s-Block Elements

To match List I (Alkali Metals) with List II (Emission Wavelength in nm), we need to know the characteristic emission lines of these elements. Here is the information for the typical emission wavelengths:

• Lithium (Li): 670.8 nm
• Sodium (Na): 589.2 nm
• Rubidium (Rb): 780.0 nm
• Cesium (Cs): 455.5 nm

Now let's match these with the given options:

• A. Li (III): 670.8 nm
• B. Na (I): 589.2 nm
• C. Rb (IV): 780.0 nm
• D. Cs (II): 455.5 nm

Therefore, the correct answer is:

Option B

(A)-(III), (B)-(I), (C)-(IV), (D)-(II)

Alkali metals are part of Group 1 on the periodic table and include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). As you move down the group, the atomic radius increases due to the addition of energy levels (shells). While you might think that the increased atomic mass would correspond to an increase in density, the increase in atomic radius generally outpaces the increase in atomic mass, leading to a decrease in density.

The densities (at room temperature) of the mentioned alkali metals are :

• Sodium (Na) : 0.97 g/cm³
• Potassium (K) : 0.86 g/cm³
• Rubidium (Rb) : 1.53 g/cm³
• Cesium (Cs) : 1.93 g/cm³

So, the order of densities is : K < Na < Rb < Cs

The alkali metals are the elements in Group 1 of the periodic table. They are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs).

The melting point of these metals decreases down the group as the size of the atoms increases. This is because the metallic bonding weakens as the atomic size increases, resulting in a lower melting point.

Therefore, the alkali metal with the lowest melting point is cesium (Cs).

So, the correct option is Option D, Cs.

Let's consider each item in List-I and match it with an appropriate item from List-II:

A. K (Potassium): Potassium ions are crucial for the operation of the sodium-potassium pump in nerve cells. So, A matches with III.

B. KCl (Potassium Chloride): Potassium chloride is widely used in agriculture as a fertilizer. So, B matches with II.

C. KOH (Potassium Hydroxide): Potassium hydroxide is a strong base, but it is not typically used in a sodium-potassium pump, as a fertilizer, for thermonuclear reactions, or as a CO₂ absorbent. However, as per the given options and its closest possible use, KOH is often used to absorb CO₂ in certain applications like rebreathing gas masks. So, C matches with IV.

D. Li (Lithium): Lithium is used in thermonuclear reactions, specifically in fusion reactions. So, D matches with I.

Therefore, the correct option is Option B: A-III, B-II, C-IV, D-I.

As you move from top to bottom in a group (vertical column) on the periodic table, the metallic character increases. This is because elements at the bottom of a group have more electron shells than those at the top. These extra shells shield the outermost electrons from the pull of the nucleus, which makes it easier for these electrons to be lost, a characteristic of metals.

On the other hand, as you move from left to right across a period (horizontal row) on the periodic table, the metallic character decreases. This is because the nuclear charge increases as you move across a period, which pulls the electrons closer to the nucleus and makes them less likely to be lost.

Given this, for the elements K (Potassium), Be (Beryllium), and Ca (Calcium), the order of metallic character should be K > Ca > Be. Potassium (K) is furthest down (group 1, period 4), calcium (Ca) is above potassium (group 2, period 4), and beryllium (Be) is above calcium (group 2, period 2). So, the correct order should be Option C: K > Ca > Be.

• Washing soda is a hydrated form of sodium carbonate, with the chemical formula Na₂CO₃·10H₂O. This means that each molecule of washing soda contains 10 water molecules.


• Soda ash is the anhydrous form of sodium carbonate, with the chemical formula Na₂CO₃. This means that it does not contain any water molecules.

Soda ash is typically produced by heating washing soda to drive off the water molecules. Washing soda can be produced by dissolving soda ash in water and then recrystallizing it.

The correct answer is Option D: 10 and 0.

Lime (Calcium oxide, CaO) reacts exothermally with water to form slaked lime (Calcium hydroxide, Ca(OH)$_2$):

CaO (s) + H$_2$O (l) $\rightarrow$ Ca(OH)$_2$ (s)

This slaked lime has low solubility in water and is used for the test of carbon dioxide (CO$_2$). In this test, the carbon dioxide reacts with the calcium hydroxide to form calcium carbonate (CaCO$_3$), an insoluble white precipitate:

Ca(OH)$_2$ (aq) + CO$_2$ (g) $\rightarrow$ CaCO$_3$ (s) + H$_2$O (l)

Further reaction of calcium carbonate with carbon dioxide forms calcium bicarbonate [Ca(HCO$_3$)$_2$], a soluble compound:

CaCO$_3$ (s) + CO$_2$ (g) + H$_2$O (l) $\rightarrow$ Ca(HCO$_3$)$_2$ (aq)

Thus, 'A' in this case is slaked lime. So, the correct option is:

Option D : Slaked lime

Statement I : "Lithium and Magnesium do not form superoxide"

This statement is correct. Lithium and magnesium are smaller in size compared to other elements in their respective groups. Although lithium can form superoxide under certain conditions, in general, the smaller size of these two elements makes it harder for them to form stable superoxides.

Statement II : "The ionic radius of Li⁺ is larger than the ionic radius of Mg²⁺"

This statement is still correct. Despite both lithium and magnesium being small in size, when they lose electrons to form ions, the Mg²⁺ ion is smaller than the Li⁺ ion due to the loss of two electrons (and thus an energy level) in magnesium compared to the loss of one electron in lithium.

So, based on the information, the answer is :

Option C : Both Statement I and Statement II are correct.

Cement is manufactured by heating limestone (calcium carbonate) with small amounts of other materials, such as clay, to a high temperature in a kiln in a process known as calcination, which produces clinker. The clinker is then cooled and ground into a fine powder that we know as cement.

However, the clinker is highly reactive, and when mixed with water, it would harden immediately. This would not be practical in a construction setting because it doesn't allow enough time for the cement mix to be poured, shaped, or smoothed out.

Gypsum ($\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$) is added to this clinker before it's ground into cement. The role of gypsum is to extend the time it takes for the cement to set, giving the workers enough time to work with the cement before it hardens.

The gypsum reacts with the clinker during the final grinding process, controlling the rate at which the cement will set. If no gypsum was added, the cement would set as soon as water is added, making it almost impossible to use in construction.

The electrolysis of brine, which is a solution of sodium chloride (NaCl) in water, involves the migration of ions towards electrodes. When an electric current is passed through the solution, positive sodium ions ($Na^+$) move towards the cathode (negative electrode), and negative chloride ions ($Cl^-$) move towards the anode (positive electrode).

At the same time, water ($H_2O$), which also dissociates into $H^+$ and $OH^-$ ions, also contribute to the reactions at the electrodes.

At the anode, chloride ions are oxidized to chlorine gas:

$2Cl^- \rightarrow Cl_2(g) + 2e^-$

At the cathode, water is reduced to hydrogen gas and hydroxide ions, instead of sodium ions being reduced to sodium metal. This is because water is more easily reduced than sodium ions:

$2H_2O + 2e^- \rightarrow H_2(g) + 2OH^-$

The sodium ions remain in the solution and combine with the hydroxide ions to form sodium hydroxide:

$Na^+ + OH^- \rightarrow NaOH$

So the products of the electrolysis of brine are chlorine gas, hydrogen gas, and sodium hydroxide. Hydrochloric acid (HCl) is not produced during this process because the hydrogen and chloride ions do not combine in the electrolyte solution to form HCl. They are separately reduced and oxidized at the cathode and anode respectively.

Hydration enthalpy is the amount of energy released when one mole of gaseous ions is completely surrounded by water molecules. The process is exothermic, hence the enthalpy change is negative.

Generally, smaller ions and ions with higher charge have larger hydration enthalpies due to the stronger attraction to the water molecules. This is because the hydration enthalpy depends on the charge density of the ion.

Among the given options, $\mathrm{Be}^{2+}$ is the smallest ion and it has the same charge as the other ions. Hence, it has the highest charge density and will have the highest hydration enthalpy.

$\mathrm{BeCl}_2$ forms a polymeric structure in the solid state due to coordination bonds formed by chlorine donating a lone pair of electrons to the beryllium atoms.

In the vapour phase at moderate temperatures, $\mathrm{BeCl}_2$ exists as a dimer, forming a bridge structure with two chlorine atoms connected to two beryllium atoms.

At very high temperatures, $\mathrm{BeCl}_2$ exists as monomers due to the high kinetic energy of the molecules, which overcomes the dimerisation tendency.

"Both (A) and (R) are correct but (R) is not the correct explanation of (A)" is the correct answer

Gypsum is indeed used for making fireproof wall boards, but the reason given in (R) that gypsum is unstable at high temperatures is not the correct explanation for why it is used for this purpose. Instead, gypsum is fire-resistant because it does not support combustion and does not emit toxic fumes when exposed to fire.

A. Beryllium oxide is amphoteric in nature.

B. Beryllium carbonate is kept in the atmosphere of CO₂ because it is thermally less stable.

C. Beryllium sulphate is readily soluble in water due to high degree of hydration.

D. Beryllium shows anomalous behaviour due to small size, high ionization energy and high value of $\phi $ (polarising power).

$\mathrm{BeCl_2}$ is a covalent molecule

So, it is soluble in organic solvents, rest are ionic compounds.

$\mathrm{4LiNO_3}$ $\buildrel {\mathrm{Heat}} \over \longrightarrow $ $\mathrm{2Li_2O+4NO_2+O_2}$

$\mathrm{LiNO_3}$ on heating produces $\mathrm{Li_2O,NO_2}$ and $\mathrm{O_2}$

BeSO$_4$ and MgSO$_4$ are readily soluble in water.