Alkali metals (Group 1): a family because they react with water the same way
An alkali metal is an element in Group 1 (Li, Na, K, Rb, Cs, Fr): soft metals, stored under oil. They are recognised as one family because they all react with water in the same way — the metal floats and moves on the surface, giving a metal hydroxide plus hydrogen gas. The hydroxide dissolves to an alkaline solution (universal indicator purple, phenolphthalein pink). The shared type of products is the evidence for the family.
In air: alkali metals tarnish and burn to metal oxides
A freshly cut alkali metal is shiny but quickly goes dull (tarnishes) as it reacts with oxygen and water vapour — hence storage under oil. Heated in air or oxygen they burn to a metal oxide (e.g. ). Valid observations with water: floats, moves/darts, gets smaller and disappears, and (sodium, potassium) melts into a silvery ball. State symbols: (s) metal, (l) water, (aq) hydroxide, (g) hydrogen.
Reactivity increases DOWN Group 1 — used to predict other metals
Going down Group 1 the metals get more reactive: lithium reacts steadily, sodium vigorously (melts into a ball), potassium fastest (ignites, lilac flame). Reason: each atom has one outer electron; further down it is further from the nucleus, so lost more easily. Use the trend to predict: rubidium and caesium (below potassium) react with water even more violently — explosively. A prediction must state the direction of the trend as its reason.
Drawn from real examiner reports.
"Fizzing / bubbles" scores zero
When asked what you see as an alkali metal reacts with water, "fizzing" or "bubbles" gain no marks: the hydrogen is already implied, so the bubbling just repeats it. Examiners want the metal's own behaviour — floats, moves/darts, gets smaller, disappears, melts into a silvery ball (Na, K). Anything already given in the stem cannot be re-credited.
"Fizzing/bubbles" (synonymous with the hydrogen), and any observation already in the stem, not credited (Jun24 1CR Q4a; Jun23 Q5ai).
Reactivity and radius both increase down
Down Group 1 the atomic radius increases (extra shells) AND the reactivity increases — the two rise together. Some candidates wrongly link a decreasing radius to increasing reactivity, reversing the trend. Correct chain: more shells, so the outer electron is further out, held less strongly and lost more easily. Keep both statements pointing the same way.
Stating reactivity increases as the radius decreases was the wrong way round; both increase down the group (Jun24 1CR Q4cii).
Sodium flame is yellow, not red
Sodium's flame colour is a recurring slip: it is yellow (yellow-orange), not red — red is lithium's colour. Writing red for sodium loses the mark. (Potassium's reaction with water shows a lilac flame.) The full flame-test procedure sits in the chemical-tests topic; here just fix the fact that sodium is yellow and lithium is red/crimson.
Sodium flame is yellow; a few wrote red and lost the mark (lithium is red/crimson) (Jun24 1CR Q4b; Jun23 Q5b).
Water gives the hydroxide, oxygen the oxide
Match the product to the reactant. With water an alkali metal gives the metal hydroxide (e.g. NaOH) + hydrogen; with oxygen/air it gives the metal oxide (e.g. ). A common slip is to write the oxide as the product of the water reaction, or to forget the hydrogen. The hydroxide makes the solution alkaline; the oxide is the dry-air product.
A comparison must actually compare
"Explain why potassium is more reactive than sodium" needs a comparative answer. "Potassium is very reactive" says nothing about sodium and scores little — you must say potassium is more reactive than sodium, with the reason (its outer electron is further from the nucleus, so lost more easily). To compare two metals, name both and use more/less reactive than.
Build observations from a checklist
Pick observations from a checklist: floats; moves/darts; melts into a silvery ball (Na, K, not Li); gets smaller/disappears; potassium — lilac flame/sparks. Never write fizzing or bubbles — they repeat the hydrogen. Give several, as marking points exceed the marks.
Predict/explain: direction + reason
Trend answers need two parts: the direction (reactivity increases down the group) and the electron reason (more shells, so the outer electron is further out and lost more easily). For a prediction, place the metal in the group, then apply the direction — no reason caps the marks.
Use the general word equations
Anchor answers on two equations: metal + water gives metal hydroxide + hydrogen; metal + oxygen gives metal oxide. Naming the right products (and the alkaline hydroxide solution) frames the family evidence. For a symbol equation, remember oxygen is diatomic and balance last.
Reaction with water (the reaction that shows they are one family): The hydroxide turns universal indicator purple / phenolphthalein pink (alkaline). Where state symbols are (s) solid, (l) liquid, (aq) aqueous, (g) gas.
Reaction with air / oxygen (forms a metal oxide):
The reactivity trend — increases DOWN the group (Li < Na < K < Rb < Cs).
Define an alkali metal.
Sodium reacts with water to form sodium hydroxide and hydrogen.
Balance the equation:
