Question 1. Why should a magnesium ribbon be cleaned before burning in air?
Answer 1 : Magnesium is a very reactive metal like Na , Ca etc. When magnesium is exposed to air, it reacts with oxygen to form a layer of magnesium oxide.
2Mg + O2 ——->2 MgO
This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen.The magnesium ribbon is cleaned with sand paper to remove this layer so that the underlying metal can be exposed to air.
Question 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
Answer 2 :
1) H2 + Cl2 ——–> 2 HCl
2) 3BaCl2 + Al2(SO4)3——–> 3 BaSO4 + 2 AlCl3
3) 2 Na + 2 H2O —–> 2 NaOH + H2
Question 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.
1)BaCl2 (aq) + Na2SO4 (aq) ———-> BaSO4 (s) + 2 NaCl (aq)
2) NaOH (aq) + HCl (aq) ——> NaCl (aq) + H2O(l)
1. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
1) The substance X is Calcium Oxide.Its Chemical formula is CaO.
2) Calcium oxide reacts vigorously with water to form Calcium hydroxide (slaked lime)
CaO + H2O —-> Ca(OH)2
Question 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.
Answer 2 Water contains two parts hydrogen and one part oxygen. During the electrolysis of water , hydrogen and oxygen get separated by the electricity.Since hydrogen goes to one test tube and oxygen goes to another test tube , the amount of gas collected in one of the test tube is double of the amount collected in the other.
Question 1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer 1 : Iron is more reactive than copper.When an iron nail is dipped in copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.Therefore the blue colour of copper sulphate solution fades and green colour appears.
Fe + CuSO4 ——> FeSO4 + Cu
Question 2 Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer 2 : Na2CO3 + CaCl2 ——->CaCO3 + 2 NaCl
Sodium carbonate reacts with calcium chloride to from calcium carbonate and sodium chloride.In this reaction, Sodium carbonate and calcium chloride exchange ions to form calcium carbonate and sodium chloride.Hence, it is a double displacement reaction.
Question 3 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)
1) Sodium is oxidised as it gains oxygen and oxygen is reduced.
2) Copper oxide is reduced to copper while hydrogen gets oxidised to water.
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Question 1 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)
Answer 1 : (i) (a) and (b)
Question 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.
Answer 2 : (d) displacement reaction
Question 3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the
(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.
Answer 3 : (a) Hydrogen gas and iron chloride are produced
Question 4. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer 4 A reaction which has equal number of atoms of all the elements on both the sides of chemical equation is called a balanced chemical equation.
The law of conservation of mass states that mass can neither be created nor destroyed.Hence in a chemical reaction, the total mass of reactants should be equal to the total mass of the products.It means that total number of atoms of each element should be equal on both sides of a chemical equation.Hence, it is for this reason that chemical equation should be balanced.
Question 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.
Answer 5 :
a) 3 H2 (g) + N2 (g) ——> 2 NH3 (g)
b) 2 H2S (g) + 3 O2 (g) ——>2 SO2 (g) + 2 H2O (l)
c) 3 BaCl2 ( aq) + 3 Al2(SO4)3 (aq) ——> 2 AlCl3 (aq) + 3 BaSO4 (s)
d) 2 K (s) + 2 H2O (l) ———->2 KOH (aq) + H2 (g)
Question 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 + HCl
Answer 6 :
(a) 2 HNO3 + Ca(OH)2 → Ca(NO3)2 +2 H2O
(b)2 NaOH + H2SO4 → Na2SO4 +2 H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2 HCl
Question 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
Answer 7 :
a) Ca(OH)2 (aq) + CO2 (g) ——-> CaCO3 (s) + H2O(l)
b) Zn (s) + 2 AgNO3 (aq) ——> Zn(NO3)2 (aq) + 2 Ag (s)
c) 2 Al (s) + 3 CuCl2 (aq) ——> 2 AlCl3 (aq) + 2 Cu(s)
d) BaCl2 (aq) + K2SO4 (aq) ——> BaSO4 (s) + KCl(l)
Question 8. 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)
Answer 8 : (a) KBr(aq) + BaCl2 (aq)—-> 2 KI(aq) + BaBr2(s)
It is a double displacement reaction.
(b) ZnCO3 (s) ——–> ZnO(s) + CO2 (g)
It is a decomposition reaction.
(c) H2 (g) + Cl2 (g) ——-> 2 HCl (g)
It is a combination reaction.
(d) Mg(s) + 2 HCl (g) ——> MgCl2 (aq) + H2 (g)
It is a displacement reaction
Question 9. What does one mean by exothermic and endothermic reactions? Give examples.
Answer 9 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.
Examples:Burning of methane in air gives heat and light
CH4(g) + 2O2(g) —–> CO2(g) + 2H2O(g)
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.
Example: CaCO3(s) —–> CaO(s) + CO2(g)
Question 10. Why is respiration considered an exothermic reaction? Explain.
Answer 10 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
Question 11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer 11 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.
AB + Energy → A + B
2H2O(l) → 2H2(g)+O2(g) During electrolysis
A + B → AB + Energy
2CO + O2 → 2CO2 + Energy
Question 12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
(1) Thermal decomposition:
2FeSO4 (s) ——-> Fe2O3 (aq) + SO2 (g) + SO3 (g)
(2) Decomposition by light:
2AgCl(s) ——> 2Ag(s) + Cl2(g)
(3) Decomposition by electricity:
2Al2O3(aq) → 4Al(s) + 3O2(g)
Question 13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Difference between displacement and double displacement reactions is as follows :
|Displacement reaction||Double displacement reaction|
|In a displacement reaction, a more reactive element replaces a less reactive element from a compound.||In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds|
|In a displacement reaction, one displacement takes place.|
A + BX → AX + B
|In a double displacement reaction, two displacements take place to complete the reaction.|
AB + CD → AC + BD
|Change of colour takes place, no precipitate forms||Precipitates are formed|
|Metals react with salt solution of another metal||Salt solutions of two different metals react with each other.|
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
2KBr(aq) + BaCl2 → 2KCl(aq) + BaBr2(s)
Question 14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer 14 2AgNO3(aq)+ Cu(s) → Cu(NO3)2(aq) + 2Ag(s)
Silver nitrate Copper Copper nitrate silver
Question 15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer 15 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.
Question 16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation is the gain of oxygen.
For example: 2Cu+O2 → 2 CuO
Here copper is oxidised to copper oxide.
(b) Reduction is the loss of oxygen
CO2 + H2 ——> CO + H2O
Carbon-dioxide (CO2) is reduced to Carbon monoxide (CO)
Question 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.
Answer 17 X is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is:
2Cu + O2 → 2CuO
Question 18. Why do we apply paint on iron articles?
Answer 18 Iron articles are painted to 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.
Question 19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer 19 Nitrogen is an inert gas and does not easily react with oils and fats. 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.
Question 20. Explain the following terms with one example each.
(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.
Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, especially those of iron. Corrosion makes the metals scaly and decreases their 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.
Example : Iron in the presence of moisture react 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