IB Chemistry SLTopic 3 — Classifying the ElementsPaper 1 & 2Trends~9 min read
Group 1 Metals Reacting with Water
The alkali metals are the most famous demonstration in school chemistry, and they are also a perfect test of whether you really understand periodic trends. Every one of them does the same reaction — only the violence changes, and the trend in violence is something you can predict from first principles.
📚 What you need to know
Group 1 = the alkali metals: lithium, sodium, potassium, rubidium, caesium, francium.
They all have one valence electron, configuration ending in ns1, and form 1+ ions.
They are soft, shiny when freshly cut, good conductors, with low melting points and low densities for metals.
With water they give a metal hydroxide + hydrogen gas, and the solution turns alkaline.
Reactivity increases down the group, because the outer electron is further out and better shielded, so it is lost more easily.
They also react vigorously with the halogens to form ionic salts.
What the alkali metals are like
They do not look like the metals you are used to. They are soft enough to cut with a knife, and the freshly cut surface is bright and shiny for only a moment before it dulls — that is the metal reacting with oxygen and water vapour in the air. This is why they are stored under oil.
Soft, and getting softer down the group
Low melting points, falling down the group as the atoms get bigger and the metallic bonding weakens
Low densities — lithium, sodium and potassium all float on water
Good conductors of heat and electricity, thanks to the delocalised electrons in metallic bonding
The physical trend and the chemical trend have the same cause. Bigger atoms mean the outer electron is held more loosely — which weakens metallic bonding (lower melting point) and makes the electron easier to give away (higher reactivity).
The reaction with water
Every Group 1 metal does the same thing: it gives its outer electron away, forms a 1+ ion, and displaces hydrogen from water.
The general reaction
metal + water → metal hydroxide + hydrogen
The hydroxide dissolves to give an alkaline solution — usually around pH 13 or 14. That is where the name “alkali metal” comes from, and it is why universal indicator turns purple in the demonstration.
Same equation each time; only the vigour changes. The outer electron gets easier to lose as you go down.
Lithium
Floats and fizzes steadily. Bubbles of hydrogen come off, and a colourless solution of lithium hydroxide is left behind. It does not melt.
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)
Sodium
Faster. Enough heat is released to melt the sodium into a small silver ball, which skates around the surface pushed by the escaping hydrogen. The solution goes strongly alkaline.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Potassium
Faster again, and now the heat released is enough to ignite the hydrogen. The metal melts, dashes across the surface, and burns with a lilac flame.
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Observations worth memorising: effervescence (hydrogen), the metal melting into a ball (exothermic), a colourless solution forming (soluble hydroxide), and the temperature of the water rising.
Why reactivity increases down the group
All of these reactions are the metal losing one electron. So the question “which is most reactive?” becomes “which loses that electron most easily?” — and that is a trend you already know.
🧩 The chain of reasoning examiners want
Going down the group, each atom has one more electron shell.
So the outer electron is further from the nucleus…
…and there is more shielding from the extra inner shells.
Nuclear charge does increase, but these two effects outweigh it.
So the attraction between nucleus and outer electron is weaker.
The electron is lost more easily → the metal is more reactive.
Another way to say the same thing: ionisation energy decreases down the group, and lower ionisation energy means a more reactive metal. This increasing ease of losing electrons is also described as increasing metallic character.
Reaction with the halogens
The alkali metals also react vigorously with Group 17 elements. One metal atom gives one electron to one halogen atom, producing an ionic metal halide salt.
Sodium and chlorine
2Na(s) + Cl2(g) → 2NaCl(s)
The same trend applies: the reaction gets more vigorous down Group 1, for exactly the same reason — the outer electron is easier to hand over.
WORKED EXAMPLE
Write a balanced equation with state symbols for rubidium reacting with water, and predict two observations.
Same pattern as every Group 1 metal2Rb(s) + 2H₂O(l) → 2RbOH(aq) + H₂(g)Rb is below K, so it is MORE reactive than potassium1. Extremely rapid effervescence — the hydrogen ignites, possibly explosively.2. The metal melts and disappears almost instantly, leaving a colourless alkaline solution.
WORKED EXAMPLE
A student adds a Group 1 metal to water and the resulting solution turns universal indicator purple. Explain this observation with an equation.
Purple means strongly alkaline, so OH⁻ ions have formedThe metal hydroxide is soluble and fully dissociates2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)MOH(aq) gives M⁺(aq) + OH⁻(aq), so the pH rises to about 13–14.
💡 Exam tip
The equation is the same shape every time: 2M + 2H2O → 2MOH + H2. Learn it once, swap the symbol.
Observations means what you would see, hear or feel — fizzing, melting, flame colour, heat. “It reacts faster” is not an observation.
Explaining reactivity needs distance, shielding and nuclear charge, not just “it’s bigger”.
Remember to balance: two metal atoms and two waters, because the metal is 1+ and water supplies one H each.
⚠️ Common mix-up
Group 1 reactivity increases down the group — the opposite of Group 17. Don’t apply one rule to both.
The gas produced is hydrogen, not oxygen.
Alkali metals are stored under oil, not water. Water is what they react with.
Increasing nuclear charge down the group is real, but it is outweighed by shielding and distance — say so, don’t ignore it.
The lilac flame comes from potassium; it is the burning hydrogen coloured by the metal, not the metal itself reacting with air.
Up next: Group 17 Elements and Halide Ions — the mirror image of this page, where the trend runs the other way and for a very good reason.
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