answer 47:
(a) oxidation
Removal of hydrogen from the substance is known as oxidation.


Answer 48:
(c) oxidation as well as combination
In the corrosion of iron metal, iron metal reacts with water and oxygen and forms hydrated oxide called rust.Addition of oxygen to the metal is a oxidation reaction.
4Fe + 3O2 + 2xH2O → 2Fe2O3.xH2O
Since iron metal combines with water and oxygen to corrode, it is also a combination reaction, in which iron metal, water and oxygen combine to form a single compound.


Answer 49:
(d) rancidity
The term used to indicate the development of unpleasant smell and taste in fat and oil containing foods due to aerial oxidation is rancidity.


Answer 50:
(c) N2
Nitrogen is an unreactive gas that prevents the spoilage of potato chips by preventing the oxidation of fat and oil present in the potato chips.


Answer 51:
(c) BaCl2 solution
BaCl2 + H2SO4 → BaSO4 + 2HCl
White ppt.


Answer 52:
(c) AgNO3 solution
On mixing aqueous solutions of silver nitrate and sodium chloride, a white curdy precipitate of silver chloride is formed. The chemical reaction is given below:
AgNO3 + NaCl → AgCl + NaNO3


Answer 53:
(d) combination reaction as well as oxidation reaction
In the above reaction, metal combines with the oxygen to form a single compound, so it is a combination reaction. Moreover, addition of oxygen takes place, so it is also a oxidation reaction.


Answer 54:
(d) an oxidation reaction that is exothermic
The food that we eat breaks down into simple compounds during digestion. For example, potatoes, bread and rice, which contain carbohydrates, breaks down to give glucose. Glucose combines with oxygen to give energy, so it is a oxidation reaction because addition of oxygen takes place; since energy is released in this process, it is also an exothermic reaction.
The chemical reaction is given as follows:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy


Answer 55:
(c) AgCl
The chemical reaction is given as follows:
2AgCl (s) → 2Ag (s) + Cl2 (g).


Answer 56:
(c) double displacement reactionIn the given reaction, exchange of the ions takes place between both the reactants, so it is a double displacement reaction.


Exercise : Solution of Questions on page Number : 49


Answer 57:
(c) redox reaction as well as displacement reaction
In the given reaction, magnesium takes oxygen and gets oxidised, whereas copper loses oxygen and gets reduced, so it is a redox reaction. Also magnesium, being more reactive metal, displaces copper from the copper solution and hence it is a displacement reaction.


Answer 58:
(a) The iron salt is ferrous sulphate crystal.
(b) Decomposition reaction takes place during the heating of iron salt.
(c) The chemical reaction involved is given below:
2FeSO4 (s) → Fe2O3 (s) + SO2 (g) + SO3 (g)


Answer 59:
(a) Lead salt is lead nitrate.
(b) The brown fumes is nitrogen dioxide gas.
(c) The chemical reaction is given as follows:
2Pb(NO3)2 (s) → 2PbO (s) + 4NO2 (g) + O2 (g)


Answer 60:
(a) Combination reaction takes place in the first case.
(b) Decomposition by electricity takes place in the second case.


Answer 61:
(a) Metal X is zinc.
(b) Metal Y is copper.
(c) Metal salt YSO4 is copper sulphate.
(d) Displacement reaction takes place. The chemical equation is given as follows:
Zn (s) + CuSO4 (aq) → ZnSO4 (s) + Cu (s)
(e) The alloy is brass.


Answer 62:
(a) The metal X is copper.
(b) The gas Y2 is H2.
(c) The compound XO is CuO.
(d) The compound Y2O is H2O.
(e) The chemical reaction is given as follows:
CuO + H2 → Cu + H2O
(f) The above reaction is a displacement reaction as well as a redox reaction.


Answer 63:
(a) Metal X is silver.
(b) Salt XNO3 is AgNO3.
(c) The compound Y is AgCl.
(d) AgNO3 (aq) + NaCl (aq) → NaNO3 (aq) + AgCl (s)
(e) The above chemical reaction is a double displacement reaction as well as a precipitation reaction.


Exercise : Solution of Questions on page Number : 50


Answer 64:
(a) Metal X is copper and metal Y is sodium.
(b) The salt is copper sulphate; its formula is CuSO4.
(c) The salt is sodium sulphate; its formula is Na2SO4.
(d) Compound Z is barium sulphate; its formula is BaSO4.
(e) The salt that turns the solution green is copper chloride; its formula is CuCl2.


Answer 65:
(a) The salt XSO4 is CuSO4.
(b) The colour of the salt CuSO4 is blue.
(c) The black precipitate XS is copper sulphide (CuS).
(d) CuSO4 + H2S → CuS + H2SO4
(e) Double displacement reaction takes place in this case.


Answer 66:
(a) Metal X is copper.
(b) Salt YNO3 is AgNO3.
(c) Metal Y is silver.
(d) The salt X(NO3)2 is Cu(NO3)2.
(e) Displacement reaction takes place between metal X and salt solution YNO3.


Answer 67:
(a) Metal M is silver (Ag).
(b) Gas X2 is chlorine (Cl2).
(c) Metal salt MX is AgCl.
(d) Solution of sodium chloride and silver nitrate on mixing together can produce a precipitate of salt MX.
(e) Decomposition reaction by light takes place.
2AgCl (s) → 2Ag (s) + Cl2 (g).


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