Answer The Following Question.
1. Why should a magnesium ribbon be cleaned before burning in air?
Ans: Magnesium is a highly reactive metal that quickly forms a thin, stable layer of magnesium oxide when exposed to oxygen in the air. This oxide layer prevents further reaction of magnesium with oxygen. To ensure proper combustion, the magnesium ribbon is cleaned with sandpaper to remove this oxide layer, exposing the pure magnesium underneath for efficient burning.

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
Ans :
(i) H2+Cl2
2HCl
(ii)  3BaCl
+ Al2(SO4) 3BaSO4 +  2ACl3
(iii) 2Na +2H2O
2NaOH  + H2↑.

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.
Ans :
(i) BaCl2(aq) + Na2SO4 (aq)
BaSO4 (s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl (aq)
NaCl (aq) + H2O(l)

4. 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.
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)

5. 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.
In the electrolysis of water, the gas collected in double the amount is hydrogen. This is because water (H₂O) is composed of 2 parts hydrogen and 1 part oxygen by volume. During electrolysis, water splits into hydrogen and oxygen gases in this same ratio, resulting in twice the amount of hydrogen gas being collected compared to oxygen. 2H2O(l) 2H2(g) + O2(g)

6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
When an iron nail is dipped into copper sulphate solution, a chemical reaction occurs where iron displaces copper from the solution. This happens because iron is more reactive than copper. As a result, iron sulphate, which is green in colour, is formed, and copper is deposited. This causes the blue colour of copper sulphate solution to change to green. Fe(s) + CuSO(aq) ⟶ FeSO4(aq) + Cu(s)

7. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.
Na2CO3(aq) + CaCl2(aq) ⟶ CaCO3(s) + 2NaCl(aq)

8. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na+O2
2Na2O
(ii) CuO+H2 Cu(s)+H2O
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 waterH2O

9. 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 : As statement (a) and (b) are incorrect, answer (i) is correct.

10. Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction. Chemical Reactions and Equations 15
(c) decomposition reaction.
(d) displacement reaction.
Solution : This is an example of displacement reaction because Fe in FeO3 has been displaced by Al. Hence correct answer is (d).

11. 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. The reaction is as follows:
Fe(s) + 2HCl(aq) ⟶ FeCl2(aq) + H2

12. What is a balanced chemical equation? Why should chemical equations be balanced?
Ans: A balanced chemical equation is one in which the number of atoms of each element is the same on both the reactant and product sides of the equation.
Chemical equations should be balanced to comply with the law of conservation of mass, which states that matter can neither be created nor destroyed in a chemical reaction. A balanced equation also accurately represents the relative quantities of reactants and products involved in the reaction.

13. 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 :
(i) 3H2 + N2 ⟶ 2NH3
(ii) 2H2S + 3O2 ⟶ 2H2O + 2SO2
(iii)3BaCl2 + Al2(SO4)3  → 2AlCl3 + 3BaSO4
(iv) 2K+2H2O→ 2KOH+H2

 

14. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)+ H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl+AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Ans :
(a) 2HNO+ Ca(OH)2 ⟶ Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 ⟶ Na2SO+ 2H2O
(c)  NaCl + AgNO3 ⟶ AgCl + NaNO3
(d) BaCl2 + H2SO4 ⟶ BaSO4 + 2HCl


15. 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

16. 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) + BaI2(aq)  →  2Kl(aq)  +  BaBr2(s)
Type : Double displacement reaction
(b) ZnCO3 (s)  →  ZnO (s) + CO2 (g)
Type : Decomposition reaction
(c) H2 (g) +  Cl2 (g)  →  2HCl(g)
Type : Combination reaction
(d) Mg (s)  +  2HCl (aq)  →  MgCl2 (aq)  +  H2 (g)
Type : Displacement reaction

 

17. What does one mean by exothermic and endothermic reactions? Give examples?
Ans :
Exothermic reactions : Reactions in which heat is released are called exothermic reactions. These reactions are represented by writing “+ Heat” on the product side of the chemical equation.
Example :
(i) C (s) + O2 (g)  →  CO(g)  +  Heat
(ii) N2 (g)  +  3H2 (g) 
(ii) N2 (g)  +  3H2 (g)  →  2NH3 (g) + Heat
Endothermic reactionsReactions in which heat is absorbed are called endothermic reactions. These reactions are represented by writing “– Heat” on the product side of the chemical equation.
Examples :
(i) C (s)   +  2S (s) → CS2 (l) – Heat
(ii) N2 (g)  +  O2 (g)  →  2NO(g)  –  Heat

18. Why is respiration considered an exothermic reaction? Explain?
Ans :
Respiration is considered an exothermic reaction because it releases energy.
The food we eat is broken down into simpler molecules like glucose during digestion. In the cells, glucose (C6H12O6) reacts with oxygen (O2) in a process called respiration. This reaction produces carbon dioxide (CO2), water (H2O), and releases energy, which the body uses to perform various functions.
The chemical equation for respiration:
C6H12O6 (aq)+6O2 (g)→6CO2(g)+6 H2O(l)+ Energy
Since energy is released in the process, respiration is an exothermic reaction.
 

19. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions?
Ans : 
A combination reaction is when two or more molecules combine to form a single, larger molecule. On the other hand, a decomposition reaction is when a larger molecule breaks down into two or more smaller molecules. Simply put, decomposition reactions are the opposite of combination reactions.
Most decomposition reactions are endothermic, meaning they need energy (like heat, light, or electricity) to break the bonds of the larger molecule.
Few examples of decomposition reactions are:
ZnCO3  ZnO + CO2
CaCO3 + Energy CaO + CO2
2HgO 2Hg + O2

20. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Ans :
(a)Decomposition reaction where heat is supplied (Thermal decomposition):
CaCO3 (Heat) CaO + CO2
Thermal decomposition is a reaction where heat provides the energy required to break down a compound.
 
(b) Decomposition reaction where electricity is supplied (Electrolytic decomposition):
2H2O(l) 2H2(g) + O2(g)
Electrolytic decomposition is a reaction where electrical energy provides the activation energy to split a compound.
 
(c) Decomposition reaction where light is supplied (Photo decomposition):
2H2O2 2H2O + O2
Photodecomposition is a reaction where light energy causes the breakdown of a substance.

21. What is the difference between displacement and double displacement reactions? Write equations for these reactions?
Ans :
In a displacement reaction, a more reactive element displaces a less reactive element from its compound. Only a single displacement occurs. In a double displacement reaction, there is an exchange of ions between two compounds, forming two new compounds.
Key Difference:
Displacement Reaction: One element displaces another.
Double Displacement Reaction: Ions of two compounds exchange places.
Example:
Displacement reaction
Mg + 2HCl MgCl+ H2
Double displacement reaction
2KBr + BaI2  2KI + BaBr2


22. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal.
Write down the reaction involved?
Ans :
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

23. What do you mean by a precipitation reaction? Explain by giving examples.
Ans :
A precipitation reaction occurs when two solutions containing soluble salts are mixed, leading to a double displacement reaction. In this reaction, the ions from the two compounds exchange places, and one of the products formed is insoluble in water. This insoluble product, called a precipitate, settles down at the bottom of the container.
 Few examples of precipitation reactions are:
CdSO4(aq) + K2S(aq) CdS(s) + K2SO4(aq)
2NaOH(aq) + MgCl2(aq) 2NaCl(aq) + Mg(OH)2(s)

24. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction

Ans:
(a) Oxidation: Oxidation refers to the gain of oxygen or the loss of hydrogen in a chemical reaction.
For Examples
(i) CO2  + H2  CO + H2O
In this reaction, hydrogen (H2) is oxidized to water (H2O), meaning it gains oxygen.
(ii) 2Cu + O2  2CuO
Here, copper (Cu) is oxidized to copper oxide (CuO), meaning it gains oxygen.
 
(b) Reduction: Reduction refers to the loss of oxygen or the gain of hydrogen in a chemical reaction.
For example
(i)ZnO + C Zn + CO
Zinc oxide (ZnO) is reduced to zinc (Zn) as it loses oxygen.
(ii)CuO + H2  Cu + H2O
Copper oxide (CuO) is reduced to copper (Cu) as it loses oxygen and gains hydrogen.

25. 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 brown coloured element ‘X ‘ is copper. On heating in air it forms copper oxide, which is black in colour.
2Cu(s)   +    O2→  2CuO
Brown      (from air)            Black

26. Why do we apply paint on iron articles?
Ans :
We apply paint on iron articles to prevent rusting. The paint forms a protective layer that keeps the iron from coming into contact with atmospheric oxygen and moisture, both of which are necessary for the formation of rust. By preventing this contact, rusting is inhibited.

27. Oil and fat containing food items are flushed with nitrogen. Why?
Ans :
Oil and fat-containing food items are flushed with nitrogen to prevent rancidity. Nitrogen is an inert gas, meaning it does not react with the food items. On the other hand, oxygen can react with oils and fats, leading to oxidation, which causes rancidity. By flushing the food packaging with nitrogen, the oxygen is removed, thus preventing the oxidation process and keeping the food fresh for longer.

28. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Ans :
(a) Corrosion:
Corrosion is the process in which materials, usually metals, deteriorate due to a chemical reaction with environmental factors such as air, moisture, or chemicals.
Example:
Iron reacts with oxygen and moisture to form hydrated iron oxide, commonly known as rust:
4Fe + 3O2 + nH2O 2Fe2O3, nH2O

(b) Rancidity:
Rancidity is the condition that occurs when fats and oils in food undergo oxidation, leading to the development of unpleasant smells and tastes. This spoils the food, making it unfit for consumption.
Example:
Rancidity occurs in foods like butter or fried snacks when they are exposed to air, leading to the breakdown of fats. It can be prevented by using antioxidants or by flushing the packaging with nitrogen to remove oxygen.
 

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