Group I: soft, shiny, low-density metals
The alkali metals (Li, Na, K, Rb, Cs) are shiny when freshly cut but tarnish quickly in air, so are stored under oil. They are soft enough to cut with a knife, and get softer down the group. Densities are low - Li, Na and K float on water - and increase down the group. Melting points are low for metals and fall down the group. The headline trend, asked in almost every question, is reactivity, which increases down the group.
Group I + water: hydroxide + hydrogen
Every Group I metal reacts with water to give the metal hydroxide (an alkali) plus hydrogen: 2M + 2H2O -> 2MOH + H2 (M = Li, Na, K), more vigorous down the group. Lithium floats and fizzes steadily; sodium melts to a silvery ball and darts about, fizzing faster; potassium reacts vigorously, its hydrogen igniting with a lilac flame. In every case the solution is alkaline (pH > 7), turning universal indicator blue/purple - never acidic.
(Extended) Why reactivity rises down Group I
Down Group I each atom has one more electron shell, so the single outer electron lies further from the nucleus and is more shielded. Both weaken the pull on it. Reactivity depends on how readily the atom loses that outer electron to form an M+ ion, so it increases down the group. Mark-scheme chain: more shells, outer electron further out, more shielding, weaker attraction, electron lost more easily, greater reactivity.
Drawn from real examiner reports.
Alkali metals give alkaline solutions
A Group I metal and water give the metal hydroxide (LiOH, NaOH, KOH) plus hydrogen. The hydroxide dissolves to give hydroxide ions, so the solution is alkaline, pH > 7. Seeing fizzing, some candidates call it acidic - but the name alkali metal signals they make alkalis. Marks need alkaline, or pH > 7, or named hydroxide; acidic scores zero.
W22 P13 Q25; W22 P23 Q25
Flame colours: Li red, Na yellow, K lilac
The three Group I flame-test colours must be exact: lithium red (crimson), sodium a persistent bright yellow, potassium lilac (violet). Candidates often give potassium as brown, orange or pink - none score - or swap it with lithium's red. Only one clear colour word is needed, and lilac or violet are both accepted for potassium; pink is not.
W22 P32 Q5(c)(iii)
Use a flame test, not silver nitrate
To identify a Group I metal, use a flame test, not a precipitation test. Candidates wrongly propose silver nitrate or ammonia - but silver nitrate (with nitric acid) tests for chloride ions, not the metal, and ammonia is used elsewhere. Method: clean a wire in acid, dip it in the sample, hold it in a roaring Bunsen flame and read the colour against the Li/Na/K chart.
W22 P41 Q2(c)(i)
Group I densities are low; they float
Two points on Group I density. First, lithium, sodium and potassium are all below 1 g/cm3 (less than water), which is why they float - unusual for a metal. Second, density broadly increases down the group, so writing decreases (copying the halogen trend) loses the mark. State low densities that increase down Group I.
W22 P13 Q25; W22 P23 Q25
Describe observations, not products
Asked what you observe when sodium meets water, write what you see: it floats, fizzes, darts about, melts into a silvery ball and dissolves. Naming a product - hydrogen is produced, sodium hydroxide forms - is not an observation and earns no mark. Sinks is wrong; sodium floats, density below water's. Keep product names for equations.
Answering Group I trend questions
State direction first: reactivity increases down Group I. Evidence: potassium reacts with water more vigorously than sodium, then lithium. Extended chain: more shells, outer electron further out, more shielded, weaker pull, lost more easily.
Use the 2:2:2:1 water equation
Every Group I metal with water: 2M + 2H2O -> 2MOH + H2 - just substitute the metal, e.g. 2K + 2H2O -> 2KOH + H2. Never leave M + H2O -> MOH + H2 (3 H right, 2 left). State symbols if asked: (s) metal, (l) water, (aq) hydroxide, (g) hydrogen.
Carry out a flame test cleanly
Give the method, not the colour: clean a platinum/nichrome wire in dilute HCl, heat clean, dip in the sample, then hold in the roaring Bunsen flame and read colour. Omitting cleaning loses a mark; silver nitrate tests chloride, not the metal.
Group I (the alkali metals): Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), francium (Fr). IGCSE focuses on Li, Na and K.
Physical properties of Group I metals:
| Property | Observation |
|---|---|
| Appearance | Shiny when freshly cut; rapidly tarnish (oxidise) in air |
| Hardness | Soft — can be cut with a knife; softness increases down the group |
| Density | Low — Li, Na and K all float on water (density < 1 g/cm³) |
| Melting point | Relatively low for metals; decreases down the group |
| Storage | Stored under oil to prevent reaction with air/water |
Reactivity trend: Reactivity increases going down Group I.
State the two products formed when any Group I metal reacts with water.
Lithium reacts with water.
Balance the following equation for this reaction:
Show your balancing clearly.
