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Potassium reacting with water. Close-up footage of a sample of potassium reacting with water. This sample reacts very vigorously, producing a bright flame. The three stable alkali metals are lithium, sodium and potassium. All three react vigorously with water, forming their respective hydroxides and hydrogen gas. Their reactivity increases with atomic number. The most reactive is potassium. The next most reactive is sodium. Lithium is the least reactive of the three. In all three cases, the alkali produced by the reaction forms a strongly basic (alkaline) solution. This can be detected by the presence of the indicator phenolphthalein that turns pink in the presence of an alkali (as seen here).
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