Perchlorate ion as a large low charge density anion
Not all alkali metal salts are soluble. The larger alkali metal ion form precipitates with large low charge density anions. Caesium ion with perchlorate ion is an example:

Cs⁺(aq) + ClO₄⁺(aq) → CsClO₄(s)

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Relative reactivity of lithium and sodium with water
Lithium is added to water containing phenolphthalein indicator. The water turns the characteristic purple color of the base form of the indicator.

2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)

The experiment is repeated with sodium. The reaction is more rapid. Also, the heat of reaction is sufficient to melt the sodium giving a globule of molten sodium skittering around.

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

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Flame test
The alkali metals have characteristic colors in a flame. Here we show the distinctive scarlet color of lithium; the yellow color of sodium; and the pale violet color of potassium.

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Solubilities of alkali metal ions
The larger alkali metal ions form precipitates with large, low-charge anions. The standard test for the larger alkali metal cations is the hexanitritocobaltate(III) ion:

3K⁺(aq) + [Co(NO₂)₆]^3-(aq) → K₃[Co(NO₂)₆](s)

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