Chemistry 6.1-6.5 - Group 1 metals

Reading families from position

A group is a vertical column in the periodic table. Elements in the same group have related outer-electron arrangements and therefore often have similar chemical properties.

In the group numbering used for this qualification:

This positional pattern lets you classify an element before knowing its individual properties. Hydrogen is often drawn above Group 1, but it is a non-metal and is not an alkali metal; the alkali metals begin with lithium.

Only the position and family names of Group 7 and Group 0 are needed here. Their properties and reactions belong to later Chemistry 6 points. Everything taught in Chemistry 6.1-6.5 is common to Foundation and Higher tier and is assessed on Combined Science Chemistry 2 (1SC0/2CF or 1SC0/2CH).

Shared physical properties

The alkali metals include lithium, sodium, potassium, rubidium and caesium. Their position in one group signals similarities, but Chemistry 6.2 names two physical properties in particular.

Lithium, sodium and potassium can be cut with a knife by a trained demonstrator. That controlled observation supports the description soft; it is not an invitation to handle these hazardous metals.

Alkali metals also have relatively low melting points compared with most metals. For scale, lithium melts at about $180.5\,^{\circ}\mathrm{C}$, sodium at about $97.8\,^{\circ}\mathrm{C}$ and potassium at about $63.5\,^{\circ}\mathrm{C}$. These values are low for metals, but all three substances are still solids at $20\,^{\circ}\mathrm{C}$. "Low melting point" does not mean "liquid at room temperature".

The heat released during a water reaction can melt sodium or potassium into a moving ball. That visible melting is evidence about temperature and melting point; it is not the reason the metal reacts chemically.

What happens in water

Start with the macroscopic evidence: what can be seen when a tiny piece of metal is placed on cold water.

All three pieces eventually disappear because the metal is used up. The bubbles are hydrogen, not oxygen. The metal hydroxide dissolves in the water, making an alkaline solution; universal indicator can turn purple in the product solution.

At the particle level, each Group 1 atom loses its one outer electron and forms a $1+$ ion. Here M stands for a Group 1 metal. The dissolved product contains $\ce{M+}$ and $\ce{OH-}$ ions, while pairs of hydrogen atoms form hydrogen molecules, $\ce{H2}$.

At the symbolic level, the shared reaction is:

alkali metal + water → metal hydroxide + hydrogen

$\ce{2M(s) + 2H2O(l) -> 2MOH(aq) + H2(g)}$

For sodium, this becomes:

$\ce{2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)}$

The coefficients conserve matter: both sides contain two metal atoms, four hydrogen atoms and two oxygen atoms. Changing a subscript in $\ce{H2O}$ or NaOH would change the substance, so equations are balanced by changing coefficients only.

Reading a teacher demonstration

This is not a mandatory practical in Chemistry 6.1-6.5. In school it is appropriately carried out as a teacher demonstration, because the metals are corrosive and highly flammable and the hydrogen produced can ignite.

• Purpose: compare the qualitative vigour of lithium, sodium and potassium reacting with water and identify the shared products.
• Arrangement: a large trough is half-filled with fresh water and a little universal indicator; a close safety screen separates the trough from the class. A trained demonstrator uses dry forceps to transfer one very small, freshly cut piece at a time, with the stock bottle closed before the metal reaches the water.
• Independent variable: identity of the metal. Dependent evidence: fizzing intensity, movement, melting or ignition, and the time taken for the visible metal to disappear. Controls: water volume, depth and starting temperature, trough geometry, observation distance and, as far as the risk assessment safely allows, exposed surface area.
• Recording and processing: agree an observation scale before starting, record the same features for each metal and order the metals from least to most vigorous. Video at a fixed frame rate can make movement and disappearance time easier to compare. Repetition should occur only if the teacher's current risk assessment permits; replaying a recording is safer than repeating an unnecessary hazardous demonstration.
• Safety: everyone wears eye protection; learners remain 2-3 m away; at least two close screens or a suitable cover control hot alkaline splashes and burning material. Only tiny prepared pieces are used, dry forceps prevent accidental contact with water, and no learner handles the metals. Any school procedure must follow the current local/CLEAPSS risk assessment.
• Limitation and improvement: safer demonstrations use smaller pieces for the more reactive metals, so mass and surface area are not perfectly controlled and the result is not a precise rate comparison. Freshly cutting each sample limits different oxide coatings; keeping the water conditions fixed and reviewing slow-motion video makes the qualitative comparison more consistent, but the conclusion should remain an order of reactivity rather than a numerical rate.