Q.Which of the following is the strongest base ?

True — alkali metals impart characteristic colours to the flame because their single loosely-bound valence electron is easily excited by flame heat and emits visible light when it falls back to the ground state.

Alkali metals (Group 1: Li, Na, K, Rb, Cs) have a very low ionisation enthalpy, so their outermost (ns^1) electron is easily excited to a higher energy level by the thermal energy of a flame. When this electron falls back to its ground state, it emits energy in the form of visible light of a characteristic wavelength (colour), which is specific to each element:

• Lithium: crimson red …

CsOH is the strongest base of the set.

For alkali metal hydroxides, basic strength increases down the group: LiOH < NaOH < KOH < RbOH < CsOH. Going down, the metal atom becomes larger and more electropositive, the M-OH bond becomes more ionic, and the hydroxide releases OH- more readily. Caesium, the most electropositive of the listed metals, therefore gives t …

An alkali-metal fire is put out with dry sand — option (b).

From NCERT Class 11 Chemistry (s-Block Elements): alkali metals are highly reactive.

• Water: 2Na + 2H₂O → 2NaOH + H₂ (explosive) — cannot be used.
• CO₂ / CCl₄: hot alkali metals react with these too, so they are unsafe. …

Going down group 1, the type of oxide formed on burning in excess air changes: Li -> normal oxide, Na -> peroxide, K (and below) -> superoxide.

When alkali metals are burnt in excess air/oxygen, they form different types of binary oxygen compounds depending on the metal's position in the group:

• Lithium forms the normal oxide, Li2O.
• Sodium forms the peroxide, Na2O2 (containing the peroxide ion, O2^2-).
• Potassium, Rubidium, and Caesium form the superoxide, MO2 (e.g., KO2), containing the superoxide ion, O2^-. …