Virtual Lab: Metals and Water
Sodium fizzes, potassium ignites, caesium explodes. Why?
Summary
Drop alkali and alkaline earth metals into water and watch the reactions. The metal reacts with water to form a hydroxide and hydrogen gas; the solution turns basic and heat is released. The further down the periodic table you go, the more violent the reaction: lithium fizzes calmly, potassium bursts into flame, caesium explodes. In game mode, put five hidden metals in order of reactivity.
In depth
Group 1 (alkali) metals carry a single electron in their outermost shell and give it up easily: 2M + 2H₂O → 2MOH + H₂. Going down the group the atomic radius grows, this outer electron moves further from the nucleus and is removed more easily, which is why reactivity increases from Li to Cs. The reaction is exothermic: because of their low melting points, sodium and potassium melt into a ball that skims across the water, and potassium ignites the hydrogen released with a lilac flame. Lithium, sodium and potassium are lighter than water and float; rubidium and caesium sink and react explosively. Group 2 (alkaline earth) metals have to give up two electrons, so they are slower: M + 2H₂O → M(OH)₂ + H₂. Beryllium does not react because of its oxide layer, magnesium fizzes noticeably only in hot water, and calcium's poorly soluble hydroxide turns the solution milky. The hydroxide ions formed raise the pH, which is why phenolphthalein turns pink.
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