Alkali Metals

The key trend in Group 1 is that reactivity increases going down the group. This means the element higher up (like Li) is less reactive, and the element lower down (like Cs) is more reactive.

From top to bottom in Group 1: Li (period 2), Na (period 3), K (period 4), Cs (period 6). Since reactivity increases going down, the correct order of increasing reactivity is Li → Na → K → Cs.

As you go down the group, each element has more electron shells. The outer electron is further from the nucleus and more shielded by inner electrons, making it easier to lose. Easier to lose the outer electron = more reactive.

The correct order of increasing reactivity is Li → Na → K → Cs, which matches the first option.

The question asks for the order of increasing reactivity, meaning the least reactive element should come first and the most reactive last.

This option goes from most reactive to least reactive (decreasing order), which is the reverse of what the question asks. Eliminate this option.

This option places Li after Na, which is wrong — Li is above Na in Group 1 and therefore less reactive than Na. The order is not consistent with the group trend.

This order is not consistent with positions in the periodic table at all. Cs should be the most reactive and Li the least, but this option places Li last and K first, which is incorrect.

The only remaining option, Li → Na → K → Cs, correctly reflects the trend that reactivity increases going down Group 1.

The observations are: (1) the universal indicator turns violet, confirming an alkaline product; (2) a gas is produced that ignites with a lilac flame. The lilac flame is the crucial clue.

Recall the flame colours: Lithium = crimson/red, Sodium = yellow/orange, Potassium = lilac/violet. A lilac flame is the characteristic flame colour for potassium.

When potassium reacts with water, the hydrogen gas produced ignites immediately and burns with a lilac flame. This matches the description perfectly: $2K+2H_{2}O\to 2KOH+H_2$

The lilac flame is the definitive identifier — the alkali metal used was potassium (K).

The most specific clue in the question is that the gas ignites with a lilac flame. Each alkali metal has a characteristic flame colour that can be used for identification.

Sodium produces a yellow/orange flame, not lilac. Eliminate sodium.

Lithium produces a crimson/red flame, not lilac. Eliminate lithium.

Rubidium produces a red-violet flame. While similar, the distinctly lilac flame described is the classic identifier for potassium in MYP chemistry. Eliminate rubidium.

Potassium is the alkali metal with the characteristic lilac/violet flame, confirming it is the correct answer.