An alkali metal reacts with water to give an alkaline metal hydroxide and hydrogen gas, for example 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g); the reaction becomes more vigorous down the group.
Group 1 metals, the alkali metals, react with water to produce a soluble metal hydroxide and hydrogen gas. The solution formed is alkaline, which is where the family name comes from. This reaction is the standard way the syllabus shows that reactivity increases down a group, so it is a favourite in the Periodic Table chapter of 4541.
The balanced equation
The general pattern is: alkali metal + water → metal hydroxide + hydrogen. With correct formulae and state symbols:
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Each metal atom loses its single valence electron, so the metal ion carries a single positive charge and two metal atoms are needed to release one molecule of hydrogen. Because water is a covalent molecule, the ionic equation keeps it whole and shows the metal hydroxide separating into ions:
2Na(s) + 2H2O(l) → 2Na+(aq) + 2OH−(aq) + H2(g)
The hydroxide ions make the final solution alkaline, turning red litmus blue and giving a high pH.
Conditions required
No heating, catalyst or special apparatus is needed, a small, freshly cut piece of the metal is simply added to water at room temperature. Only a small piece is used because the reaction is strongly exothermic and, with the more reactive metals, can be violent. The metals are stored under oil because they react with both water vapour and oxygen in the air.
What you observe
The pattern of observations is the key to the whole topic. Lithium floats, moves slowly and fizzes gently, giving off a gas. Sodium reacts faster: the heat released melts it into a shiny silvery ball that darts about the surface with a hissing sound, and the fizzing is brisk. Potassium is the most vigorous: it ignites the hydrogen produced, burning with a lilac flame, moves very rapidly and may spit or explode. In every case a gas is released, the metal disappears, and if universal indicator is added the solution turns blue or purple, confirming it is alkaline.
Where it appears in the SPM exam
In 4541/1 you compare the vigour of the three metals or predict the products. In 4541/2 you write the balanced equation, describe and explain the increasing reactivity down the group in terms of atomic size and the ease of losing the valence electron, and identify the alkaline solution and the hydrogen gas. Safety reasoning, why only a small piece is used, why a safety screen is placed in front, is also commonly asked, and the hydrogen produced can be tested with a lighted splint for a “pop”.
How we teach it
Our teachers make sure the explanation of the trend is precise: down the group the atomic radius increases, the valence electron is farther from the nucleus and better shielded, so it is released more easily and the metal is more reactive. Students most often lose marks by describing the observations without explaining the trend, or by forgetting that the product solution is alkaline. Linking each observation back to “how easily the atom loses its one electron” keeps the whole answer coherent and correct.
Quick summary
An alkali metal plus water gives a metal hydroxide and hydrogen, and the reaction gets more vigorous from lithium to potassium. Learn the three balanced equations, the ionic equation with its hydroxide ions, the ordered observations, and the size-and-shielding explanation of the reactivity trend, and this reliable topic is fully covered.
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