Text Book Back Questions and Answers - Chapter 5 Alkali and Alkaline Earth Metals 11th Chemistry Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

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On April 24, 2024, 11:35 AM

Alkali and Alkaline Earth
Textual Evaluation Solved
I. Choose The Correct Answer:
Question 1.
For alkali metals, which one of the following trends are incorrect?
(a) Hydration energy: $\mathrm{Li}>\mathrm{Na}>\mathrm{K}>\mathrm{Rb}$
(b) Ionization energy: $\mathrm{Li}>\mathrm{Na}>\mathrm{K}>\mathrm{Rb}$
(c) Density : $\mathrm{Li}<\mathrm{Na}<\mathrm{K}<\mathrm{Rb}$
(d) Atomic size : $\mathrm{Li}<\mathrm{Na}<\mathrm{K}<\mathrm{Rb}$
Answer:
(c) Density : $\mathrm{Li}<\mathrm{Na}<\mathrm{K}<\mathrm{Rb}$
Potassium is lighter than sodium. The correct order of density is
$\mathrm{Li}<\mathrm{K}<\mathrm{Na}<\mathrm{Rb}<\mathrm{Cs}$
$0.54<0.86<0.97<1.53<1.90\left(\right.$ in $\mathrm{g} \mathrm{cm}^3$ ).
Question 2.
Which of the following statements are incorrect?
(a) $\mathrm{Li}+$ has minimum degree of hydration among alkali metal cations.
(b) The oxidation state of $\mathrm{K}$ in $\mathrm{KO}_2$ is +1 .
(c) Sodium is used to make $\mathrm{Na} / \mathrm{Pb}$ alloy.
(d) $\mathrm{MgSO}_4$ is readily soluble in water.

Answer:
(a) $\mathrm{Li}+$ has minimum degree of hydration among alkali metal cations.
$\mathrm{Li}+$ has maximum degree of hydration among alkali metal cations.
$
\mathrm{Li}^{+}>\mathrm{Na}^{+}>\mathrm{K}^{+}>\mathrm{Rb}^{+}>\mathrm{Cs}^{+}
$
Question 3.
Which of the following compounds will not evolve $\mathrm{H}_2$ gas on reaction with alkali metals?
(a) ethanoic acid
(b) ethanol
(c) phenol

(d) none of these
Answer:
(d) none of these
Hint:
All these compounds reacts with alkali metals to evolve hydrogen gas.
Question 4.
Which of the following has the highest tendency to give the reaction Aqueous -

(a) $\mathrm{Na}$
(b) $\mathrm{Li}$
(c) $\mathrm{Rb}$
(d) $\mathrm{K}$
Answer:
(b) $\mathrm{Li}$.
Hint:
Hydration energy of $\mathrm{Li}^{+}$is more and hence $\mathrm{Li}^{+}$is stabilized in aqueous medium.
Question 5.
Sodium is stored in ............
(a) alcohol
(b) water
(c) kerosene
(d) none of these
Answer:
(c) kerosene
Question 6.
$\mathrm{RbO}_2$ is
(a) superoxide and paramagnetic
(b) peroxide and diamagnetic
(c) superoxide and diamagnetic
(d) peroxide and paramagnetic
Answer:
(a) superoxide and paramagnetic
Hint:

$\mathrm{RbO}_2$ is a super oxide which contains $\mathrm{Rb}^{+}$and $\mathrm{O}_2$ ions. $\mathrm{O}_2$ - contains one unpaired electron and hence it is paramagnetic.
Question 7.
Find the wrong statement
(a) sodium metal is used in organic qualitative analysis
(b) sodium carbonate is soluble in water and it is used in inorganic qualitative analysis
(c) potassium carbonate can be prepared by Solvay process
(d) potassium bicarbonate is acidic salt
Answer:
(c) Potassium carbonate can be prepared by Solvay process
Hint:
Potassium carbonate cannot be prepared by Solvay process. Potassium bicarbonate is fairly soluble in water and does not precipitate out.

Question 8.
Lithium shows diagonal relationship with
(a) sodium
(b) magnesium
(c) calcium
(d) aluminium
Answer:
(b) magnesium (diagram pending)
Question 9.
In case of alkali metal halides, the ionic character increases in the order
(a) $\mathrm{MF}<\mathrm{MCl}<\mathrm{MBr}<\mathrm{MI}$
(b) $\mathrm{MI}<\mathrm{MBr}<\mathrm{MCl}<\mathrm{MF}$
(c) $\mathrm{MI}<\mathrm{MBr}<\mathrm{MF}<\mathrm{MCl}$
(d) none of these
Answer:
(b) $\mathrm{MI}<\mathrm{MBr}<\mathrm{MCl}<\mathrm{MF}$
Hint:
Ionic character (difference in electronegativity) $\mathrm{MI}<\mathrm{MBr}<\mathrm{MCl}<\mathrm{MF}$
Question 10.
In which process, fused sodium hydroxide is electrolysed for extraction of sodium?
(a) Castner's process
(b) cyanide process
(c) Down process
(d) All of these
Answer:

$
\mathrm{NaOH} \rightleftharpoons \mathrm{Na}^{+}+\mathrm{OH}^{-}
$
Cathode $: \mathrm{Na}^{+}+\mathrm{e}^{-} \rightarrow \mathrm{Na}$
Anode $: 2 \mathrm{OH}^{-} \rightarrow \mathrm{H}_2 \mathrm{O}+1 / 2 \mathrm{O}_2+2 \mathrm{e}^{-}$
Question 11.
The product obtained as a result of a reaction of nitrogen with $\mathrm{CaC}_2$ is (NEET Phase I)
(a) $\mathrm{Ca}(\mathrm{CN})_3$
(b) $\mathrm{CaN}_2$
(c) $\mathrm{Ca}(\mathrm{CN})_2$
(d) $\mathrm{Ca}_3 \mathrm{~N}_2$
Answer:
(c) $\mathrm{Ca}(\mathrm{CN})_2$
Hint:

Question 12.
Which of the following has highest hydration energy?
(a) $\mathrm{MgCl}_2$
(b) $\mathrm{CaCl}_2$
(c) $\mathrm{BaCl}_2$
(d) $\mathrm{SrCl}_2$
Answer:
(a) $\mathrm{MgCl}_2$
Hint:
The order of hydration energy of alkaline earth metal is $\mathrm{Be}^{2+}>\mathrm{Mg}^{2+}>\mathrm{Ca}^{2+}>\mathrm{Sr}^{2+}$ $>\mathrm{Ba}^{2+}$
Question 13.
Match the flame colours of the alkali and alkaline earth metal salts in the bunsen burner
(p) Sodium - (1) Brick red
(q) Calcium - (2) Yellow
(r) Barium - (3) Violet
(s) Strontium - (4) Apple green
(t) Cesium - (5) Crimson red

(u) Potassium - (6) Blue
(a) $\mathrm{p}-2, \mathrm{q}-1, \mathrm{r}-4, \mathrm{~s}-5, \mathrm{t}-6, \mathrm{u}-3$
(b) $\mathrm{p}-1, \mathrm{q}-2, \mathrm{r}-4, \mathrm{~s}-5, \mathrm{t}-6, \mathrm{u}-3$
(c) $\mathrm{p}-4, \mathrm{q}-1, \mathrm{r}-2, \mathrm{~s}-3, \mathrm{t}-5, \mathrm{u}-6$
(d) $\mathrm{p}-6, \mathrm{q}-5, \mathrm{r}-4, \mathrm{~s}-3, \mathrm{t}-1, \mathrm{u}-2$
Answer:
(a) $\mathrm{p}-2, \mathrm{q}-1, \mathrm{r}-4, \mathrm{~s}-5, \mathrm{t}-6, \mathrm{u}-3$
(p) sodium - yellow (2)
(p) calcium - brick red (1)
(r) barium - apple green (4)
(s) strontium - crimson red (5)
(t) cesium - blue (6)
(u) potassium - violet (3)
Question 14.
Assertion : Generally alkali and alkaline earth metals form superoxides Reason : There is a single bond between $\mathrm{O}$ and $\mathrm{O}$ in superoxides.
(a) both assertion and reason are true and reason is the correct explanation of assertion
(b) both assertion and reason are true but reason is not the correct explanation of assertion
(c) assertion is true but reason is false
(d) both assertion and reason are false
Answer:
(d) both assertion and reason are false
Hint:
Among alkali and alkaline earth metals, $\mathrm{K}, \mathrm{Rb}$ and $\mathrm{Cs}$ alone forms superoxides. Superoxide $\mathrm{O}_2$ has 3 electron bond.
Question 15.
Assertion : $\mathrm{BeSO}_4$ is soluble in water while $\mathrm{BaSO}_4$ is not
Reason: Hydration energy decreases down the group from $\mathrm{Be}$ to $\mathrm{Ba}$ and lattice energy remains almost constant.
(a) both assertion and reason are true and reason is the correct explanation of assertion
(b) both assertion and reason are true but reason is not the correct explanation of assertion
(c) assertion is true but reason is false

(d) both assertion and reason are false
Answer:
(a) both assertion and reason are true and reason is the correct explanation of assertion
Question 16.
Which is the correct sequence of solubility of carbonates of alkaline earth metals?
(a) $\mathrm{BaCO}_3>\mathrm{SrCO}_3>\mathrm{CaCO}_3>\mathrm{MgCO}_3$
(b) $\mathrm{MgCO}_3>\mathrm{CaCO}_3>\mathrm{SrCO}_3>\mathrm{BaCO}_3$
(c) $\mathrm{CaCO}_3>\mathrm{BaCO}_3>\mathrm{SrCO}_3>\mathrm{MgCO}_3$
(d) $\mathrm{BaCO}_3>\mathrm{CaCO}_3>\mathrm{SrCO}_3>\mathrm{MgCO}_3$
Answer:
(b) $\mathrm{MgCO}_3>\mathrm{CaCO}_3>\mathrm{SrCO}_3>\mathrm{BaCO}_3$
Hint:
Solubility of carbonates decreases down the group.
Question 17.
In context with beryllium, which one of the following statements is incorrect?
(a) It is rendered passive by nitric acid
(b) It forms $\mathrm{Be}_2 \mathrm{C}$
(c) Its salts are rarely hydrolyzed
(d) Its hydride is electron deficient and polymeric
Answer:
(c) Its salts are rarely hydrolyzed
Hint:
Correct statement is beryllium salts are easily hydrolyzed

Question 18.
The suspension of slaked lime in water is known as (NEET Phase - II)
(a) lime water
(b) quick lime
(c) milk of lime
(d) aqueous solution of slaked lime
Answer:
(c) milk of lime
Hint:
Slaked lime $\mathrm{Ca}(\mathrm{OH})_2$. The suspension is called milk of lime and the clear solution is called lime water
Question 19.
A colourless solid substance (A) on heating evolved $\mathrm{CO}_2$ and also gave a white
residue, soluble in water. Residue also gave $\mathrm{CO}_2$ when treated with dilute $\mathrm{HCl}$.
(a) $\mathrm{Na}_2 \mathrm{CO}_3$
(b) $\mathrm{NaHCO}_3$
(c) $\mathrm{CaCO}_3$
(d) $\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2$
Answer:
(b) $\mathrm{NaHCO}_3$
Hint:

Question 20.
The compound $\mathrm{X})$ on heating gives a colourless gas and a residue that is dissolved in water to obtain (B). Excess of $\mathrm{CO}_2$ is bubbled through aqueous solution of $\mathrm{B}, \mathrm{C}$ is formed. Solid $(\mathrm{C})$ on heating gives back $\mathrm{X}$. (B) is ........
(a) $\mathrm{CaCO}_3$
(b) $\mathrm{Ca}(\mathrm{OH})_2$
(c) $\mathrm{Na}_2 \mathrm{CO}_3$
(d) $\mathrm{NaHCO}_3$
Answer:
(b) $\mathrm{Ca}(\mathrm{OH})_2$
Solution:
$
\begin{aligned}
& \mathrm{CaCO}_3 \stackrel{\Delta}{\rightarrow} \mathrm{CaO}+\mathrm{CO}_2 \\
& \mathrm{CaO}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2 \\
& \mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2 \rightarrow \mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O}
\end{aligned}
$
Question 21.
Which of the following statement is false ? (NEET - Phase -1)
(a) $\mathrm{Ca}^{2+}$ ions are not important in maintaining the regular beating of the heart
(b) $\mathrm{Mg}^{2+}$ ions are important in the green parts of the plants
(c) $\mathrm{Mg}^{2+}$ ions form a complex with ATP
(d) $\mathrm{Ca}^{2+}$ ions are important in blood clotting

Answer:
(a) $\mathrm{Ca}^{2+}$ ions are not important in maintaining the regular beating of the heart Hint: $\mathrm{Ca}^{2+}$ ion plays an important role in maintaining regular heart beat.
Question 22.
The name 'Blue John' is given to which of the following compounds?
(a) $\mathrm{CaH}_2$
(b) $\mathrm{CaF}_2$
(c) $\mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2$
(d) $\mathrm{CaO}$
Answer:
(b) $\mathrm{CaF}_2$
Hint:
'Blue john' $-\mathrm{CaF}_2$ (A variety of fluorite)
Question 23.
Formula of gypsum is
(a) $\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$
(b) $\mathrm{CaSO}_4 \cdot \frac{1}{2} 22 \mathrm{H}_2 \mathrm{O}$
(c) $3 \mathrm{CaSO}_4 \cdot \mathrm{H}_2 \mathrm{O}$
(d) $2 \mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$
Answer:
(a) $\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$
Question 24.
When $\mathrm{CaC}_2$ is heated in atmospheric nitrogen in an electric furnace the compound formed is
(a) $\mathrm{Ca}(\mathrm{CN})_2$
(b) $\mathrm{CaNCN}$
(c) $\mathrm{CaC}_2 \mathrm{~N}_2$
(d) $\mathrm{CaNC}_2$
Answer:
(b) $\mathrm{CaNCN}$

Question 25.
Among the following the least thermally stable is
(a) $\mathrm{K}_2 \mathrm{CO}_3$
(b) $\mathrm{Na}_2 \mathrm{CO}_3$
(c) $\mathrm{BaCO}_3$
(d) $\mathrm{Li}_2 \mathrm{CO}_3$
Answer:
(d) $\mathrm{Li}_2 \mathrm{CO}_3$
Hint:
$\mathrm{Li}_2 \mathrm{CO}_3$ is least stable.
II. Write brief answer to the following questions
Question 26.
Why sodium hydroxide is much more water soluble than chloride?
Answer:
$
\mathrm{NaOH}+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{Na}^{+}+\mathrm{OH}^{-}
$
1. This reaction is an exothermic reaction. Sodium hydroxide being a strong base, completely dissociated in aqueous medium. The heat evolved increases the stability. This phenomenon is strong enough to prove that sodium hydroxide crystals are readily dissolved in water.
2. $\mathrm{NaCl}$ is geologically stable. If kept dry, it will remain a free flowing solid for years. Water can dissolve $\mathrm{NaCl}$ because the $\mathrm{Na}^{+}$ions are attracted by $\mathrm{OH}^{-}$in water and $\mathrm{Cl}^{-}$ions are attracted by $\mathrm{H}^{+}$in water. The solubility of $\mathrm{NaCl}$ does not increase the temperature. Based on this, $\mathrm{NaOH}$ is much more soluble due to exothermic nature than sodium chloride.
Question 27.
Explain what to meant by efflorescence?
Answer:
1. Efflorescence is the formation of powdery deposit on the surface of rock as a result of loss of moisture or water on exposure to air.

2. Efflorescence is the formation of whitish powdery deposit on the surface of rocks like gypsum in dry regions. It is formed as mineral rich water, rises to the surface through capillary action and then evaporates.
3. Gypsum crystals are sometimes found to occur in the form that resembles the petals of flower. This happens mostly in arid areas or desert terrains, where there is rapid loss of water. This phenomenon is called as efflorescence.
Question 28.
Write the chemical equations for the reactions involved in Solvay process of preparation of sodium carbonate.
Answer:
Solvay process:
The Solvay process is represented by the below chemical equations:

Question 29.
An alkali metal (x) forms a hydrated sulphate, $\mathrm{X}_2 \mathrm{SO}_2 .10 \mathrm{H}_2 \mathrm{O}$. Is the metal more likely to he sodium (or) potassium.
Answer:
$\mathrm{X}$ forms $\mathrm{X}_2 \mathrm{SO}_2 .10 \mathrm{H}_2 \mathrm{O}$. The metal is more likely be sodium. So $\mathrm{X}$ is $\mathrm{Na}_2 \mathrm{SO}_4$. $10 \mathrm{H}_2 \mathrm{O}$. It is otherwise called as Glauber's salt.
Question 30 .
Write balanced chemical equation for each of the following chemical reactions.
(i) Lithium metal with nitrogen gas
(ii) Heating solid sodium bicarbonate
(iii) Rubidium with oxygen gas
(iv) Solid potassium hydroxide with $\mathrm{CO}_2$
(v) Heating calcium carbonate
(vi) Heating calcium with oxygen

Answer:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

Question 31.
Discuss briefly the similarities between beryllium and aluminium.
Answer:
Diagonal relationship:
As observed in alkali metals, beryllium shows a diagonal relationship with aluminium. Similarities between beryllium and aluminium:
1. Beryllium and aluminium have same electronegativity values.
2. Their changes per unit area is closer.
3. $\mathrm{BeCl}_2$ and $\mathrm{AlCl}_3$ forms dimeric structure. Both are soluble in organic solvents and are strong Lewis acids.
4. $\mathrm{Be}(\mathrm{OH})_2$ and $\mathrm{Al}(\mathrm{OH})_3$ dissolves in excess alkali to give beryllate ion $\left[\mathrm{Be}(\mathrm{OH})_4\right]^{2-}$ and aluminate ion $\left[\mathrm{Al}(\mathrm{OH})_4\right]^{-}$, respectively.
5. $\mathrm{Be}$ and $\mathrm{Al}$ ions have strong tendency to form complexes, e.g. $\mathrm{BeF}_4{ }^{2-}$ and $\mathrm{AlF}_6^{3-}$
6. Both $\mathrm{Be}(\mathrm{OFI})_2$ and $\mathrm{Al}(\mathrm{OH})_3$ are amphoteric in nature.
7. Carbide of $\mathrm{Be}$ - beryllium carbide $\left(\mathrm{Be}_2 \mathrm{C}\right)$ and $\mathrm{Al}$ - aluminium carbide $\left(\mathrm{Al}_4 \mathrm{C}_3\right)$ give methane on hydrolysis.
8. Both beryllium and aluminium are rendered passive by nitric acid.
Question 32.
Give the systematic names for the following:

1. milk of magnesia
2. lye
3. lime
4. caustic potash
5. washing soda
6. soda ash and
7. trona.
Answer:
1. Milk of magnesia $-\mathrm{Mg}(\mathrm{OH})_2-$ Magnesium hydroxide
2. Lye $-\mathrm{NaOH}-$ Sodium hydroxide
3. Lime $-\mathrm{Ca}(\mathrm{OH})_2$ Calcium hydroxide
4. Caustic potash $-\mathrm{KOH}-$ Potassium hydroxidc
5. Washing soda $-\mathrm{Na}_2 \mathrm{CO}_3 .10 \mathrm{H}_2 \mathrm{O}-$ Sodium carbonate decahydrate
6. Soda ash $-\mathrm{Na}_2 \mathrm{CO}_3-$ Sodium carbonate (anhydrous)
7. Trona $-\mathrm{NaCO}_3 \cdot \mathrm{NaHCO}_3 \cdot 2 \mathrm{H}_2 \mathrm{O}-$ Sodium sesqui carbonate
Question 33.
Substantiate lithium fluoride has the lowest solubility among group one metal fluorides.
Answer:
Lithium fluoride has high lattice enthalpy due to the small size of $\mathrm{Li}^{+}$and $\mathrm{F}^{-}$. So, due to the high lattice enthalpy, $\mathrm{LiF}$ is less soluble in water.
Question 34.
Mention the uses of Plaster of Paris.
Answer:
- The largest use of Plaster of Paris is in the building industry as well as plasters.
- It is used for immobilizing the affected part of organ, where there is a bone fracture or sprain.
- It is also employed in dentistry, in ornamental work and for making casts of statues and busts.

Question 35.
Beryllium halides are covalent whereas magnesium halides are ionic why?
Answer:
Beryllium ion $\left(\mathrm{Be}^{2+}\right)$ is smaller in size and it is involved in equal sharing of
electrons with halogens to form covalent bond, whereas magnesium ion $\left(\mathrm{Mg}^{2+}\right)$ is bigger and it is involved in transfer of electrons to form ionic bond.
Question 36.
Alkaline earth metal (A), belongs to $3^{\text {rd }}$ period reacts with oxygen and nitrogen to form compound (B) and $(\mathrm{C})$ respectively. It undergo metal displacement reaction with $\mathrm{AgNO}_3$ solution to form compound (D).
Answer:
1. An alkaline earth (A) metal belongs to third period is magnesium (Mg).
2. Magnesium reacts with oxygen to form magnesium oxide (MgO) (B).

3. Magnesium reacts with nitrogen to form magnesium nitride $\mathrm{Mg}_3 \mathrm{~N}_2(C)$.

4. Magnesium undergoes metal displacement reaction with $\mathrm{AgNO}_3$ solution to form magnesium nitrate $\mathrm{Mg}\left(\mathrm{NO}_3\right)_3$ (D).

Question 37.
Write balanced chemical equation for the following processes:
(a) heating calcium in oxygen
(b) heating calcium carbonate
(c) evaporating a solution of calcium hydrogen carbonate
(d) heating calcium oxide with carbon

Answer:

(a)

(b)

(c)

(d)

Question 38 .
Explain the important common features of group 2 elements. Important common features of group 2 elements.
Answer:
1. Group 2 elements except beryllium are commonly known as alkaline earth metals because their oxides and hydroxides are alkaline in nature and these metal oxides are found in the Earth's crust.
2. Many alkaline earth metals are used in creating colours and used in fireworks.
3. Their general electronic configuration is $\mathrm{ns}^2$.
4. Atomic and ionic radii of alkaline earth metals are smaller than alkali meta on moving down the group, the radii increases.
5. These elements exhibit +2 oxidation state in their compounds.
6. Alkaline earth metals have higher ionizatoin enthalpy values than alkali metals and they are less electropositive than alkali metals.
7. Hydration enthalpies of alkaline earth metals decreases as we go down the group.
8. Electronegativity values of alkaline earth metals decrease down the group.
9. Alkaline earth metal salts moistened with concentrated hydrochloric acid g a characteristic coloured flame, when heated on a platinum wire in a flame
Question 39.
Discuss the similarities between beryllium and aluminium.
Answer:
Diagonal relationship:
As observed in alkali metals, beryllium shows a diagonal relationship with aluminium. Similarities between beryllium and aluminium:

- Beryllium and aluminium have same electronegativity values.
- Their changes per unit area is closer.
- $\mathrm{BeCl}_2$ and $\mathrm{AlCl}_3$ forms dimeric structure. Both are soluble in organic solvents and are strong Lewis acids.
- $\mathrm{Be}(\mathrm{OH})_2$ and $\mathrm{Al}(\mathrm{OH})_3$ dissolves in excess alkali to give beryllate ion $\left[\mathrm{Be}(\mathrm{OH})_4\right]^{2-}$ and aluminate ion $\left[\mathrm{Al}(\mathrm{OH})_4\right]^{-}$, respectively.
- $\mathrm{Be}$ and $\mathrm{Al}$ ions have strong tendency to form complexes, e.g. $\mathrm{BeF}_4{ }^{2-}$ and $\mathrm{AlF}_6{ }^{3-}$
- Both $\mathrm{Be}(\mathrm{OFI})_2$ and $\mathrm{Al}(\mathrm{OH})_3$ are amphoteric in nature.
- Carbide of $\mathrm{Be}$ - beryllium carbide $\left(\mathrm{Be}_2 \mathrm{C}\right)$ and $\mathrm{Al}$ - aluminium carbide $\left(\mathrm{Al}_4 \mathrm{C}_3\right)$ give methane on hydrolysis.
- Both beryllium and aluminium are rendered passive by nitric acid.
Question 40.
Why alkaline earth metals are harder than alkali metals?
Answer:
1. The strength of metallic bond in alkaline earth metals is higher than alkali metals due to the presence of 2 electrons in its outermost shell as compared to alkali metals, which have only 1 electron in valence shell. Therefore, alkaline earth metals are harder than alkali metals.
2. The alkaline earth metals have greater nuclear charge and more valence electrons, thus metallic bonding is more effective. Due to this they are harder than alkali metals.
Question 41.
How is plaster of paris prepared?
Answer:
Plaster of paris is a hemihydrate of calcium sulphate $\mathrm{CaSO}_4 \cdot \mathrm{H}_2 \mathrm{O}$. It is obtained by heating gypsum at $393 \mathrm{~K}$.

Question 42.
Give the uses of gypsum.
Answer:
- The Alabaster variety of gypsum was used by the sculptors.
- Gypsum is used in making dry walls or plaster boards.
- Gypsum is used in the production of Plaster of Paris, which is used as a sculpting material
- Gypsum is used in making surgical and orthopedic casts.
- It plays an important role in agriculture as a soil additive, conditioner and fertilizer
- Gypsum is used in toothpaste, shampoo and hair products.
- Calcium sulphate acts as a coagulator in making tofu.
- It is also used in baking as a dough conditioner.
- Gypsum is a component of Portland cement, where it acts as a hardening retarder to control the speed at which concrete sets.
- Gypsum is used to give colour to cosmetics and drugs.
- Gypsum plays a very important role in wine making.
Question 43.
Describe briefly the biological importance of calcium and magnesium. Answer:
1. An adult body contains about $25 \mathrm{~g}$ of $\mathrm{Mg}$ and $1200 \mathrm{~g}$ of $\mathrm{Ca}$. The daily requirement in the human body has been estimated to be 200-300 $\mathrm{mg}$.
2. Magnesium is the co-factor of all enzymes that utilize ATP in phosphate transfer and energy release.
3. The main pigment for the absorption of light in plants is chlorophyll which contains magnesium.
4. About $99 \%$ of body calcium is present in bones and teeth.

5. Calcium plays important roles in neuromuscular function, intemeuronal transmission, cell membrane integrity and blood coagulation.
6. The calcium concentration in plasma is regulated at about $100 \mathrm{mgL}^{-1}$. It is maintained by two hormones: calcitonin and parathyroid hormone.
7. Deficiency of magnesium results into convulsion and neuromuscular irritation.
8. $2 \%$ of adult weight is made up of calcium. Calcium phosphate is present in teeth and Calcium carbonate is present in bones. They make the teeth and bone hard.
9. Water in the human body such as inside the cell and in the blood contain dissolved calcium ions. These ions are involved in making muscles move and in sending electricity around the brain and along the nerves.
10. Magnesium is an essential element in both plant and animal life.
Question 44 .
Which would you expect to have a higher melting point, magnesium oxide or
magnesium fluoride? Explain your reasoning.
Answer:
- Magnesium oxide has very strong ionic bonds as compared to magnesium fluoride.
- $\mathrm{Mg}^{2+}$ and $\mathrm{O}^{2-}$ have charges of +2 and -2 , respectively.
- Oxygen ion is smaller than fluoride ion.
- The smaller the ionic radii, the smaller the bond length in $\mathrm{MgO}$ and the bond is stronger than $\mathrm{MgF}_2$.
- Due to more strong bond nature in $\mathrm{MgO}$, it has high melting point than $\mathrm{MgF}_2$.