2nd Year CHAPTER 2 (s-Block Elements) Answer And Question
Short And Simple Questions And Answer
Q1: Why is the aqueous solution of Na2CO3 alkaline in nature?
Ans: The aqueous solution of Na2CO3 is alkaline because it produces NaOH and H2CO3 when dissolved in water: Na2CO3(s)+2H2O(l)→NaOH(aq)+H2CO3(aq) Since H2CO3 is a weak acid and NaOH is a strong base, the solution is alkaline.
Q2: How do LiNO3 and NaNO3 differ in the application of heat? Provide corresponding equations?
Ans: LiNO3, upon heating, decomposes to form Li2O and NO2 gas. On the other hand, NaNO3 gives NaNO2 and oxygen gas, as shown in the equations:
24LiNO3→2Li2O+4NO2
O2 22NaNO3→2Na+NO2
Q3: Why is a solution of Na2O in water alkaline? Justify the statement?
Ans: The solution of Na2O in water is alkaline because it produces NaOH, which is a strong alkali: Na2O+H2O→2NaOH
Q4: Why is 2% gypsum added to cement?
Ans: 2% gypsum is added to cement to slow down its hardening process. Gypsum increases the setting time of cement.
Q5: What happens when?
a. Li2CO3 is heated
b. Na2CO3 is heated
Ans: a. Li2CO3 ͢ 2Li2O + CO2
b. 2Na2CO3 ͢ 2NaNO2 + O2
Q6: Give the formulas of Natron and Halite?
| Natron: Na2CO3⋅10H2O |
| Halite: NaCl |
7. Write two points of difference of Beryllium with its family members?
Ans: 1. Beryllium metal is almost as hard as iron and hard enough to scratch the glass. The other alkaline
earth metals are much softer than beryllium.
2. Beryllium is the only member of its group which reacts with alkalies to give hydrogen. The other
members do not react with alkalies.
Be (s) + NaOH (aq) ͢ Na2Be2O (s) + H2 (g)
Q8: Prove that decomposition of lithium nitrate gives different products than nitrates of other alkali
metals?
Ans: Decomposition of lithium nitrate gives different products than nitrates of other alkali metals as
shown below:
4 LiNO3 ͢ 2Li2O + 4NO2 + O2
2NaNO3 ͢ 2NaNO2 + O2
Q9: How lime and sand are used to make glass?
Ans: Lime reacts with sand at high temperature to form calcium silicate (CaSiO3), which is the basis for glass manufacture. The addition of lime increases the hardness and chemical durability of glass.Ans: Lime reacts with sand at high temperature to form calcium silicate (CaSiO3), which is the basis for glass manufacture. The addition of lime increases the hardness and chemical durability of glass.
Q10: Why is lime added to acidic soil?
Ans: Lime is added to acidic soil to neutralize its acidity. This application of lime helps increase the amount of readily soluble phosphorus in the soil.Ans: Lime is added to acidic soil to neutralize its acidity. This application of lime helps increase the amount of readily soluble phosphorus in the soil.
Q11: What is the action of litmus with an aqueous solution of Na2CO3?
Ans: The aqueous solution of Na2CO3 is alkaline, and it turns litmus solution red due to the formation of the strong alkali NaOH.Ans: The aqueous solution of Na2CO3 is alkaline, and it turns litmus solution red due to the formation of the strong alkali NaOH.
Na2CO3 + 2H2O→NaOH + H2CO3Na2CO3 + 2H2O→NaOH + H2CO3
Q12: What is Plaster of Paris?
Ans: When gypsum is heated under carefully controlled conditions, it loses three quarters of water of
crystallization. The resulting product is called Plaster of Paris.
2CaSO4. 2H2O (s) ͢ (CaSO4)2.H2O (s) + 3H2O
Gypsum ͢ Plaster of Paris
Q13: What is Plaster of Paris? Give its two applications?
Ans: When gypsum is heated under carefully controlled conditions, it loses three quarters of water of
crystallization. The resulting product is called Plaster of Paris.
2CaSO4. 2H2O (s) ͢ (CaSO4)2.H2O (s) + 3H2O
Gypsum Plaster of Paris
Applications:
1. It is used for making plaster walls, casts of statuary, coins, etc.
͢2. It is used in surgery. Plaster of Paris bandages are used for holding in place fractured bones after they
have been set.
Q14: Why lime water turns milky by passing CO2 gas but becomes clear with excess of CO2?
Ans: Lime water turns milky by passing CO2 gas due to formation of insoluble CaCO3
Ca(OH)2 (s) + CO2 ͢ CaCO3 (s) + H2O (l)
On further addition of CO2, insoluble CaCO3 changes into soluble CaHCO3
CaCO3 (s) + H2O (l) + CO2 ͢ CaHCO3 (aq)
Q15: Why KO2 is used in breathing equipment used by mountaineers and astronauts?
Ans: KO2 is used in breathing equipment used by mountaineers and astronauts because it has ability to
absorb carbon dioxide while giving out oxygen at the same time.
4KO2 (s) + 2CO2 ͢ 2K2CO3 (s) + 3O2 (g)
Q16: Give four points in which lithium differs from its own family members?
Ans: 1.Lithium reacts very slowly with water while other alkali metals react violently.
2.Lithium hydride is more stable than the hydrides of other alkali metals.
3.The lithium salts of anions with high charge density are generally less soluble in water than those of
the other alkali metals, e.g. LiOH, LiF etc.
4.Lithium is the least reactive metal of all the alkali metals.
Q17: Reaction of alkali metal oxide with water is acid-base reaction and not an oxidation-reduction
reaction. Justify?
Ans: Alkali metal oxides dissolve in water to give strong alkaline solution. This reaction is an acid base
reaction and not an oxidation reduction reaction since no element undergoes a change in its oxidation
number. It has been shown by the following reaction:
Li2O (s) + H2O (l) ͢ 2LiOH (aq)
Q18: Give two major problems that may arise in Nelson’s cell?
Ans: 1. Chlorine produced can react with hydroxide ions in cold giving hypochlorite ions.
Cl2 (g) + 2OH-
(aq) ͢ OCl-
(aq) + Cl-
(aq) + H2O (l)
2.Hydroxide ions may be attracted towards anode, where they can be discharged releasing oxygen
gas. This oxygen gas may contaminate the chlorine and renders it impure.
Q19: Give advantages of Down’s Cell?
Ans: 1.The metallic fog is not produced.
2.Liquid sodium can easily be collected at 6000C.
3.Material of the cell is not attacked by the products formed during the electrolysis.
Q20: Why alkali and alkaline earth metals are the most reactive elements of the periodic table?
Ans: Since ionization energies of alkali and alkaline earth metals are very low and they have large
atomic sizes, therefore they are the most reactive elements of the periodic table.
Q21: BeO is Amphoteric. Prove it?
Ans: BeO is amphoteric as it can behave both as an acid as well as a base as shown by the following
equation s:
BeO + H2SO4 ͢ BeSO4 + H2O
BeO + NaOH ͢ Na2BeO2 +H2O
Q22: What is lime water and milk of magnesia?
Ans:
| Lime water Ca(OH)2 |
| Milk of magnesia Mg(OH)2 |
Q23: How gypsum is converted into plaster of Paris?
Ans: When gypsum is heated under carefully controlled conditions, it loses three quarters of water of
crystallization. The resulting product is called Plaster of Paris.
2CaSO4. 2H2O (s) ͢ (CaSO4)2.H2O (s) + 3H2O
Gypsum Plaster of Paris
Q24: What is lime mortar? How is it formed?
Ans: Lime mortar or Ordinary mortar is prepared by mixing freshly prepared slaked lime (one volume)
with sand (three or four volumes) and water to form a thick paste. This material when placed between
the stones and bricks hardens or sets, thus binding the blocks firmly together. The equations for the
chemical reactions which take place when lime mortar hardens are:
CaO (s) + H2O (l) ͢ Ca(OH)2 (s)
Ca(OH)2 (s) + CO2 (g) ͢ CaCO3 (s) + H2O (l)
Ca(OH)2 (s) + SiO2 (s) ͢ CaSiO3 (s) + H2O (l)
