Alkali Metals and Water

Introduction

The Group 1 metals got their family name from the way they react with water. Drop a small piece of any of them into water and it produces a solution that turns universal indicator purple: an alkali . At the same time a gas is released, and the metal gets used up. Watching lithium, sodium and potassium react one after another is one of the clearest ways to see a trend on the periodic table, because each metal reacts more violently than the one above it.

Core explanation

The general reaction. Every alkali metal reacts with water in the same way:

alkali metal + water → metal hydroxide + hydrogen

For sodium, the balanced symbol equation is:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

The same pattern holds for the rest of the group: 2Li + 2H₂O → 2LiOH + H₂, and 2K + 2H₂O → 2KOH + H₂. Because every alkali metal has one valence electron and forms a 1+ ion , the formulas of the products always have the same shape. Learn one equation and you can write them all by swapping the symbol.

Why the solution is alkaline. The metal hydroxide dissolves in the water. Sodium hydroxide, for example, exists in solution as Na⁺ ions and OH⁻ (hydroxide) ions . A solution containing an excess of hydroxide ions has a pH above 7, so it is alkaline. With universal indicator the water turns from green to blue or purple.

What happens to the atoms. Each metal atom loses its single outer electron and becomes a positive ion: Na → Na⁺ + e⁻. Water molecules accept these electrons, producing hydroxide ions and hydrogen gas. So the metal is oxidised (it loses electrons) and the water is reduced.

Observations for each metal. Teachers normally demonstrate these reactions using only a tiny piece of metal, a large volume of water and a safety screen, because the reactions become dangerous quickly.

Metal What is seen --- --- Lithium Floats, fizzes steadily, moves slowly around the surface, gradually disappears Sodium Floats, melts into a shiny ball, whizzes around rapidly, fizzes strongly, disappears quickly Potassium Floats, melts, hydrogen ignites at once with a lilac flame, may spit or crackle, disappears very quickly

Rubidium and caesium react explosively and are never used in school demonstrations.

Explaining the observations. The metals float because their densities are less than water (lithium about 0.53 g/cm³, sodium about 0.97 g/cm³, potassium about 0.86 g/cm³). They fizz because hydrogen is produced. Sodium and potassium melt because the reaction releases heat and their melting points are low (sodium 98 °C, potassium 63 °C). They move because the gas pushes them across the surface. They disappear because the metal is turned into ions that dissolve.

The trend. Reactivity with water increases down the group: Li < Na < K < Rb < Cs. The reason, which depends on how easily the outer electron is lost, is explored in detail on the next page.

Step-by-step reasoning

To write the equation for any Group 1 metal with water:

1. Write the word equation: metal + water → metal hydroxide + hydrogen. 2. The metal ion is 1+ and hydroxide is 1−, so the hydroxide formula is MOH (for example LiOH, KOH). 3. Hydrogen is diatomic, so write H₂. 4. Balance: two metal atoms and two water molecules give two hydroxide units and one H₂ molecule. 5. Add state symbols: (s), (l), (aq), (g).

Visual explanation

Imagine three bowls of water with indicator side by side. In the first, a small lithium piece drifts and fizzes gently, with a faint purple trail. In the second, a sodium ball skates quickly, leaving a streak of purple. In the third, potassium bursts into a lilac flame and the whole bowl turns purple within seconds.

Real-world analogy

Think of three people each holding one balloon on a string. The first grips it tightly, the second loosely, the third barely at all. In a breeze the third lets go almost instantly. The alkali metals lower down "hold" their single outer electron less tightly, so they give it away faster.

Real-world example

Sodium hydroxide, the product of sodium reacting with water, is made industrially on a huge scale (by electrolysis of brine, not from sodium metal). It is used to make soap, paper and bleach, and in drain cleaners, because hydroxide ions break down fats and grease.

Why?

Why are alkali metals stored under oil? They react with water vapour and oxygen in the air, quickly forming a dull coating of oxide and hydroxide. A layer of oil keeps air and moisture away, so the metal stays usable and the risk of an uncontrolled reaction is reduced.

Common misconception

"The gas given off is oxygen, because water contains oxygen." The gas is hydrogen. The oxygen from the water ends up in the hydroxide ions in solution. A lighted splint gives a squeaky pop, which is the test for hydrogen, not oxygen.

Worked example

Question: Write a balanced symbol equation, with state symbols, for rubidium reacting with water, and predict the pH of the resulting solution.

Reasoning: Rubidium is in Group 1, so it forms Rb⁺ and the hydroxide is RbOH. Following the same pattern as sodium: 2Rb(s) + 2H₂O(l) → 2RbOH(aq) + H₂(g). RbOH is a soluble hydroxide, so the solution is strongly alkaline.

Answer: 2Rb(s) + 2H₂O(l) → 2RbOH(aq) + H₂(g); pH well above 7 (around 13–14 for a concentrated solution).

Quick check

1. Name the two products when potassium reacts with water. Answer: Potassium hydroxide and hydrogen.

Exam focus

Examiners expect precise observations linked to explanations: "floats because it is less dense than water", "fizzes because hydrogen gas is produced", "melts because the reaction is exothermic and the metal has a low melting point". Always balance the symbol equation with two metal atoms and one H₂, and state that the solution is alkaline because of OH⁻ ions.

Advanced insight

The lilac flame above potassium is not the potassium burning in the usual sense. Heat from the reaction ignites the hydrogen, and potassium atoms vaporised in the flame emit light at characteristic wavelengths, giving the lilac colour also used in flame tests. Sodium in a hot enough reaction gives a yellow-orange flame for the same reason.

Summary

Alkali metals react with water to form a metal hydroxide and hydrogen gas, for example 2Na + 2H₂O → 2NaOH + H₂. The hydroxide dissolves to give an alkaline solution containing OH⁻ ions. The metals float, fizz and dissolve; sodium and potassium also melt, and potassium ignites the hydrogen. Reactivity increases down the group from lithium to caesium.

Practice questions

1. Write the word equation for lithium reacting with water. Answer: Lithium + water → lithium hydroxide + hydrogen. 2. Explain why sodium melts during its reaction with water but lithium does not. Answer: The reaction of sodium is faster and releases heat more quickly, and sodium has a low melting point (98 °C), so it melts; lithium reacts more slowly and has a higher melting point (about 181 °C). 3. What colour does universal indicator turn in the water after the reaction, and why? Answer: Blue or purple, because the metal hydroxide dissolves to give OH⁻ ions, making the solution alkaline. 4. Put caesium, lithium and potassium in order of increasing reactivity with water. Answer: Lithium, potassium, caesium. 5. Balance the equation: K + H₂O → KOH + H₂. Answer: 2K + 2H₂O → 2KOH + H₂.