d) both assertion and reason are false

The reactivity of alkali metals is due to their low ionisation enthalpy. the lonisation enthalpy decreases down the group. Hence the reactivity of alkali metal increases from Li to Cs.
The high reactivity of alkali metals is proved by their reaction with oxygen add halogen which are most electro negative.

The heat in the flame excites the valence electron to a higher energy-level. When it drops back to its actual energy level, the excess energy is emitted as light, whose wavelength is in the visible region. This appears as a characteristic colour to the particular metal.

Mg (OH)₂ ; Magnesium hydroxide.

Efflorescence is the spontaneous loss of water by a hydrated salt, which occurs when the aqueous vapor pressure of the hydrate is greater than the partial pressure of the water vapour in the air. This is the property of salts
Ex: Glauber's salt- Na₂SO₄·10H₂O
Epsom salt - MgSO₄·7H₂O

(a) Reason: Li⁺ ion being the smallest in size will combine with small N^3- ion to form the stable crystal.
6Li + N₂ ⟶ 2Li₃N

i) The fact that a small cation can stabilize a  small anion and a large cation can stabilize a large anion explains the formation and stability of these oxides
(ii) The size of Li⁺ ion is very small and it has a strong positive field around it
(iii) It can combine with only small anion, O^2- ion, resulting in the formation of monoxide (Li₂O)

Reasons for anomalous behaviour of lithium,
(i) Extremely small in size
(ii) Greater polarizing power of ion
(iii) Least electropositive character
(iv) Non-availability of d-orbitals

In a group, the density increases with increase in atomic number. But the density of ₁₉K is less than ₁₁Na.
Reason:
Atomic volume ofK is nearly twice of Na, but its mass (39) is not exactly double of Na (23).
Since density is inversely related to volume, the greater volume K has lower density than Na

The alkali metals and their salts impart characteristic colour to an oxidizing flame.
This is because the heat from the flame excites the valence electron to a higher energy level.
When the excited electron comes back to the ground state there is emission of radiation in the visible region

The high solubility of lithium salts is due to strong solvation of small size Li⁺ ion

1. Gypsum is used in making drywalls or plaster boards.
2. Another important use of gypsum is the production of plaster of Paris. Gypsum is heated to about 300 degree Fahrenheit to produce plaster of paris, which is also known as gypsum plaster. It is mainly used as a sculpting material.
3. Gypsum is used in making surgical and orthopedic casts, such as surgical splints and casting moulds.
4. Gypsum plays an important role in agriculture as a soil additive, conditioner, and fertilizer. It helps loosen up compact or clay soil, and provides calcium and sulphur, which are essential for the healthy growth of a plant.
5. Gypsum is used in toothpastes, shampoos, and hair products.
6. Gypsum is a component of portland cement, where it acts as a hardening retarder to control the speed at which concrete sets.

6Li + N₂ ⟶ 2Li₃ N

(i) Solvay process - in this process ammonia is converted to ammonium carbonate, which is then converted to ammonium bicarbonate by passing excess carbon dioxide in sodium chloride solution saturated with ammonia.
(ii) The ammonium bicarbonate formed reacts with soctium chloride to give sodium bicarbonate. As sodium bicarbonate has poor solubility, it gets precipitated.
(iii) The sodium bicarbonate is isolated and is heated to give sodium carbonate.
(iv) The equations involved in this process is as below:
i) \(2NH_3 + H_2O + CO_2 ⟶\underset{Ammonium\ carbonate}{ (NH_4)2CO_3}\)
ii) \((NH_4)2CO_3 + H_2O + CO_2 ⟶ \underset{Ammonium\ bicarbonate}{2NH_4HCO_3}\)
iii) \(NH_4HCO_3 + NaCl⟶\underset{Ammonium\ chloride}{NH_4Cl + NaHCO_3}\)
iv) \(2NaHCO_3\xrightarrowΔ\underset{Sodium\ carbonate}{Na_2CO_3 }+ CO_2 + H_2O\)

(i) An alkali metal (A) belongs to period number II and group number I is lithium.
(ii) Lithium reacts with oxygen to form simple oxide lithium oxide (B).
\(4Li + O_2 ⟶ \underset{Lithium\ oxide (B)}{2Li_2O}\)
(iii) Lithium reacts with water to form lithium hydroxide with liberation of hydrogen.
\(2Li + 2H_2O ⟶ \underset{Lithium\ hydroxide (C)}{2LiOH + H_2}\)
(iv) Lithium directly react with carbon to form an ionic compound lithium carbide.
\(2Li + 2C ⟶ \underset{Lithium\ carbide}{Li_2C_2}\)