Reactivity trends of the alkali metals

These demonstrations show the similarity of the physical and chemical properties of the alkali metals and the trend in reactivity down group 1 of the Periodic Table. 

The experiments take about 10–20 minutes if everything is prepared in advance. Advance preparation includes cutting pieces of alkali metals to the recommended size, filling water troughs and setting up safety screens. You should try the experiments in advance if you have not done them before.

Equipment

Apparatus

All experiments

• Goggles or a face shield for the demonstrator, eye protection for the audience
• Tweezers or forceps
• Filter paper
• Beaker, 150 cm³
• Ceramic tile
• Scalpel or sharp knife to cut the metals

Experiment one

• Protective gloves (preferably nitrile)
• Small iron nail
• Power supply, ~6 V
• Light bulb in holder (for testing conductivity)
• Electrical leads
• Petri dishes, with lids
• Adhesive tape (to seal Petri dishes)

Experiment two

• One or more large glass troughs, 5 dm³ capacity
• Safety screens, at least x2
• Glass or Perspex sheets to cover troughs (optional)

Chemicals

Small bottles of oil containing small pieces of the following metal (Note 1):

• Lithium (HIGHLY FLAMMABLE, CORROSIVE), 5 mm cubes
• Sodium (HIGHLY FLAMMABLE, CORROSIVE), 4 mm cubes
• Potassium (HIGHLY FLAMMABLE, CORROSIVE), 3 mm cubes

Access to:

• Ethanol (HIGHLY FLAMMABLE) or Industrial denatured alcohol (IDA) (HIGHLY FLAMMABLE, HARMFUL), 150 cm³ (Note 2)
• 2-methylpropan-2-ol (HIGHLY FLAMMABLE, HARMFUL), 150 cm³ (Note 2)
• Detergent, 1 drop
• Universal indicator solution (HIGHLY FLAMMABLE)

Technical notes

1. A technician should prepare the pieces of metal and store them under oil. Using the tweezers, remove a large piece of the alkali metal from the oil. Ensure that conditions are dry. Place the metal on a tile and, using a scalpel or sharp knife, cut pieces of lithium (5 mm cubes), sodium (4 mm cubes) and potassium (3 mm cubes). Place the small pieces in separate bottles of oil, labelled with the metal name and the hazard symbol. Cut each alkali metal separately and return the larger piece to its bottle before starting the next one.
2. Place any apparatus used to cut (and later handle) the metal (filter paper, scalpels etc) in a trough of water after use. Small pieces of alkali metal for disposal should be allowed to react fully with ethanol (for lithium and sodium) or 2-methylpropan-2-ol (for potassium) until fizzing stops, before washing away with water.

Health and safety

• Read our standard health & safety guidance
• Demonstrator to wear goggles or a face shield. Pupils to be 2–3 m away and wearing eye protection.
• Lithium, Li(s), sodium, Na(s) and potassium, K(s), (all HIGHLY FLAMMABLE, CORROSIVE) – see CLEAPSS Hazcards HC058a.
• Ethanol, C₂H₅OH(l), (HIGHLY FLAMMABLE or, if IDA, HIGHLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040a.
• 2-methylpropan-2-ol, (CH₃)₃COH(l) – see CLEAPSS Hazcard HC084a.
• Universal indicator solution (FLAMMABLE) – see CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book.

Procedure

Experiment 1: physical properties 

Do the following with a small piece of each metal, lithium, sodium and potassium

1. Remove the metal from the oil bottle with tweezers.
2. Cut the metal with a scalpel (lithium is the hardest to cut). Place any pieces that will not be used back into the oil.
3. Wearing protective gloves, squeeze the remaining metal in your gloved hand to show its softness. Lithium is the hardest to mould.
4. Drop each metal from a height of a few centimetres on to a piece of filter paper on the bench. Compare with a piece of iron of about the same size. Note the gentle impacts that reveal that the metals have a low density.
5. Use the circuit to show that the metals conduct electricity well.
6. Place the piece of metal in a petri dish and pass around the class. They must NOT touch the piece of metal – tape the lid shut if necessary.
7. Dispose of the lithium and sodium in ethanol, and dispose of the potassium in 2-methylpropan-2-ol.

Experiment 2: reaction with water

1. Fill the trough(s) about half-full of water. Add a drop of detergent (to stop the metals sticking to the side). Place enough Universal indicator solution into each, with stirring, until the colour is clearly visible.
2. At least two safety screens should be used, as close to the troughs as possible. Alternatively use glass plates to cover the trough(s).
3. Ensure that stock bottles are closed (and no metal comes into contact with splashed water) before the reaction begins.
4. Ensure that the students cannot pick up pieces of the alkali metals.
5. Using dry tweezers, remove a small piece of lithium from the stock bottle, place the lithium on a filter paper and close the bottle. Use the filter paper to wipe off the oil.
6. Using tweezers drop the piece of metal on to the water surface in a trough (and place the cover over the trough).
7. Repeat with a piece of filter paper floated on the surface of the water, placing another piece of lithium on the paper.
8. In both cases, the lithium should float and fizz – giving off hydrogen. The water in the trough turns alkaline. The lithium does not catch fire.
9. Use a fresh trough (or fresh water) for the next metal. Note that the reaction heats up the water.
10. Repeat with sodium. This time the piece on the water floats, moves around more vigorously and fizzes. It may catch fire if it sticks to the side of the trough. On the filter paper it will only catch fire. It burns with a yellow flame and gives off a white smoke. TAKE CARE when sodium and potassium catch fire – they give off corrosive oxides as smokes. The students must be far enough away not to breathe these (or the troughs should be covered).
11. Replace the water and repeat with potassium. Do not use the filter paper. Be careful as potassium is very reactive. The potassium moves around vigorously, melts, fizzes and catches fire, burning with a lilac flame.
12. Be careful to ensure that no traces of the metals remain on the tweezers, filter paper etc. The simplest way is to place them in one of the troughs of water to react. Small pieces of metals can be disposed of in ethanol (for lithium and sodium) or 2-methylpropan-2-ol (for potassium) and the residue flushed down the sink with when all reaction has ceased.

Teaching notes

The reaction with water can be done with the trough on an overhead projector. The projector should first be focussed on a matchstick floating in the water. Alternatively use a flexicam linked to a projector or a TV screen.

The reactions are:

2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)

where M represents the alkali metal. The solutions of the hydroxides formed are alkaline.

The reactions clearly show that the reactivity sequence is lithium < sodium < potassium.