Experiment: Reaction of Group 1 metals with water

The reaction of the Group 1 alkali metals with water is the showpiece experiment of the Periodic Table chapter, because it makes a group trend visible in a single lesson. Lithium, sodium and potassium each react with water, but not equally, and comparing them shows clearly that reactivity increases down the group. This guide sets out the method, the observations, and the Paper 3 skills the practical assessment rewards, with safety treated as the first priority throughout.

Aim

To investigate and compare the reactivity of the Group 1 metals lithium, sodium and potassium with water, and to relate the trend in reactivity to their position in the group.

Apparatus and materials

• Small pieces of lithium, sodium and potassium, stored under oil
• Trough or large basin of water
• Forceps and a white tile
• Knife and filter paper
• Red litmus paper or phenolphthalein indicator
• Safety screen and safety goggles

Procedure

1. Half fill a trough with water and, if using phenolphthalein, add a few drops so that any alkali formed can be seen.
2. Place a safety screen between the trough and the observers.
3. Using forceps, take a small piece of lithium from the oil and dry it on filter paper.
4. Cut a very small piece with a knife on a tile, then drop this piece onto the surface of the water and observe.
5. When the reaction is over, test the resulting solution with red litmus paper, or note the colour if phenolphthalein was used.
6. Repeat steps 3 to 5 with a fresh, equally small piece of sodium.
7. Repeat again with an equally small piece of potassium, taking extra care because it reacts most vigorously.
8. Compare the vigour of the three reactions and record the observations for each metal.

Expected observations

Each metal floats on the water and moves about the surface while giving off a gas. Lithium reacts steadily, fizzing gently as it moves. Sodium reacts faster, melting into a shiny ball that darts across the surface with fizzing. Potassium reacts most vigorously of the three: it melts, moves rapidly, and the hydrogen produced ignites with a lilac flame, sometimes with sparks. After each reaction the solution turns red litmus blue, or turns phenolphthalein pink, showing that an alkaline solution has formed.

Inference and conclusion

Each Group 1 metal reacts with water to form a metal hydroxide, which is an alkali, and hydrogen gas, for example, sodium gives sodium hydroxide and hydrogen. The alkaline solution explains the litmus and phenolphthalein results. The vigour of the reaction increases from lithium to sodium to potassium, so reactivity increases down Group 1. This is because, going down the group, the atoms have more shells, so the single outer electron is further from the nucleus and more shielded; it is therefore lost more easily, and losing that electron is what makes an alkali metal react.

Science process skills (Paper 3 style)

Making a hypothesis. The lower a Group 1 metal is in the group, the more vigorously it reacts with water.

Identifying variables. The manipulated variable is the type of Group 1 metal; the responding variable is the vigour of the reaction with water; the controlled variables include the size of the metal piece, the volume and temperature of the water.

Tabulating data. Draw a table with a row for each metal and columns for the observations and for the litmus or indicator result, so the three reactions can be compared directly.

Making inferences. From the increasing vigour down the group, infer that the outer electron is lost more easily in larger atoms; from the indicator change, infer that an alkali is formed.

Operational definition. Reactivity is operationally defined here as how vigorously the metal reacts with water, judged by the speed of movement, the rate of gas release and whether the hydrogen ignites.

Safety precautions

• Use only very small pieces of the metals, because the reaction is vigorous and larger pieces are dangerous.
• Stand a safety screen between the trough and the observers, because hydrogen may ignite and spit.
• Handle the metals with forceps and never with bare hands, because they are corrosive and react with moisture on the skin.
• Wear safety goggles, because the alkali formed is corrosive and can splash.

Common errors

• Using too large a piece. This makes the reaction violent and unsafe; always cut a tiny piece.
• Not drying the metal. Oil left on the surface slows the start and makes the comparison unfair; dry each piece on filter paper.
• Touching the metal with fingers. This is dangerous and contaminates the sample; use forceps.
• Uneven piece sizes. If the pieces differ greatly in size, the comparison of reactivity is not fair; keep them similar.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this experiment to fix the idea of a group trend and its explanation: a student who can link the increasing vigour to atomic size and the ease of losing the outer electron can answer periodic-trend questions across SPM Chemistry. Because comparing reactivity and writing the word and balanced equations are recurring Paper 3 and Paper 2 tasks (Paper 3 is a practical test assessing science process skills), understanding both the observations and the reason here secures marks throughout the Periodic Table topic.